DOS Reference chapter 4

       **  Programmer's Technical Reference for MSDOS and the IBM PC **
                USA copyright TXG 392-616  ALL RIGHTS RESERVED
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ DOSREF (tm) ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
                     ISBN 1-878830-02-3 (disk-based text)
                    Copyright (c) 1987, 1991 Dave Williams
                        ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
                        ³ Shareware Version, 12/20/91 ³
                        ³  Please Register Your Copy  ³
                        ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ


                           C H A P T E R    F O U R

                       DOS INTERRUPTS AND FUNCTION CALLS

       note: The registered version of this chapter is twice this size.



DOS REGISTERSÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

 DOS uses the following registers, pointers, and flags when it executes
interrupts and function calls:
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
³GENERAL REGISTERS  ³ register ³                  definition                   ³
³                   ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
³                   ³    AX    ³  accumulator (16 bit)                         ³
³                   ³    AH    ³  accumulator high-order byte (8 bit)          ³
³                   ³    AL    ³  accumulator low order byte (8 bit)           ³
³                   ³    BX    ³  base (16 bit)                                ³
³                   ³    BH    ³  base high-order byte (8 bit)                 ³
³                   ³    BL    ³  base low-order byte (8 bit)                  ³
³                   ³    CX    ³  count (16 bit)                               ³
³                   ³    CH    ³  count high order byte (8 bit)                ³
³                   ³    CL    ³  count low order byte (8 bit)                 ³
³                   ³    DX    ³  data (16 bit)                                ³
³                   ³    DH    ³  date high order byte (8 bit)                 ³
³                   ³    DL    ³  data low order byte (8 bit)                  ³
ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
³SEGMENT REGISTERS  ³ register ³                  definition                   ³
³                   ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
³                   ³    CS    ³  code  segment (16 bit)                       ³
³                   ³    DS    ³  data  segment (16 bit)                       ³
³                   ³    SS    ³  stack segment (16 bit)                       ³
³                   ³    ES    ³  extra segment (16 bit)                       ³
ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
³INDEX REGISTERS    ³ register ³                  definition                   ³
³                   ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
³                   ³    DI    ³  destination index (16 bit)                   ³
³                   ³    SI    ³  stack       index (16 bit)                   ³
ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
³SEGMENT REGISTERS  ³ register ³                  definition                   ³
³                   ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
³                   ³    CS    ³  code  segment (16 bit)                       ³
³                   ³    DS    ³  data  segment (16 bit)                       ³
³                   ³    SS    ³  stack segment (16 bit)                       ³
³                   ³    ES    ³  extra segment (16 bit)                       ³
ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
³INDEX REGISTERS    ³ register ³                  definition                   ³
³                   ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
³                   ³    DI    ³  destination index (16 bit)                   ³
³                   ³    SI    ³  stack       index (16 bit)                   ³
ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
³POINTERS           ³ register ³                  definition                   ³
³                   ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
³                   ³    SP    ³  stack pointer (16 bit)                       ³
³                   ³    BP    ³  base pointer (16 bit)                        ³
³                   ³    IP    ³  instruction pointer (16 bit)                 ³
ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
³FLAGS                   AF, CF, DF, IF, OF, PF, SF, TF, ZF                    ³
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

 These registers, pointers, and flags are "lowest common denominator" 8088-8086
CPU oriented. DOS makes no attempt to use any of the special or enhanced
instructions availible on the later CPUs which will execute 8088 code, such as
the 80186, 80286, 80386, or NEV V20, V30, V40, or V50.

 When DOS takes control after a function call, it switches to an internal
stack. Registers which are not used to return information (other than AX) are
preserved. The calling program's stack must be large enough to accomodate the
interrupt system - at least 128 bytes in addition to other interrupts.
 DOS actually maintains three stacks -
stack 1: 384 bytes (in DOS 3.1)
         for functions 00h and for 0Dh and up, and for ints 25h and 26h.

stack 2: 384 bytes (in DOS 3.1)
         for function calls 01h through 0Ch.

stack 3: 48 bytes (in DOS 3.1)
         for functions 0Dh and above. This stack is the initial stack used by
         the int 21h handler before it decides which of the other two to use.
         It is also used by function 59h (get extended error), and 01h to 0Ch if
         they are called during an int 24h (critical error) handler. Functions
         33h (get/set break flag), 50h (set process ID), 51h (get process ID)
         and 62h (get PSP address) do not use any DOS stack under DOS 3.x
         (under 2.x, 50h and 51h use stack number 2).

 IBM and Microsoft made a change back in DOS 3.0 or 3.1 to reduce the size of
DOS. They reduced the space allocated for scratch areas when interrupts are
being processed. The default seems to vary with the DOS version and the
machine, but 8 stack frames seems to ring a bell. That means that if you get
more than 8 interrupts at the same time, clock, disk, printer spooler,
keyboard, com port, etc., the system will crash. It seems to happen usually on
a network. STACKS=16,256 means allow 16 interrupts to interrupt each other and
allow 256 bytes for each for scratch area. Eight is marginal.

 DOS 3.2 does some different stack switching than previous versions. The
interrupts which are switched are 02h, 08h, 09h, 0Ah, 0Bh, 0Ch, 0Dh, 0Eh, 70h,
72h, 73h, 74h, 75h, 76h, and 77h. DOS 3.2 has a special check in the
initialization code for a PCjr and don't enable stack switching on that machine.



INTERRUPTSÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

 Microsoft recommends that a program wishing to examine or set the contents of
any interrupt vector use the DOS function calls 35h and 25h provided for those
purposes and avoid referencing the interrupt vector locations directly.
 DOS reserves interrupt numbers 20h to 3Fh for its own use. This means absolute
memory locations 80h to 0FFh are reserved by DOS. The defined interrupts are as
follows with all values in hexadecimal.


ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
³Interrupt 21h  Function Call Request                                         ³
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
(0:0084h)
 DOS provides a wide variety of function calls for character device I/O, file
management, memory management, date and time functions,execution of other
programs, and more. They are grouped as follows:

          call              description
        00h             program terminate
        01h-0Ch         character device I/O, CP/M compatibility format
        0Dh-24h         file management,      CP/M compatibility format
        25h-26h         nondevice functions,  CP/M compatibility format
        27h-29h         file management,      CP/M compatibility format
        2Ah-2Eh         nondevice functions,  CP/M compatibility format
        2Fh-38h         extended functions
        39h-3Bh         directory group
        3Ch-46h         extended file management
        47h             directory group
        48h-4Bh         extended memory management
        54h-57h         extended functions
        5Eh-5Fh         networking
        60h-62h         extended functions
        63h-66h         enhanced foreign language support


List of DOS services:   * = undocumented
        00h     terminate program
        01h     get keyboard input
        02h     display character to STDIO
        03h     get character from STDAUX
        04h     output character to STDAUX
        05h     output character to STDPRN
        06h     direct console I/O - keyboard to screen
        07h     get char from std I/O without echo
        08h     get char from std I/O without echo, checks for ^C
        09h     display a string to STDOUT
        0Ah     buffered keyboard input
        0Bh     check STDIN status
        0Ch     clear keyboard buffer and invoke keyboard function
        0Dh     flush all disk buffers
        0Eh     select disk
        0Fh     open file with File Control Block
        10h     close file opened with File Control Block
        11h     search for first matching file entry
        12h     search for next matching file entry
        13h     delete file specified by File Control Block
        14h     sequential read from file specified by File Control Block
        15h     sequential write to file specified by File Control Block
        16h     find or create firectory entry for file
        17h     rename file specified by file control block
        18h*    unknown
        19h     return current disk drive
        1Ah     set disk transfer area (DTA)
        1Bh     get current disk drive FAT
        1Ch     get disk FAT for any drive
        1Dh*    unknown
        1Eh*    unknown
        1Fh*    read DOS disk block, default drive
        20h*    unknown
        21h     random read from file specified by FCB
        22h     random write to file specified by FCB
        23h     return number of records in file specified by FCB
        24h     set relative file record size field for file specified by FCB
        25h     set interrupt vector
        26h     create new Program Segment Prefix (PSP)
        27h     random file block read from file specified by FCB
        28h     random file block write to file specified by FCB
        29h     parse the command line for file name
        2Ah     get the system date
        2Bh     set the system date
        2Ch     get the system time
        2Dh     set the system time
        2Eh     set/clear disk write VERIFY
        2Fh     get the Disk Transfer Address (DTA)
        30h     get DOS version number
        31h     TSR, files opened remain open
        32h*    read DOS Disk Block
        33h     get or set Ctrl-Break
        34h*    INDOS  Critical Section Flag
        35h     get segment and offset address for an interrupt
        36h     get free disk space
        37h*    get/set option marking character (SWITCHAR)
        38h     return country-dependent information
        39h     create subdirectory
        3Ah     remove subdirectory
        3Bh     change current directory
        3Ch     create and return file handle
        3Dh     open file and return file handle
        3Eh     close file referenced by file handle
        3Fh     read from file referenced by file handle
        40h     write to file referenced by file handle
        41h     delete file
        42h     move file pointer (move read-write pointer for file)
        43h     set/return file attributes
        44h     device IOCTL (I/O control) info
        45h     duplicate file handle
        46h     force a duplicate file handle
        47h     get current directory
        48h     allocate memory
        49h     release allocated memory
        4Ah     modify allocated memory
        4Bh     load or execute a program
        4Ch     terminate prog and return to DOS
        4Dh     get return code of subprocess created by 4Bh
        4Eh     find first matching file
        4Fh     find next matching file
        50h*    set new current Program Segment Prefix (PSP)
        51h*    puts current PSP into BX
        52h*    pointer to the DOS list of lists
        53h*    translates BPB (Bios Parameter Block, see below)
        54h     get disk verification status (VERIFY)
        55h*    create PSP: similar to function 26h
        56h     rename a file
        57h     get/set file date and time
        58h     get/set allocation strategy             (DOS 3.x)
        59h     get extended error information
        5Ah     create a unique filename
        5Bh     create a DOS file
        5Ch     lock/unlock file contents
        5Dh*    network
        5Eh*    network printer
        5Fh*    network redirection
        60h*    parse pathname
        61h*    unknown
        62h     get program segment prefix (PSP)
        63h*    get lead byte table                     (DOS 2.25)
        64h*    unknown
        65h     get extended country information        (DOS 3.3)
        66h     get/set global code page table          (DOS 3.3)
        67h     set handle count                        (DOS 3.3)
        68h     commit file                             (DOS 3.3)
        69h     disk serial number                      (DOS 4.0)
        6Ah     unknown
        6Bh     unknown
        6Ch     extended open/create                    (DOS 4.0)


CALLING THE DOS SERVICESÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

 The DOS services are invoked by placing the number of the desired function in
register AH, subfunction in AL, setting the other registers to any specific
requirements of the function, and invoking int 21h.

 On return, the requested service will be performed if possible. Most codes
will return an error; some return more information. Details are contained in
the listings for the individual functions. Extended error return may be
obtained by calling function 59h (see 59h).

 Register settings listed are the ones used by DOS. Some functions will return
with garbage values in unused registers. Do not test for values in unspecified
registers; your program may exhibit odd behavior.

 DS:DX pointers are the data segment register (DS) indexed to the DH and DL
registers (DX). DX always contains the offset address, DS contains the segment
address.

 The File Control Block services (FCB services) were part of DOS 1.0. Since
the release of DOS 2.0, Microsoft has recommended that these services not be
used. A set of considerably more enhanced services (handle services) were
introduced with DOS 2.0. The handle services provide support for wildcards and
subdirectories, and enhanced error detection via function 59h.

 The data for the following calls was compiled from various Intel, Microsoft,
IBM, and other publications. There are many subtle differences between MSDOS
and PCDOS and between the individual versions. Differences between the
versions are noted as they occur.

 There are various ways of calling the DOS functions. For all methods, the
function number is loaded into register AH, subfunctions and/or parameters are
loaded into AL or other registers, and call int 21 by one of the following
methods:
 A) call interrupt 21h directly  (the recommended procedure)
 B) perform a long call to offset 50h in the program's PSP.
     1) This method will not work under DOS 1.x
     2) Though recommended by Microsoft for DOS 2.0, this method takes more
        time and is no longer recommended.
 C) place the function number in CL and perform an intrasegment call to
    location 05h in the current code segment. This location contains a long
    call to the DOS function dispatcher.
     1) IBM recommends this method be used only when using existing programs
        written for different calling conventions. (such as converting CP/M
        programs). This method should be avoided unless you have some specific
        use for it.
     2) AX is always destroyed by this method.
     3) This method is valid only for functions 00h-24h.

 There are also various ways of exiting from a program. (assuming it is not
intended to be a TSR). All methods except call 4Ch must ensure that the
segment register contains the segment address of the PSP.
 A) Interrupt 21h, function 4Ch (Terminate with Result Code). This is the
    "official" recommended method of returning to DOS.
 B) Interrupt 21h, function 00h (Exit Program). This is the early style
    int 21 function call. It simply calls int 20h.
 C) Interrupt 20h (Exit).
 D) A JMP instruction to offset 00h (int 20h vector) in the Program Segment
    Prefix. This is just a roundabout method to call int 20h. This method
    was set up in DOS 1.0 for ease of conversion for CP/M programs. It is no
    longer recommended for use.
 E) A JMP instruction to offset 05h (int 21 vector) in the Program Segment
    Prefix, with AH set to 00h or 4Ch. This is another CP/M type function.




INT 21H   DOS services
          Function (hex)

* Indicates Functions not documented in the IBM DOS Technical Reference.
 Note some functions have been documented in other Microsoft or licensed OEM
documentation.


Function  00h   Terminate Program
      Ends program, updates, FAT, flushes buffers, restores registers
entry   AH      00h
        CS      segment address of PSP
return  none
note 1) Program must place the segment address of the PSP control block in CS
        before calling this function.
     2) The terminate, ctrl-break,and critical error exit addresses (0Ah, 0Eh,
        12h) are restored to the values they had on entry to the terminating
        program, from the values saved in the program segment prefix at
        locations PSP:000Ah, PSP:000Eh, and PSP:0012h.
     3) All file buffers are flushed and the handles opened by the process are
        closed.
     4) Any files that have changed in length and are not closed are not
        recorded properly in the directory.
     5) Control transfers to the terminate address.
     6) This call performs exactly the same function as int 20h.
     7) All memory used by the program is returned to DOS.


Function  01h     Get Keyboard Input
        Waits for char at STDIN (if nescessary), echoes to STDOUT
entry   AH      01h
return  AL      ASCII character from STDIN (8 bits)
note 1) Checks char for Ctrl-C, if char is Ctrl-C, executes int 23h.
     2) For function call 06h, extended ASCII codes require two function calls.
        The first call returns 00h as an indicator that the next call will be an
        extended ASCII code.
     3) Input and output are redirectable. If redirected, there is no way to
        detect EOF.


Function  02h   Display Output
      Outputs char in DL to STDOUT
entry   AH      02h
        DL      8 bit data (usually ASCII character)
return  none
note 1) If char is 08 (backspace) the cursor is moved 1 char to the left
        (nondestructive backspace).
     2) If Ctrl-C is detected after input, int 23h is executed.
     3) Input and output are redirectable. If redirected, there is no way to
        detect disk full.


Function  03h   Auxiliary Input
      Get (or wait until) character from STDAUX
entry   AH      03h
return  AL      char from auxiliary device
note 1) AUX, COM1, COM2 is unbuffered and not interrupt driven
     2) This function call does not return status or error codes. For greater
        control it is recommended that you use ROM BIOS routine (int 14h) or
        write an AUX device driver and use IOCTL.
     3) At startup, PC-DOS initializes the first auxiliary port (COM1) to 2400
        baud, no parity, one stop bit, and an 8-bit word. MSDOS may differ.
     4) If Ctrl-C is has been entered from STDIN, int 23h is executed.


Function  04h   Auxiliary Output
      Write character to STDAUX
entry   AH      04h
        DL      char to send to AUX
return  none
note 1) This function call does not return status or error codes. For greater
        control it is recommended that you use ROM BIOS routine (int 14h) or
        write an AUX device driver and use IOCTL.
     2) If Ctrl-C is has been entered from STDIN, int 23h is executed.
     3) Default is COM1 unless redirected by DOS.
     4) If the device is busy, this function will wait until it is ready.


Function  05h   Printer Output
      Write character to STDPRN
entry   AL      05h
        DL      character to send
return  none
note 1) If Ctrl-C is has been entered from STDIN, int 23h is executed.
     2) Default is PRN or LPT1 unless redirected with the MODE command.
     3) If the printer is busy, this function will wait until it is ready.


Function  06h   Direct Console I/O
      Get character from STDIN; echo character to STDOUT
entry   AH      06h
        DL      0FFh for console input, or 00h-0FEh for console output
return  ZF      set   (1) = no character
                clear (0) = character recieved
        AL      character
note 1) Extended ASCII codes require two function calls. The first call returns
        00h to indicate the next call will return an extended code.
     2) If DL is not 0FFh, DL is assumed to have a valid character that is
        output to STDOUT.
     3) This function does not check for Ctrl-C or Ctrl-PrtSc.
     4) Does not echo input to screen
     5) If I/O is redirected, EOF or disk full cannot be detected.


Function  07h   Direct Console Input Without Echo         (does not check BREAK)
      Get or wait for char at STDIN, returns char in AL
entry   AH      07h
return  AL      character from standard input device
note 1) Extended ASCII codes require two function calls. The first call returns
        00h to indicate the next call will return an extended code.
     2) No checking for Ctrl-C or Ctrl-PrtSc is done.
     3) Input is redirectable.


Function  08h   Console Input Without Echo                (checks BREAK)
      Get or Wait for char at STDIN, return char in AL
entry   AH      08h
return  AL      char from standard input device
note 1) Char is checked for ctrl-C. If ctrl-C is detected, executes int 23h.
     2) For function call 08h, extended ASCII characters require two function
        calls. The first call returns 00h to signify an extended ASCII code.
        The next call returns the actual code.
     3) Input is redirectable. If redirected, there is no way to check EOF.


Function  09h   Print String
      Outputs Characters in the Print String to the STDOUT
entry   AH      09h
        DS:DX   pointer to the Character String to be displayed
return  none
note 1) The character string in memory must be terminated by a $ (24h)
        The $ is not displayed.
     2) Output to STDOUT is the same as function call 02h.


Function  0Ah   Buffered Keyboard Input
      Reads characters from STDIN and places them in the buffer beginning
      at the third byte.
entry   AH      0Ah
        DS:DX   pointer to an input buffer
return  none
note 1) Min buffer size = 1, max = 255
     2) Char is checked for ctrl-C. If ctrl-C is detected, executes int 23h.
     3) Format of buffer DX:
        byte       contents
         1      Maximum number of chars the buffer will take, including CR.
                Reading STDIN and filling the buffer continues until a carriage
                return (<Enter> or 0Dh) is read. If the buffer fills to one less
                than the maximum number the buffer can hold, each additional
                number read is ignored and ASCII 7 (BEL) is output to the
                display until a carriage return is read. (you must set this
                value)
         2      Actual number of characters received, excluding the carriage
                return, which is always the last character. (the function sets
                this value)
         3-n    Characters received are placed into the buffer starting here.
                Buffer must be at least as long as the number in byte 1.
     4) Input is redirectable. If redirected, there is no way to check EOF.
     5) The string may be edited with the standard DOS editing commands as it
        is being entered.
     6) Extended ASCII characters are stored as 2 bytes, the first byte being
        zero.


Function  0Bh   Check Standard Input (STDIN) status
      Checks for character availible at STDIN
entry   AH      0Bh
return  AL      0FFh    if a character is availible from STDIN
                00h     if no character is availible from STDIN
note 1) Checks for Ctrl-C. If Ctrl-C is detected, int 23h is executed
     2) Input can be redirected.
     3) Checks for character only, it is not read into the application
     4) IBM reports that this call does not work properly under the DOSSHELL
        program in DOS 4.00 and 4.01. DOSSHELL will return all zeroes. This
        function works correctly from the command line or application.


Function  0Ch   Clear Keyboard Buffer & Invoke a Keyboard Function       (FCB)
      Dumps buffer, executes function in AL (01h,06h,07h,08h,0Ah only)
entry   AH      0Ch
        AL      function number (must be 01h, 06h, 07h, 08h, or 0Ah)
return  AL      00h     buffer was flushed, no other processing performed
                other   any other value has no meaning
note 1) Forces system to wait until a character is typed.
     2) Flushes all typeahead input, then executes function specified by AL (by
        moving it to AH and repeating the int 21 call).
     3) If AL contains a value not in the list above, the keyboard buffer is
        flushed and no other action is taken.


Function  0Dh   Disk Reset
      Flushes all currently open file buffers to disk
entry   AH      0Dh
return          none
note 1) Does not close files. Does not update directory entries; files changed
        in size but not closed are not properly recorded in the directory
     2) Sets DTA address to DS:0080h
     3) Should be used before a disk change, Ctrl-C handlers, and to flush
        the buffers to disk.


Function  0Eh   Select Disk
      Sets the drive specified in DL (if valid) as the default drive
entry   AL      0Eh
        DL      new default drive number (0=A:,1=B:,2=C:,etc.)
return  AL      total number of logical drives (not nescessarily physical)
note 1) For DOS 1.x and 2.x, the minimum value for AL is 2.
     2) For DOS 3.x and 4.x, the minimum value for AL is 5.
     3) The drive number returned is not nescessarily a valid drive.
     4) For DOS 1.x: 16 logical drives are availible, A-P.
        For DOS 2.x: 63 logical drives are availible. (Letters are only used for
                     the first 26 drives. If more than 26 logical drives are
                     used, further drive letters will be other ASCII characters
                     ie {,], etc.
        For DOS 3.x: 26 logical drives are availible, A-Z.
        For DOS 4.x: 26 logical drives are availible, A-Z.


Function  0Fh   Open Disk File                                            (FCB)
      Searches current directory for specified filename and opens it
entry   AH      0Fh
        DS:DX   pointer to an unopened FCB
return  AL      00h     if file found
                0FFh    if file not not found
note 1) If the drive code was 0 (default drive) it is changed to the actual
        drive used (1=A:,2=B:,3=C:, etc). This allows changing the default drive
        without interfering with subsequent operations on this file.
     2) The current block field (FCB bytes C-D, offset 0Ch) is set to zero.
     3) The size of the record to be worked with (FCB bytes E-F, offset 0Eh) is
        set to the system default of 80h. The size of the file (offset 10h) and
        the date (offset 14h) are set from information obtained in the root
        directory. You can change the default value for the record size (FCB
        bytes E-F) or set the random record size and/or current record field.
        Perform these actions after the open but before any disk operations.
     4) The file is opened in compatibility mode.
     5) Microsoft recommends handle function call 3Dh be used instead.
     6) This call is also used by the APPEND command in DOS 3.2+
     7) Before performing a sequential disk operation on the file, you must
        set the Current Record field (offset 20h). Before performing a random
        disk operation on the file, you must set the Relative Record field
        (offset 21h). If the default record size of 128 bytes is incorrect, set
        it to the correct value.


Function  10h  Close File                                              (FCB)
     Closes a File After a File Write
entry   AH      10h
        DS:DX   pointer to an opened FCB
return  AL      00h     if the file is found and closed
                0FFh    if the file is not found in the current directory
note 1) This function call must be done on open files that are no longer needed,
        and after file writes to insure all directory information is updated.
     2) If the file is not found in its correct position in the current
        directory, it is assumed that the diskette was changed and AL returns
        0FFh. This error return is reportedly not completely reliable with DOS
        version 2.x.
     3) If found, the directory is updated to reflect the status in the FCB, the
        buffers to that file are flushed, and AL returns 00h.


Function  11h   Search For First Matching Entry                           (FCB)
      Searches current disk & directory for first matching filename
entry   AH      11h
        DS:DX   pointer to address of FCB
return  AL      00h     successful match
                0FFh    no matching filename found
note 1) The FCB may contain the wildcard character ? under Dos 2.x, and ? or *
        under 3.x and 4.x.
     2) The original FCB at DS:DX contains information to continue the search
        with function 12h, and should not be modified.
     3) If a matching filename is found, AL returns 00h and the locations at the
        Disk Transfer Address are set as follows:
        a) If the FCB provided for searching was an extended FCB, then the first
           byte at the disk transfer address is set to 0FFh followed by 5 bytes
           of zeroes, then the attribute byte from the search FCB, then the
           drive number used (1=A, 2=B, etc) then the 32 bytes of the directory
           entry. Thus, the disk transfer address contains a valid unopened FCB
           with the same search attributes as the search FCB.
        b) If the FCB provided for searching was a standard FCB, then the first
           byte is set to the drive number used (1=A,2=b,etc), and the next 32
           bytes contain the matching directory entry. Thus, the disk transfer
           address contains a valid unopened normal FCB.
     4) If an extended FCB is used, the following search pattern is used:
        a) If the FCB attribute byte is zero, only normal file entries are
           found. Entries for volume label, subdirectories, hidden or system
           files, are not returned.
        b) If the attribute byte is set for hidden or system files, or
           subdirectory entries, it is to be considered as an inclusive search.
           All normal file entries plus all entries matching the specified
           attributes are returned. To look at all directory entries except the
           volume label, the attribute byte may be set to hidden + system +
           directory (all 3 bits on).
        c) If the attribute field is set for the volume label, it is considered
           an exclusive search, and ONLY the volume label entry is returned.
     5) This call is also used by the APPEND command in DOS 3.2+


Function  12h   Search For Next Entry Using FCB                          (FCB)
      Search for next matching filename
entry   AH      12h
        DS:DX   pointer to the unopened FCB specified from the previous Search
                First (11h) or Search Next (12h)
return  AL      00h     if matching filename found
                0FFh    if matching filename was not found
note 1) After a matching filename has been found using function call 11h,
        function 12h may be called to find the next match to an ambiguous
        request. For DOS 2.x, ?'s are allowed in the filename. For DOS 3.x
        and 4.x, global (*) filename characters are allowed.
     2) The DTA contains info from the previous Search First or Search Next.
     3) All of the FCB except for the name/extension field is used to keep
        information nescessary for continuing the search, so no disk operations
        may be performed with this FCB between a previous function 11h or 12h
        call and this one.
     4) If the file is found, an FCB is created at the DTA address and set up to
        open or delete it.


Function  13h   Delete File Via FCB                                       (FCB)
      Deletes file specified in FCB from current directory
entry   AH      13h
        DS:DX   pointer to address of FCB
return  AL      00h     file deleted
                0FFh    if file not found or was read-only
note 1) All matching current directory entries are deleted. The global filename
        character "?" is allowed in the filename.
     2) Will not delete files with read-only attribute set
     3) Close open files before deleting them.
     4) Requires Network Access Rights


Function  14h   Sequential Disk File Read                                 (FCB)
      Reads record sequentially from disk via FCB
entry   AH  14h
        DS:DX   pointer to an opened FCB
return  AL      00h     successful read
                01h     end of file (no data read)
                02h     Data Transfer Area too small for record size specified
                        or segment overflow
                03h     partial record read, EOF found
note 1) The record size is set to the value at offset 0Eh in the FCB.
     2) The record pointed to by the Current Block (offset 0Ch) and the Current
        Record (offset 20h) fields is loaded at the DTA, then the Current Block
        and Current Record fields are incremented.
     3) The record is read into memory at the current DTA address as specified
        by the most recent call to function 1Ah. If the size of the record and
        location of the DTA are such that a segment overflow or wraparound would
        occur, the error return is set to AL=02h
     4) If a partial record is read at the end of the file, it is passed to the
        requested size with zeroes and the error return is set to AL=03h.


Function  15h   Sequential Disk Write                                     (FCB)
      Writes record specified by FCB sequentially to disk
entry   AH      15h
        DS:DX   pointer to address of FCB
return  AL      00h     successful write
                01h     diskette full, write canceled
                02h     disk transfer area (DTA) too small or segment wrap
note 1) The data to write is obtained from the disk transfer area
     2) The record size is set to the value at offset 0Eh in the FCB.
     3) This service cannot write to files set as read-only
     4) The record pointed to by the Current Block (offset 0Ch) and the Current
        Record (offset 20h) fields is loaded at the DTA, then the Current Block
        and Current Record fields are incremented.
     5) If the record size is less than a sector, the data in the DTA is written
        to a buffer; the buffer is written to disk when it contains a full
        sector of data, the file is closed, or a Reset Disk (function 0Dh) is
        issued.
     6) The record is written to disk at the current DTA address as specified
        by the most recent call to function 1Ah. If the size of the record and
        location of the DTA are such that a segment overflow or wraparound would
        occur, the error return is set to AL=02h


Function  16h   Create A Disk File                                        (FCB)
      Search and open or create directory entry for file
entry   AH      16h
        DS:DX   pointer to an FCB
return  AL      00h     successful creation
                0FFh    no room in directory
note 1) If a matching directory entry is found, the file is truncated to zero
        bytes.
     2) If there is no matching filename, a filename is created.
     3) This function calls function 0Fh (Open File) after creating or
        truncating a file.
     4) A hidden file can be created by using an extended FCB with the attribute
        byte (offset FCB-1) set to 2.


Function  17h   Rename File Specified by File Control Block              (FCB)
      Renames file in current directory
entry   AH      17h
        DS:DX   pointer to an FCB (see note 4)
return  AL      00h     successfully renamed
                0FFh    file not found or filename already exists
note 1) This service cannot rename read-only files
     2) The "?" wildcard may be used.
     3) If the "?" wildcard is used in the second filename, the corresponding
        letters in the filename of the directory entry are not changed.
     4) The FCB must have a drive number, filename, and extension in the usual
        position, and a second filename starting 6 bytes after the first, at
        offset 11h.
     5) The two filenames cannot have the same name.
     6) FCB contains new name starting at byte 17h.


Function  18h  Internal to DOS
 *   Unknown
entry   AH      18h
return  AL      0


Function  19h   Get Current Disk Drive
      Return designation of current default disk drive
entry   AH      19h
return  AL      current default drive (0=A, 1=B,etc.)
note    Some other DOS functions use 0 for default, 1=A, 2=B, etc.


Function  1Ah   Set Disk Transfer Area Address (DTA)
      Sets DTA address to the address specified in DS:DX
entry   AH      1Ah
        DS:DX   pointer to buffer
return  none
note 1) The default DTA is 128 bytes at offset 80h in the PSP. DOS uses the
        DTA for all file I/O.
     2) Registers are unchanged.
     3) No error codes are returned.
     2) Disk transfers cannot wrap around from the end of the segment to the
        beginning or overflow into another segment.


Function  1Bh   Get Current Drive File Allocation Table Information
      Returns information from the FAT on the current drive
entry   AH      1Bh
exit    AL      number of sectors per allocation unit (cluster)
        DS:BX   address of the current drive's media descriptor byte
        CX      number of bytes per sector
        DX      number of allocation units (clusters) for default drive
note 1) Save DS before calling this function.
     2) This call returned a pointer to the FAT in DOS 1.x. Beginning with
        DOS 2.00, it returns a pointer only to the table's ID byte.
     3) IBM recommends programmers avoid this call and use int 25h instead.


Function  1Ch   Get File Allocation Table Information for Specific Device
      Returns information on specified drive
entry   AH      1Ch
        DL      drive number (1=A, 2=B, 3=C, etc)
return  AL      number of sectors per allocation unit (cluster)
        DS:BX   address of media descriptor byte for drive in DL
        CX      sector size in bytes
        DX      number of allocation units (clusters)
note 1) DL = 0 for default.
     2) Save DS before calling this function.
     3) Format of media-descriptor byte:
        bits:   0       0   (clear)   not double sided
                        1   (set)     double sided
                1       0   (clear)   not 8 sector
                        1   (set)     8 sector
                2       0   (clear)   nonremovable device
                        1   (set)     removable device
                3-7     always set (1)
     4) This call returned a pointer to the FAT in DOS 1.x. Beginning with
        DOS 2.00, it returns a pointer only to the table's ID byte.
     5) IBM recommends programmers avoid this call and use int 25h instead.


Function  1Dh   Not Documented by Microsoft
 *    Unknown
entry   AH      1Dh
return  AL      0


Function  1Eh   Not Documented by Microsoft
 *    Unknown
entry   AH      1Eh
return  AL      0
note    Apparently does nothing


Function  1Fh Get Default Drive Parameter Block
 *  Same as function call 32h (below), except that the table is accessed from
    the default drive
entry   AH      1Fh
        other registers unknown
return  AL      00h     no error
                0FFh    error
        DS:BX   points to DOS Disk Parameter Block for default drive.
note 1) Unknown vector returned in ES:BX.
     2) For DOS 2.x and 3.x, this just invokes function 32h (undocumented,
        Read DOS Disk Block) with DL=0


Function  20h  Unknown
 *   Internal - does nothing?
entry   AH      20h
return  AL      0


Function  21h  Random Read from File Specified by File Control Block     (FCB)
     Reads one record as specified in the FCB into the current DTA.
entry   AH      21h
        DS:DX   address of the opened FCB
return  AL      00h     successful read operation
                01h     end of file (EOF), no data read
                02h     DTA too small for the record size specified
                03h     end of file (EOF), partial data read
note 1) The current block and current record fields are set to agree with the
        random record field. Then the record addressed by these fields is read
        into memory at the current Disk Transfer Address.
     2) The current file pointers are NOT incremented this function.
     3) If the DTA is larger than the file, the file is padded to the requested
        length with zeroes.


Function  22h  Random Write to File Specified by FCB                      (FCB)
     Writes one record as specified in the FCB to the current DTA
entry   AH      22h
        DS:DX   address of the opened FCB
return  AL      00h     successful write operation
                01h     disk full; no data written (write was canceled)
                02h     DTA too small for the record size specified (write was
                        canceled)
note 1) This service cannot write to read-only files.
     2) The record pointed to by the Current Block (offset 0Ch) and the Current
        Record (offset 20h) fields is loaded at the DTA, then the Current Block
        and Current Record fields are incremented.
     3) If the record size is less than a sector, the data in the DTA is written
        to a buffer; the buffer is written to disk when it contains a full
        sector of data, the file is closed, or a Reset Disk (function 0Dh) is
        issued.
     4) The current file pointers are NOT incremented this function.
     5) The record is written to disk at the current DTA address as specified
        by the most recent call to function 1Ah. If the size of the record and
        location of the DTA are such that a segment overflow or wraparound would
        occur, the error return is set to AL=02h


Function  23h  Get File Size                                             (FCB)
     Searches current subdirectory for matching file, returns size in FCB
entry   AH      23h
        DS:DX   address of an unopened FCB
return  AL      00h file found
                0FFh file not found
note 1) Record size field (offset 0Eh) must be set before invoking this function
     2) The disk directory is searched for the matching entry. If a matching
        entry is found, the random record field is set to the number of records
        in the file. If the value of the Record Size field is not an even
        divisor of the file size, the value set in the relative record field is
        rounded up. This gives a returned value larger than the actual file size
     3) This call is used by the APPEND command in DOS 3.2+


Function  24h  Set Relative Record Field                                  (FCB)
     Set random record field specified by an FCB
entry   AH      24h
        DS:DX   address of an opened FCB
return  Random Record Field of FCB is set to be same as Current Block
        and Current Record.
note 1) You must invoke this function before performing random file access.
     2) The relative record field of FCB (offset 21h) is set to be same as the
        Current Block (offset 0Ch) and Current Record (offset 20h).
     3) No error codes are returned.
     4) The FCB must already be opened.


Function  25h  Set Interrupt Vector
     Sets the address of the code DOS is to perform each time the specified
     interrupt is invoked.
entry   AH      25h
        AL      int number to reassign the handler to
        DS:DX   address of new interrupt vector
return  none
note 1) Registers are unchanged.
     2) No error codes are returned.
     3) The interrupt vector table for the interrupt number specified in AL
        is set to the address contained in DS:DX. Use function 35h (Get Vector)
        to get the contents of the interrupt vector and save it for later use.
     4) When you use function 25 to set an interrupt vector, DOS 3.2 doesn't
        point the actual interrupt vector to what you requested. Instead, it
        sets the interrupt vector to point to a routine inside DOS, which does
        this:
                1. Save old stack pointer
                2. Switch to new stack pointer allocated from DOS's stack pool
                3. Call your routine
                4. Restore old stack pointer
        The purpose for this was to avoid possible stack overflows when there
        are a large number of active interrupts. IBM was concerned (this was an
        IBM change, not Microsoft) that on a Token Ring network there would be
        a lot of interrupts going on, and applications that hadn't allocated
        very much stack space would get clobbered.


Function  26h  Create New Program Segment Prefix (PSP)
     This service copies the current program-segment prefix to a new memory
     location for the creation of a new program or overlay. Once the new PSP is
     in place, a DOS program can read a DOS COM or overlay file into the memory
     location immediately following the new PSP and pass control to it.
entry   AH      26h
        DX      segment number for the new PSP
return  none
note 1) Microsoft recommends you use the newer DOS service 4Bh (EXEC) instead.
     2) The entire 100h area at location 0 in the current PSP is copied into
        location 0 of the new PSP. The memory size information at location 6
        in the new segment is updated and the current termination, ctrl-break,
        and critical error addresses from interrupt vector table entries for
        ints 22h, 23h, and 24 are saved in the new program segment starting at
        0Ah. They are restored from this area when the program terminates.
     3) Current PSP is copied to specified segment


Function  27h  Random Block Read From File Specified by FCB
     Similar to 21h (Random Read) except allows multiple files to be read.
entry   AH      27h
        CX      number of records to be read
        DS:DX   address of an opened FCB
return  AL      00h     successful read
                01h     end of file, no data read
                02h     DTA too small for record size specified (read canceled)
                03h     end of file
        CX      actual number of records read (includes partial if AL=03h)
note 1) The record size is specified in the FCB. The service updates the Current
        Block (offset 0Ch) and Current Record (offset 20h) fields to the next
        record not read.
     2) If CX contained 0 on entry, this is a NOP.
     3) If the DTA is larger than the file, the file is padded to the requested
        length with zeroes.
     4) This function assumes that the FCB record size field (0Eh) is correctly
        set. If not set by the user, the default is 128 bytes.
     5) The record is written to disk at the current DTA address as specified
        by the most recent call to function 1Ah. If the size of the record and
        location of the DTA are such that a segment overflow or wraparound would
        occur, the error return is set to AL=02h


Function  28h  Random Block Write to File Specified in FCB
     Similar to 27h (Random Write) except allows multiple files to be read.
entry   AH      28h
        CX      number of records to write
        DS:DX   address of an opened FCB
return  AL      00h     successful write
                01h     disk full, no data written
                02h     DTA too small for record size specified (write canceled)
        CX      number of records written
note 1) The record size is specified in the FCB.
     2) This service allocates disk clusters as required.
     3) This function assumes that the FCB Record Size field (offset 0Eh) is
        correctly set. If not set by the user, the default is 128 bytes.
     4) The record size is specified in the FCB. The service updates the Current
        Block (offset 0Ch) and Current Record (offset 20h) fields to the next
        record not read.
     5) The record is written to disk at the current DTA address as specified
        by the most recent call to function 1Ah. If the size of the record and
        location of the DTA are such that a segment overflow or wraparound would
        occur, the error return is set to AL=02h
     6) If called with CX=0, no records are written, but the FCB's File Size
        entry (offset 1Ch) is set to the size specified by the FCB's Relative
        Record field (offset 21h).


Function  29h  Parse the Command Line for Filename
     Parses a text string into the fields of a File Control Block
entry   AH      29h
        DS:SI   pointer to string to parse
        ES:DI   pointer to memory buffer to fill with unopened FCB
        AL      bit mask to control parsing
                bit 0 = 0: parsing stops if file seperator found
                        1: causes service to scan past leading chars such as
                           blanks. Otherwise assumes the filename begins in
                           the first byte
                    1 = 0: drive number in FCB set to default (0) if string
                           contains no drive number
                        1: drive number in FCB not changed
                    2 = 0: filename in FCB set to 8 blanks if no filename in
                           string
                        1: filename in FCB not changed if string does not
                           contain a filename
                    3 = 0: extension in FCB set to 3 blanks if no extension in
                           string
                        1: extension left unchanged
                    4-7    must be zero
return  AL      00h     no wildcards in name or extension
                01h     wildcards appeared in name or extension
                0FFh    invalid drive specifier
        DS:SI   pointer to the first byte after the parsed string
        ES:DI   pointer to a buffer filled with the unopened FCB
note 1) If the * wildcard characters are found in the command line, this service
        will replace all subsequent chars in the FCB with question marks.
     2) This service uses the characters as filename separators
        DOS 1       : ; . , + / [ ] = " TAB SPACE
        DOS 2,3     : ; . , + = TAB SPACE
     3) This service uses the characters
        : ; . , + < > | / \ [ ] = " TAB SPACE
        or any control characters as valid filename separators
     4) A filename cannot contain a filename terminator. If one is encountered,
        all processing stops. The handle functions will allow use of some of
        these characters.
     5) If no valid filename was found on the command line, ES:DI +1 points
        to a blank (ASCII 32).
     6) This function cannot be used with filespecs which include a path
     7) Parsing is in the form D:FILENAME.EXT. If one is found, a corresponding
        unopened FCB is built at ES:DI


Function  2Ah  Get Date
     Returns day of the week, year, month, and date
entry   AH      2Ah
return  CX      year    (1980-2099)
        DH      month   (1-12)
        DL      day     (1-31)
        AL      weekday 00h     Sunday
                        01h     Monday
                        02h     Tuesday
                        03h     Wednesday
                        04h     Thursday
                        05h     Friday
                        06h     Saturday
note 1) Date is adjusted automatically if clock rolls over to the next day,
        and takes leap years and number of days in each month into account.
     2) Although DOS cannot set an invalid date, it can read one, such as
        1/32/80, etc.
     3) DesQview also accepts CX = 4445h and DX = 5351h, i.e. 'DESQ' as valid
     4) DOS will accept CH=0 (midnight) as a valid time, but if a file's time
        is set to exactly midnight the time will not be displayed by the DIR
        command.


Function  2Bh  Set Date
     set current system date
entry   AH      2Bh
        CX      year    (1980-2099)
        DH      month   (1-12)
        DL      day     (1-31)
return  AL      00h     no error (valid date)
                0FFh    invalid date specified
note 1) On entry, CX:DX must have a valid date in the same format as returned
        by function call 2Ah
     2) DOS 3.3 also sets CMOS clock


Function  2Ch  Get Time
     Get current system time from CLOCK$ driver
entry   AH      2Ch
return  CH      hours   (0-23)
        CL      minutes (0-59)
        DH      seconds (0-59)
        DL      hundredths of a second (0-99)
note 1) Time is updated every 5/100 second.
     2) The date and time are in binary format


Function  2Dh  Set Time
     Sets current system time
entry   AH      2Dh
        CH      hours   (0-23)
        CL      minutes (0-59)
        DH      seconds (0-59)
        DL      hundredths of seconds (0-99)
return  AL      00h     if no error
                0FFh    if bad value sent to routine
note 1) DOS 3.3 also sets CMOS clock
     2) CX and DX must contain a valid time in binary


Function  2Eh  Set/Reset Verify Switch
     Set verify flag
entry   AH      2Eh
        AL      00      to turn verify off (default)
                01      to turn verify on
return  none
note 1) This is the call invoked by the DOS VERIFY command
     2) Setting of the verify switch can be obtained by calling call 54h
     3) This call is not supported on network drives
     4) DOS checks this flag each time it accesses a disk


Function  2Fh  Get Disk Transfer Address (DTA)
     Returns current disk transfer address used by all DOS read/write operations
entry   AH      2Fh
return  ES:BX   address of DTA
note 1) The DTA is set by function call 1Ah
     2) Default DTA address is a 128 byte buffer at offset 80h in that program's
        Program Segment Prefix


Function  30h  Get DOS Version Number
     Return DOS version and/or user number
entry   AH      30h
return  AH      minor version number  (i.e., DOS 2.10 returns AX = 0A02h)
        AL      major version number
        BH      OEM ID number
                00h     IBM
                16h     DEC    (others not known)
        BL:CX   24-bit user serial number
note 1) If AL returns a major version number of zero, the DOS version is
        below 1.28 for MSDOS and below 2.00 for PCDOS.
     2) IBM PC-DOS always returns 0000h in BX and CX.
     3) OS/2 v1.0 Compatibility Box returns a value of 10 for major version.
     4) Due to the OS/2 return and the fact that some European versions of DOS
        carry higher version numbers than IBM's DOS, utilities which check
        for a DOS version should not abort if a higher version than required
        is found unless some specific problems are known.


Function  31h  Terminate Process and Stay Resident
     KEEP, or TSR
entry   AH      31h

        AL      exit code
        DX      program memory requirement in 16 byte paragraphs
return  AX      return code (retrieveable by function 4Dh)
note 1) Files opened by the application are not closed when this call is made
     2) Memory can be used more efficiently if the block containing the copy of
        the DOS environment is deallocated before terminating. This can be done
        by loading ES with the segment contained in 2Ch of the PSP and issuing
        function call 49h (Free Allocated Memory).
     3) Unlike int 27h, more than 64k may be made resident with this call


Function  32h  Read DOS Disk Block
 *   Retrieve the pointer to the drive parameter block for a drive
entry   AH      32h
        DL      drive (0=default, 1=A:, etc.).
return  AL      00h     if drive is valid
                0FFh    if drive is not valid
        DS:BX   pointer to DOS Drive Parameter Table. Format of block:
                Bytes   Type        Value
                00h     byte    Drive: 0=A:, 1=B:, etc.
                01h     byte    Unit within drive (0, 1, 2, etc.)
                02h-03h word    Bytes per sector
                04h     byte    Sectors per cluster - 1
                05h     byte    Cluster to sector shift (i.e., how far to shift-
                                left the bytes/sector to get bytes/cluster)
                06h-07h word    Number of reserved (boot) sectors
                08h     byte    Number of FATs
                09h-0Ah word    Number of root directory entries
                0Bh-0Ch word    Sector # of 1st data. Should be same as # of
                                sectors/track.
                0Dh-0Eh word    # of clusters + 1 (=last cluster #)
                0Fh     byte    Sectors for FAT
                10h-11h word    First sector of root directory
                12h-15h dword   Address of device driver header for this drive
                16h     byte    Media Descriptor Byte for this drive
                17h     byte    0FFh indicates block must be rebuilt
                                (DOS 3.x) 00h indicates block device has
                                been accessed
                18h-1Bh dword   address of next DOS Disk Block (0FFFFh means
                                last in chain)
                22h     byte    Current Working Directory (2.0 only) (64 bytes)
note 1) Use [BX+0D] to find no. of clusters (>1000H, 16-bit FAT; if not, 12-bit
        (exact dividing line is probably a little below 1000h to allow for
        bad sectors, EOF markers, etc.)
     2) Short article by C.Petzold, PC Magazine  Vol.5,no.8, and the article
        "Finding Disk Parameters" in the May 1986 issue of PC Tech Journal.
     3) This call is mostly supported in OS/2 1.0's DOS Compatibility Box. The
        dword at 12h will not return the address of the next device driver when
        in the Compatibility Box.
     4) used by CHKDSK


Function  33h  Control-Break Check
     Get or set control-break checking at CON
entry   AH      33h
        AL      00h     to test for break checking
                01h     to set break checking
                        DL      00h     to disable break checking
                                01h     to enable break checking
                02h     internal, called by PRINT.COM (DOS 3.1)
                03h     unknown
                04h     unknown
                05h     boot drive (DOS 4.0+)
return  DL      00h     if break=off
                01h     if break=on
                (if AL=05h) boot drive, A=1, B=2, etc)
        AL      0FFh    error


Function  34h  Return INDOS Flag
 *   Returns ES:BX pointing to Critical Section Flag, byte indicating whether
     it is safe to interrupt DOS.
entry   AH      34h
return  ES:BX   points to DOS "critical section flag"
note 1) If byte is 0, it is safe to interrupt DOS. This was mentioned in some
        documentation by Microsoft on a TSR standard, and PC Magazine reports
        it functions reliably under DOS versions 2.0 through 3.3. Chris
        Dunford (of CED fame) and a number of anonymous messages on the BBSs
        indicate it may not be totally reliable.
     2) The byte at ES:BX+1 is used by the Print program for this same purpose,
        so it's probably safer to check the WORD at ES:BX.
     3) Reportedly, examination of DOS 2.10 code in this area indicates that the
        byte immediately following this "critical section flag" must be 00h to
        permit the PRINT.COM interrupt to be called. For DOS 3.0 and 3.1 (except
        Compaq DOS 3.0), the byte before the "critical section flag" must be
        zero; for Compaq DOS 3.0, the byte 01AAh before it must be zero.
     4) In DOS 3.10 this reportedly changed to word value, with preceding byte.
     5) This call is supported in OS/2 1.0's DOS Compatibility Box
     6) Gordon Letwin of Microsoft discussed this call on ARPAnet in 1984. He
        stated:
        a) this is not supported under any version of the DOS
        b) it usually works under DOS 2, but there may be circumstances
           when it doesn't (general disclaimer, don't know of a specific
           circumstance)
        c) it will usually not work under DOS 3 and DOS 3.1; the DOS is
           considerably restructured and this flag takes on additional
           meanings and uses
        d) it will fail catastrophically under DOS 4.0 and forward.
           Obviously this information is incorrect since the call works fine
           through DOS 3.3. Microsoft glasnost?


Function  35h  Get Vector
     Get interrupt vector
entry   AH      35h
        AL      interrupt number (hexadecimal)
return  ES:BX   address of interrupt vector
note    Use function call 25h to set the interrupt vectors


Function  36h  Get Disk Free Space
     get information on specified drive
entry   AH      36h
        DL      drive number (0=default, 1=A:, 2=B:, etc)
return  AX      number of sectors per cluster
                0FFFFh means drive specified in DL is invalid
        BX      number of availible clusters
        CX      bytes per sector
        DX      clusters per drive
note 1) Mult AX * CX * BX for free space on disk
     2) Mult AX * CX * DX for total disk space
     3) Function 36h returns an incorrect value after an ASSIGN command. Prior
        to ASSIGN, the DX register contains 0943h on return, which is the free
        space in clusters on the HC diskette. After ASSIGN, even with no
        parameters, 0901h is returned in the DX register; this is an incorrect
        value. Similar results occur with DD diskettes on a PC-XT or a PC-AT.
        This occurs only when the disk is not the default drive. Results are as
        expected when the drive is the default drive. Therefore, the
        circumvention is to make the desired drive the default drive prior to
        issuing this function call.
     4) Int 21h, function call 36h returns an incorrect value after an ASSIGN
        command. Prior to ASSIGN, the DX register contains 0943h on return,
        which is the free space in clusters on the HC diskette. After ASSIGN,
        even with no parameters, 0901h is returned in the DX register; this is
        an incorrect value. Similar results occur with DD diskettes on a PC-XT
        or a PC-AT. This occurs only when the disk is not the default drive.
        Results are as expected when the drive is the default drive. Therefore,
        the circumvention is to make the desired drive the default drive prior
        to issuing this function call.
     5) This function supercedes functions 1Bh and 1Ch.


Function  37h  SWITCHAR / AVAILDEV
 *   Get/set option marking character (is usually "/"), and device type
entry   AH      37h
        AL      00h     read switch character (returns current character in DL)
                01h     set character in DL as new switch character
      (DOS 2.x) 02h     read device availability (as set by function AL=3) into
                        DL. A 0 means devices that devices must be accessed in
                        file I/O calls by /dev/device. A non-zero value means
                        that devices are accessible at every level of the
                        directory tree (e.g., PRN is the printer and not a file
                        PRN).
                        AL=2 to return flag in DL, AL=3 to set from DL (0 = set,
                        1 = not set).
      (DOS 2.x) 03h     get device availability, where:
        DL      00h     means /dev/ must precede device names
                01h     means /dev/ need not precede device names
return  DL      switch character (if AL=0 or 1)
                device availability flag (if AL=2 or 3)
        AL      0FFh    the value in AL was not in the range 0-3.
note 1) Functions 2 & 3 appear not to be implemented for DOS 3.x.
     2) It is documented on page 4.324 of the MS-DOS (version 2) Programmer's
        Utility Pack (Microsoft - published by Zenith).
     3) Works on all versions of IBM PC-DOS from 2.0 through 3.3.1.
     4) The SWITCHAR is the character used for "switches" in DOS command
        arguments (defaults to '/', as in "DIR/P"). '-' is popular to make a
        system look more like UNIX; if the SWITCHAR is anything other than '/',
        then '/' may be used instead of '\' for pathnames
     5) Ignored by XCOPY, PKARC, LIST
     6) SWITCHAR may not be set to any character used in a filename
     7) In DOS 3.x you can still read the "AVAILDEV" byte with subfunction 02h
        but it always returns 0FFh even if you try to change it to 0 with
        subfunction 03h.
     8) AVAILDEV=0 means that devices must be referenced in an imaginary
        subdirectory "\dev" (similar to UNIX's /dev/*); a filename "PRN.DAT"
        can be created on disk and manipulated like any other. If AVAILDEV != 0
        then device names are recognized anywhere (this is the default):
        "PRN.DAT" is synonymous with "PRN:".
     9) These functions reportedly are not supported in the same fashion in
        various implementations of DOS.
    10) used by DOS 3.3 CHKDSK, BASIC, DEBUG


Function  38h   Return Country Dependent Information
                (PCDOS 2.0, 2.1, MSDOS 2.00 only)
entry   AH      38h
        AL      function code  (must be 0 in DOS 2.x)
        DS:DX   pointer to 32 byte memory buffer for returned information
return  CF      set on error
                AX      error code (02h)
        BX      country code
        DS:DX   pointer to buffer filled with country information:
                bytes 0,1       date/time format
                                0       USA standard       H:M:S   M/D/Y
                                1       European standard  H:M:S   D/M/Y
                                2       Japanese standard  H:M:S   D:M:Y
                byte2   ASCIIZ string currency symbol
                byte3   zeroes
                byte4   ASCIIZ string thousands separator
                byte5   zeroes
                byte6   ASCIIZ string decimal separator
                byte7   zeroes
                bytes 8,1Fh  24 bytes   reserved


Function  38h   Get Country Dependent Information
                (PCDOS 3.x+, MSDOS 2.01+)
entry   AH      38h
        AL      function code
                00h     to get current country information
                01h-0FEh country code to get information for, for countries
                        with codes less than 255
                0FFh    to get country information for countries with a code
                        greater than 255
                        BX      16 bit country code if AL=0FFh
        DS:DX   pointer to the memory buffer where the data will be returned
                DX      0FFFFh if setting country code rather than getting info
return  CF      0 (clear) function completed
                1 (set) error
                   AX   error code
                        02h     invalid country code (no table for it)
        (if DX <> 0FFFFh)
        BX      country code (usually international telephone code)
        DS:DX   pointer to country data buffer
          bytes 0,1     date/time format
                        0       USA standard       H:M:S   M/D/Y
                        1       European standard  H:M:S   D/M/Y
                        2       Japanese standard  H:M:S   D:M:Y
          bytes 2-6     currency symbol null terminated
          byte  07h     thousands separator null terminated
          byte  08h     zeroes
          byte  09h     decimal separator null terminated
          byte  0Ah     zeroes
          byte  0Bh     date separator null terminated
          byte  0Ch     zeroes
          byte  0Dh     time separator null terminated
          byte  0Eh     zeroes
          byte  0Fh     bit field currency format
                        bit 0 = 0  if currency symbol precedes the value
                                1  if currency symbol is after the value
                        bit 1 = 0  no spaces between value and currency symbol
                                1  one space between value and currency symbol
                        bit 2 = 1  set if currency symbol replaces decimal pt
                        bits 3-7   not defined by Microsoft
          byte  10h     number of significant decimal digits in currency
                        (number of places to right of decimal point)
          byte  11h     time format
                        bit 0 = 0       12 hour clock
                                1       24 hour clock
                        bits 1-7 unknown, probably not used
          bytes 12h-15h address of case map routine (FAR CALL, AL = char)
                        entry   AL  ASCII code of character to be converted to
                                    uppercase
                        return  AL  ASCII code of the uppercase input character
          byte  16h     data-list separator character
          byte  17h     zeroes
          bytes 18h-21h 5 words reserved
note 1) When an alternate keyboard handler is invoked, the keyboard routine is
        loaded into user memory starting at the lowest portion of availible
        user memory. The BIOS interrupt vector that services the keyboard is
        redirected to the memory area where the new routine resides. Each new
        routine takes up about 1.6K of memory and has lookup tables that return
        values unique to each language. (KEYBxx in the DOS book)
         Once the keyboard interrupt vector is changed by the DOS keyboard
        routine, the new routine services all calls unless the system is
        returned to the US format by the ctrl-alt-F1 keystroke combination. This
        does not change the interrupt vector back to the BIOS location; it
        merely passes the table lookup to the ROM locations.
     2) Ctrl-Alt-F1 will only change systems with US ROMS to the US layout.
        Some systems are delivered with non-US keyboard handler routines in ROM
     3) Case mapping call: the segment/offset of a FAR procedure that performs
        country-specific lower-to-upper case mapping on ASCII characters 80h to
        0FFh. It is called with the character to be mapped in AL. If there is
        an uppercase code for the letter, it is returned in AL, if there is no
        code or the function was called with a value of less than 80h AL is
        returned unchanged.
     4) This call is fully implemented in MS-DOS version 2.01 and higher. It
        is in version 2.00 but not fully implemented (according to Microsoft)


Function  38h   Set Country Dependent Information
entry   AH      38h
        AL      code    country code to set information for, for countries
                        with codes less than 255
                0FFh    to set country information for countries with a code
                        greater than 255
        BX      16 bit country code if AL=0FFh
        DX      0FFFFh
return  CF      clear   successful
                set     if error
                        AX      error code (02h)


Function  39h   Create Subdirectory (MKDIR)
                Makes a subdirectory along the indicated path
entry   AH      39h
        DS:DX   address of ASCIIZ pathname string
return  flag CF 0       successful
                1       error
                        AX      error code if any  (3, 5)
note 1) The ASCIIZ string may contain drive and subdirectory.
     2) Drive may be any valid drive (not nescessarily current drive)
     3) The pathname cannot exceed 64 characters


Function  3Ah   Remove Subdirectory  (RMDIR)
entry   AH      3Ah
        DS:DX   address of ASCIIZ pathname string
return  CF      clear     successful
                set       AX      error code if any  (3, 5, 16)
note 1) The ASCIIZ string may contain drive and subdirectory.
     2) Drive may be any valid drive (not nescessarily current drive)
     3) The pathname cannot exceed 64 characters


Function  3Bh   Change Current Directory  (CHDIR)
entry   AH      3Bh
        DS:DX   address of ASCIIZ string
return  flag CF 0       successful
                1       error
        AX      error code if any (3)
note 1) The pathname cannot exceed 64 characters
     2) The ASCIIZ string may contain drive and subdirectory.
     3) Drive may be any valid drive (not nescessarily current drive)


Function  3Ch   Create A File (CREAT)
                Create a file with handle
entry   AH      3Ch
        CX      attributes for file
                00h     normal
                01h     read only
                02h     hidden
                03h     system
        DS:DX   address of ASCIIZ filename string
return  flag CF 0       successful creation
                1       error
        AX      16 bit file handle
                or error code  (3, 4, 5)
note 1) The ASCIIZ string may contain drive and subdirectory.
     2) Drive may be any valid drive (not nescessarily current drive)
     3) If the volume label or subdirectory bits are set in CX, they are ignored
     4) The file is opened in read/write mode
     5) If the file does not exist, it is created. If one of the same name
        exists, it is truncated to a length of 0.
     6) Good practice is to attempt to open a file with fn 3Dh and jump to an
        error routine if successful, create file if 3Dh fails. That way an
        existing file will not be truncated and overwritten.


Function  3Dh   Open A File
                Open disk file with handle
entry   AH      3Dh
        AL      access code byte
(DOS 2.x)       bits 0-2  file attribute
                000     read only
                001     write only
                010     read/write
                bits 3-7 should be set to zero
(DOS 3.x)       bits 0-2  file attribute
                000     read only
                001     write only
                010     read/write
                bit 3   reserved
                0       should be set to zero
                bits 4-6 sharing mode (network)
                000     compatibility mode (the way FCBs open files)
                001     read/write access denied (exclusive)
                010     write access denied
                011     read access denied
                100     full access permitted
                bit 7   inheritance flag
                0       file inherited by child process
                1       file private to child process
        DS:DX   address of ASCIIZ pathname string
return  flag CF set on error
                AX      error code
                1       error
        AX      16 bit file handle
                or error code (1, 2, 4, 5, 0Ch)
note 1) Opens any normal, system, or hidden file
     2) Files that end in a colon are not opened
     3) The rear/write pointer is set at the first byte of the file and the
        record size of the file is 1 byte (the read/write pointer can be changed
        through function call 42h). The returned file handle must be used for
        all subsequent input and output to the file.
     4) If the file handle was inherited from a parent process or was
        duplicated by DUP or FORCEDUP, all sharing and access restrictions are
        also inherited.
     5) A file sharing error (error 1) causes an int 24h to execute with an
        error code of 2


Function  3Eh   Close A File Handle
                Close a file and release handle for reuse
entry   AH      3Eh
        BX      file handle
return  flag CF 0       successful close
                1       error
        AX      error code if error (6)
note 1) When executed, the file is closed, the directory is updated, and all
        buffers for that file are flushed. If the file was changed, the time
        and date stamps are changed to current
     2) If called with the handle 00000, it will close STDIN (normally the
        keyboard).


Function  3Fh   Read From A File Or Device
                Read from file with handle
entry   AH      3Fh
        BX      file handle
        CX      number of bytes to read
        DS:DX   address of buffer
return  flag CF 0       successful read
                1       error
        AX      0       pointer was already at end of file
                        or number of bytes read
                        or error code (5, 6)
note 1) This function attempts to transfer the number of bytes specified in CX
        to a buffer location. It is not guaranteed that all bytes will be read.
        If AX < CX a partial record was read.
     2) If performed from STDIN (file handle 0000), the input can be redirected
     3) If used to read the keyboard, it will only read to the first CR
     4) The file pointer is incremented to the last byte read.


Function  40h   Write To A File Or Device
                Write to file with handle
entry   AH      40h
        BX      file handle
        CX      number of bytes to write
        DS:DX   address of buffer
return  flag CF 0       successful write
                1       error
        AX      number of bytes written
                or error code  (5, 6)
note 1) This call attempts to transfer the number of bytes indicated in CX
        from a buffer to a file. If CX and AX do not match after the write,
        an error has taken place; however no error code will be returned for
        this problem. This is usually caused by a full disk.
     2) If the write is performed to STDOUT (handle 0001), it may be redirected
     3) To truncate the file at the current position of the file pointer, set
        the number of bytes in CX to zero before calling int 21h. The pointer
        can be moved to any desired position with function 42h.
     4) This function will not write to a file or device marked read-only.
     5) May also be used to display strings to CON instead of fn 09h. This
        function will write CX bytes and stop; fn 09h will continue to write
        until a $ character is found.
     6) This is the call that DOS actually uses to write to the screen in DOS
        2.x and above.


Function  41h   Delete A File From A Specified Subdirectory  (UNLINK)
entry   AH      41h
        DS:DX   pointer to ASCIIZ filespec to delete
return  CF      0       successful
                1       error
                AX      error code if any  (2, 5)
note 1) This function will not work on a file marked read-only
     2) Wildcards are not accepted


Function  42h   Move a File Read/Write Pointer  (LSEEK)
entry   AH      42h
        AL      method code
                00h     offset from beginning of file
                01h     offset from present location
                02h     offset from end of file
        BX      file handle
        CX      most significant half of offset
        DX      least significant half of offset
return  AX      low offset of new file pointer
        DX      high offset of new file pointer
        CF      0       successful move
                1       error
                AX      error code (1, 6)
note 1) If pointer is at end of file, reflects file size in bytes.
     2) The value in DX:AX is the absolute 32 bit byte offset from the beginning
        of the file


Function   43h  Get/Set file attributes  (CHMOD)
entry   AH      43h
        AL      00h     get file attributes
                01h     set file attributes
                CX      file attributes to set
                    bit 0       read only
                        1       hidden file
                        2       system file
                        3       volume label
                        4       subdirectory
                        5       written since backup
        DS:DX   pointer to full ASCIIZ file name
return  CF      set if error
        AX      error code  (1, 2, 3, 5)
        CX      file attributes on get
                attributes:
                01h     read only
                02h     hidden
                04h     system
                0FFh    archive
note 1) This call will not change the volume label or directory bits


Function  44h   I/O Control for Devices (IOCTL)
                Get or Set Device Information
entry   AH      44h
        AL      00h     Get Device Information (from DX)
                        BX      file or device handle
                        return  DX      device info
                                        If bit 7 set: (character device)
                                           bit 0: console input device
                                               1: console output device
                                               2: NUL device
                                               3: CLOCK$ device
                                               4: device is special
                                               5: binary (raw) mode
                                               6: not EOF
                                              12: network device (DOS 3.x)
                                              14: can process IOCTL control
                                                  strings (func 2-5)
                                        If bit 7 clear: (file)
                                           bits 0-5: block device number
                                                6: file has not been written
                                               12: Network device (DOS 3.x)
                                               15: file is remote (DOS 3.x)
                01h     Set Device Information (DH must be zero for this call)
                        DX bits:
                        0    1  console input device
                        1    1  console output device
                        2    1  null device
                        3    1  clock device
                        4    1  reserved
                        5    0  binary mode - don't check for control chars
                             1  cooked mode - check for control chars
                        6    0  EOF - End Of File on input
                        7       device is character device if set, if not, EOF
                                is 0 if channel has been written, bits 0-5 are
                                block device number
                        12      network device
                        14   1  can process control strings (AL 2-5, can only be
                                read, cannot be set)
                        15   n  reserved
                02h     Read CX bytes to device in DS:DX from BX control chan
                03h     Write Device Control String
                        BX      device handle
                        CX      number of bytes to write
                        DS:DX   pointer to buffer
                        return  AX      number of bytes written
                04h     Read From Block Device (drive number in BL)
                        BL      drive number (0=default)
                        CX      number of bytes to read
                        DS:DX   pointer to buffer
                        return  AX      number of bytes read
                05h     Write to Block Device  (drive number in BL)
                        AX      number of bytes transfered
                06h     Get Input Handle Status
                        BX      file or device handle
                        return  AL      0FFh    device ready
                                        00h     device not ready
                07h     Get Output Handle Status
                        return  AL      00h     not ready
                                        0FFh    ready
                         note: for DOS 2.x, files are always ready for output
                08h     Removable Media Bit (DOS 3.x+)
                        return  AX      00h     device is removable
                                        01h     device is nonremovable
                                        0Fh     invalid drive specification
                09h     Test whether Local or Network Device in BL (DOS 3.x+)
                        BL      drive number (0=default)
                        return  DX      attribute word, bit 12 set if device is
                                        remote
                0Ah     Is Handle in BX Local or Remote? (DOS 3.x+)
                        BX     file handle
                        return DX (attribute word) bit 15 set if file is remote
                0Bh     Change Sharing Retry Count to DX (default=3), (DOS 3.x+)
                        CX     delay (default=1)
                        DX     retry count (default=3)
                0Ch     General IOCTL (DOS 3.3 [3.2?]) allows a device driver to
                        prepare, select, refresh, and query Code Pages
                0Dh     Block Device Request (DOS 3.3+)
                        BL      drive number (0=default)
                        CH      major subfunction
                        CL      minor subfunction
                                40h set device parameters
                                41h write logical device track
                                42h format and verify logical device track
                                60h get device parameters
                                61h read logical device track
                                62h verify logical device track
                        DS:DX   pointer to parameter block
                0Eh     Get Logical Device (DOS 3.3+)
                        BL      drive number (0=default)
                        return  AL=0 block device has only one logical drive
                                assigned 1..n the last letter used to reference
                                the device (1=A:,etc)
                0Fh     Set Logical Device (DOS 3.3+)
        BL      drive number:  0=default, 1=A:, 2=B:, etc.
        BX      file handle
        CX      number of bytes to read or write
        DS:DX   data or buffer
        DX      data
return  AX      number of bytes transferred
                or error code (call function 59h for extended error codes)
                or status  00h     not ready
                           0FFh    ready
        CF      set if error


Function  45h   Duplicate a File Handle (DUP)
entry   AH      45h
        BX      file handle to duplicate
return  CF      clear   AX      duplicate handle
                set     AX      error code  (4, 6)
note 1) If you move the pointed of one handle, the pointer of the other will
        also be moved.
     2) The handle in BX must be open


Function  46h   Force Duplicate of a Handle (FORCEDUP or CDUP)
                Forces handle in CX to refer to the same file at the same
                position as BX
entry   AH      46h
        BX      existing file handle
        CX      new file handle
return  CF      clear   both handles now refer to existing file
                set     error
                AX      error code (4, 6)
note 1) If CX was an open file, it is closed first
     2) If you move the read/write pointer of either file, both will move
     3) The handle in BX must be open


Function  47h   Get Current Directory
                Places full pathname of current directory/drive into a buffer
entry   AH      47h
        DL      drive (0=default, 1=A:, etc.)
        DS:SI   points to 64-byte buffer area
return  CF      clear   DS:DI   pointer to ASCIIZ pathname of current directory
                set     AX      error code (0Fh)
note   String does not begin with a drive identifier or a backslash


Function  48h   Allocate Memory
                Allocates requested number of 16-byte paragraphs of memory
entry   AH      48h
        BX      number of 16-byte paragraphs desired
return  CF      clear   AX      segment address of allocated space
                        BX      maximum number paragraphs available
                set     AX      error code (7, 8)
note    BX indicates maximum memory availible only if allocation fails


Function  49h   Free Allocated Memory
                Frees specified memory blocks
entry   AH      49h
        ES      segment address of area to be freed
return  CF      clear   successful
                set     AX      error code (7, 9)
note 1) This call is only valid when freeing memory obtained by function 48h.
     2) A program should not try to release memory not belonging to it.


Function  4Ah   Modify Allocated Memory Blocks (SETBLOCK)
                Expand or shrink memory for a program
entry   AH      4AH
        BX      new size in 16 byte paragraphs
        ES      segment address of block to change
return  CF      clear   nothing
                set     AX      error code (7, 8, 9)
                    or  BX      max number paragraphs available
note 1) Max number paragraphs availible is returned only if the call fails
     2) Memory can be expanded only if there is memory availible


Function  4Bh   Load or Execute a Program  (EXEC)
entry   AH      4Bh
        AL      00h     load and execute program. A PSP is built for the program
                        the ctrl-break and terminate addresses are set to the
                        new PSP.
               *01h     load but don't execute  (note 1)
               *01h     load but don't execute (internal, DOS 3.x & DESQview)
               *02h     load but do not execute (internal, DOS 2.x only)
                03h     load overlay (do not create PSP, do not begin execution)
        DS:DX   points to the ASCIIZ string with the drive, path, and filename
                to be loaded
        ES:BX   points to a parameter block for the load
               (AL=00h) word    segment address of environment string to be
                                passed
                       dword    pointer to the command line to be placed at
                                PSP+80h
                       dword    pointer to default FCB to be passed at PSP+5Ch
                       dword    pointer to default FCB to be passed at PSP+6Ch
              (*AL=01h) word    segment of environment (0 = use current)
                       dword    pointer to command line
                       dword    pointer to FCB 1
                       dword    pointer to FCB 2
                       dword    will hold SS:SP on return
                       dword    will hold program entry point (CS:IP) on return
              (*AL=02h) word    segment of environment (0 = use current)
                       dword    pointer to command line
                       dword    pointer to FCB 1
                       dword    pointer to FCB 2
               (AL=03h) word    segment address where file will be loaded
                        word    relocation factor to be applied to the image
return  CF      set     error
                        AX      error code (1, 2, 8, 0Ah, 0Bh)
note 1) If you make this call with AL=1 the program will be loaded as if you
        made the call with AL=0 except that the program will not be executed.
        Additionally, with AL=1 the stack segment and pointer along with the
        program's CS:IP entry point are returned to the program which made the
        4B01h call. These values are put in the four words at ES:BX+0Eh. On
        entry to the call ES:BX points to the environment address, the command
        line and the two default FCBs. This form of EXEC is used by DEBUG.COM.
     2) Application programs may invoke a secondary copy of the command
        processor (normally COMMAND.COM) by using the EXEC function.  Your
        program may pass a DOS command as a parameter that the secondary
        command processor will execute as though it had been entered from the
        standard input device.
        The procedure is:
         A. Assure that adequate free memory (17k for 2.x and 3.0, 23k for 3.1
            up) exists to contain the second copy of the command processor and
            the command it is to execute. This is accomplished by executing
            function call 4Ah to shrink memory allocated to that of your current
            requirements. Next, execute function call 48h with BX=0FFFFh. This
            returns the amount of memory availible.
        B. Build a parameter string for the secondary command processor in the
           form:
                         1 byte   length of parameter string
                        xx bytes  parameter string
                         1 byte   0Dh (carriage return)
           For example, the assembly language statement below would build the
           string to cause execution of the command FOO.EXE:
                              DB 19,"/C C:FOO",13
        C. Use the EXEC function call (4Bh), function value 0 to cause execution
           of the secondary copy of the command processor. (The drive,
           directory, and name of the command processor can be gotten from the
           COMSPEC variable in the DOS environment passed to you at PSP+2Ch.)
        D. Remember to set offset 2 of the EXEC control block to point to the
           string built above.
     3) All open files of a process are duplicated in the newly created
        process after an EXEC, except for files originally opened with the
        inheritance bit set to 1.
     4) The environment is a copy of the original command processor's
        environment. Changes to the EXECed environment are not passed back to
        the original. The environment is followed by a copy of the DS:DX
        filename passed to the child process. A zero value will cause the
        child process to inherit the environment of the calling process. The
        segment address of the environment is placed at offset 2Ch of the
        PSP of the program being invoked.
     5) This function uses the same resident part of COMMAND.COM, but makes a
        duplicate of the transient part.
     6) How EXEC knows where to return to: Basically the vector for int 22h
        holds the terminate address for the current process. When a process
        gets started, the previous contents of int 22h get tucked away in the
        PSP for that process, then int 22h gets modified. So if Process A
        EXECs process B, while Process B is running, the vector for int 22h
        holds the address to return to in Process A, while the save location in
        Process B's PSP holds the address that process A will return to when
        *it* terminates. When Process B terminates by one of the usual legal
        means, the contents of int 22h are (surmising) shoved onto the stack,
        the old terminate vector contents are copied back to int 22h vector from
        Process B's PSP, then a RETF or equivalent is executed to return control
        to process A.
     7) To load an overlay file with 4B: first, don't de-allocate the memory
        that the overlay will load into. With the other 4Bh functions, the
        opposite is true--you have to free the memory first, with function 4Ah.
        Second, the "segment address where the file will be loaded" (first item
        in the parameter block for sub-function 03) should be a paragraph
        boundary within your currently-allocated memory. Third, if the
        procedures within the overlay are FAR procs (while they execute, CS will
        be equal to the segment address of the overlay area), the relocation
        factor should be set to zero. On the other hand, if the CS register
        will be different from the overlay area's segment address, the
        relocation factor should be set to represent the difference. You
        determine where in memory the overlay file will load by using the
        segment address mentioned above. Overlay files are .EXEs (containing
        header, relocation table, and memory image).
     8) When function 00h returns, all registers are changed, including the
        stack. You must resore SS, SP, and any other required registers.


Function  4Ch   Terminate a Process (EXIT)
                Quit with ERRORLEVEL exit code
entry   AH      4Ch
        AL      exit code in AL when called, if any, is passed to next process
return  none
note 1) Control passes to DOS or calling program
     2) return code from AL can be retrieved by ERRORLEVEL or function 4Dh
     3) all files opened by this process are closed, buffers are flushed, and
        the disk directory is updated
     4) Restores Terminate vector from PSP:000Ah
                 Ctrl-C vector from PSP:000Eh
                 Critical Error vector from PSP:0012h


Function  4Dh   Get Return Code of a Subprocess (WAIT)
                Gets return code from functions 31h and 4Dh  (ERRORLEVEL)
entry   AH      4Dh
return  AL      exit code of subprogram (functions 31h or 4Ch)
        AH      circumstance which caused termination
                00h     normal termination
                01h     control-break
                02h     critical device error
                03h     terminate and stay resident (function 31h)
note    The exit code is only returned once


Function  4Eh   Find First Matching File (FIND FIRST)
entry   AH      4Eh
        CX      search attributes
        DS:DX   pointer to ASCIIZ filename (with attributes)
return  CF      set     AX      error code (2, 12h)
                clear   data block written at current DTA
                        format of block is:  (info from BIX)
  documented by Micro-  |00h   1 byte   attribute byte of search
  soft as "reserved for |01h   1 byte   drive letter for search
  DOS' use on subsquent |02h   11 bytes the search name used
  Find Next calls"      |0Ch   2 bytes  word value of last entry
  function 4Fh          |0Fh   4 bytes  dword pointer to this DTA
                        |13h   2 bytes  word directory start
                        | PC-DOS 3.10 (from INTERRUP.ARC)
                        |00h   1 byte   drive letter
                        |01h-0Bh    bytes  search template
                        |0Ch   1 byte   search attributes
                        | DOS 2.x (and DOS 3.x except 3.1?) (from INTERRUP.ARC)
                        |00h   1 byte   search attributes
                        |01h   1 byte   drive letter
                        |02h-0Ch bytes  search template
                        |0Dh-0Eh 2 bytes entry count within directory
                        |0Fh-12h   bytes reserved
                        |13h-14h 2 bytes cluster number of parent directory

                         15h   1 byte   file attribute
                         16h   2 bytes  file time
                         18h   2 bytes  file date
                         1Ah   2 bytes  low word of file size
                         1Ch   2 bytes  high word of file size
                         1Eh  13 bytes  name and extension of file found, plus
                                        1 byte of 0s. All blanks are removed
                                        from the name and extension, and if an
                                        extension is present it is preceded by a
                                        period.
note 1) Will not find volume label
     2) This function does not support network operations
     3) Wildcards are allowed in the filespec
     4) If the attribute is zero, only ordinary files are found. If the volume
        label bit is set, only volume labels will be found. Any other attribute
        will return that attribute and all normal files together.
     5) To look for everything except the volume label, set the hidden, system,
        and subdirectory bits all to 1


Function  4Fh   Find Next Matching File (FIND NEXT)
                Find next ASCIIZ file
entry   AH      4Fh
return  CF      clear   data block written at current DTA
                set     AX      error code (2, 12h)
note 1) If file found, DTA is formatted as in call 4Eh
     2) Volume label searches using 4Eh/4Fh reportedly aren't 100% reliable
        under DOS 2.x. The calls sometime report there's a volume label and
        point to a garbage DTA, and if the volume label is the only item they
        often won't find it
     3) This function does not support network operations
     4) Use of this call assumes that the original filespec contained wildcards


Function  50h   "Used Internally by DOS" - Set PSP
 *              Set new Program Segment Prefix (current Process ID)
entry   AH      50h
        BX      segment address of new PSP
return  none - swaps PSP's regarded as current by DOS
note 1) By putting the PSP segment value into BX and issuing call 50h DOS stores
        that value into a variable and uses that value whenever a file call is
        made.
     2) Note that in the PSP (or PDB) is a table of 20 (decimal) open file
        handles. The table starts at offset 18h into the PSP. If there is an
        0FFh in a byte then that handle is not in use. A number in one of the
        bytes is an index into an internal FB table for that handle. For
        instance the byte at offset 18h is for handle 0, at offset 19h handle
        1, etc. up to 13h. If the high bit is set then the file associated by
        the handle is not shared by child processes EXEC'd with call 4Bh.
     3) Function 50h is dangerous in background operations prior to DOS 3.x as
        it uses the wrong stack for saving registers.  (same as functions
        0..0Ch in DOS 2.x)
     4) Under DOS 2.x, this function cannot be invoked inside an int 28h handler
        without setting the Critical Error flag
     5) Open File information, etc. is stored in the PSP DOS views as current.
        If a program (eg. a resident program) creates a need for a second PSP,
        then the second PSP should be set as current to make sure DOS closes
        that as opposed to the first when the second application finishes.
     6) See PC Mag Vol.5, No 9, p.314 for discussion.
     7) Used by DOS 3.3 PRINT & DEBUG, DesQview 2.01, Windows 1.03, SYMDEB
        from MASM 4.0


Function  51h    "Used Internally by DOS" - Get Program Segment Prefix
 *               Returns the PSP address of currently executing program
entry   AH      51h
return  BX      address of currently executing program
                offset
                00h     program exit point
                02h     memory size in paragraphs
                04h     unused (0)
                05h     CP/M style entry point (far call to DOS)
                0Ah     terminate address (old int 22h)
                0Ch     terminate segment
                0Eh     break address (old int 23h)
                10h     break segment
                12h     error address (old int 24h)
                14h     error segment
                16h     parent PSP segment
                18h     DOS 2.0+ open files, 0FFh = unused
                2Ch     DOS 2.0+ environment segment
                2Eh     far ptr to process's SS:SP
                32h     DOS 3.x max open files
                34h     DOS 3.x openfile table address
                36h     DOS 3.x openfile table segment
                38h     unused by DOS versions <= 3.3
                50h     DOS function dispatcher (FAR routine)
                53h     unused
                55h     FCB #1 extension
                5Ch     FCB #1
                6Ch     FCB #2
                80h     command tail / default DTA buffer
note 1) Used in DOS 2.x, 3.x uses 62h
     2) Function 51h is dangerous in background operations prior to DOS 3.x as
        it uses the wrong stack for saving registers.  (same as functions
        0..0Ch in DOS 2.x)
     3) 50h and 51h might be used if you have more than one process in a PC.
        For instance if you have a resident program that needs to open a file
        you could first call 51h to save the current id and then call 50h to set
        the ID to your PSP.
     4) Under DOS 2.x, this function cannot be invoked inside an int 28h handler
        without setting the Critical Error flag
     5) Used by DOS 3.3 PRINT, DEBUG


Function  52h   "Used Internally by DOS" - IN-VARS
 *              Returns a pointer to a set of DOS data variables MCB chain,
                pointer to first device driver and a pointer to disk parameter
                blocks (first one)
entry   AH      52h
return  ES:BX   pointer to the DOS list of lists, for disk information. Does not
                access the disk, so information in tables might be incorrect if
                disk has been changed. Returns a pointer to the following array
                of longword pointers:
                Bytes   Value
                -2h,-1h segment of first memory control block
                00h-03h pointer to first DOS disk block (see function 36h)
                04h-07h Pointer to list of DOS file tables
                        dword   pointer to next file table
                        word    number of files in this table
                                35h bytes per file
                                00h-01h number of file handles referring to
                                        this file
                                02h-06h unknown
                                07h-0Ah pointer to device driver header if
                                        character device; pointer to DOS Device
                                        Control Block if block device (see
                                        fn 32h for format)
                                0Bh-1Fh unknown
                                20h-2Ah filename in FCB format (no path, no
                                        period, blank-padded)
                                2Bh-2Ch PSP segment of file's owner
                                2Dh-30h unknown -  0 always
                                31h-32h unknown
                                33h-34h unknown
                8h-0Bh  pointer to CLOCK$ device driver, whether installable or
                        resident
                0Ch-0Fh pointer to actual CON: device driver, whether
                        installable or resident
        (DOS 2.x)
                10      number of logical drives in system
                11-12   maximum bytes/block of any block device
                13-16   unknown
                17      beginning (not a pointer. The real beginning!) of NUL
                        device driver. This is the first device on DOS's linked
                        list of device drivers.
        (DOS 3.x)
                10h-11h maximum bytes/block of any block device (0200h)
                12h-15h pointer to first disk buffer
                16h-19h partially undefined: Pointer to array of drive info:
                        51h bytes per drive, starting with A: ...
                        00h-3Fh current path as ASCIIZ, starting with 'x:\'
                        40h-43h unknown    zeros always
                        44h     unknown    flags? Usually 40h, except for
                                entry after last valid entry = 00h
                        45h-48h pointer to DOS disk block for this drive
                        49h-4Ah unknown. Current track or block?
                                -1 if never accessed
                        4Bh-4Eh unknown  -1 always
                        4Fh-52h unknown   2 always
                1Ah-1Dh pointer to FCB table (if CONFIG.SYS contains FCBS=)
                1Eh-1Fh size of FCB table
                20h     number of block devices
                21h     value of LASTDRIVE command in CONFIG.SYS (default 5)
                22h     beginning (not a pointer. The real beginning!) of NUL
                        device driver. This is the first device on DOS's linked
                        list of device drivers.
note 1) This call is not supported in OS/2 1.0's DOS Compatibility Box
     2) Used by DOS 4.0 MEM.EXE, DOS 3.3 ASSIGN.COM, PRINT.COM, SUBST.EXE
     3) IMPORTANT:  The structure of this list changes with EVERY version of
        DOS.  It is only partially supported by DR-DOS, and isn't supported
        under PC-MOS, OS/2's DOS box, or Wendin-DOS.  Since the information
        changes so much, I feel it should be put in the "interesting, but
        not real useful" category.  If you depend on this stuff in production
        code, you're going to regret it.


Function  53h   "Used Internally by DOS" - Translate BPB
 *              Translates BPB (BIOS Parameter Block, see below) into a DOS Disk
                Block (see function call 32h).
entry   AH      53h
        DS:SI   pointer to BPB
        ES:BP   pointer to area for DOS Disk Block.
                Layout of Disk Block:
                bytes   value
                00h-01h bytes per sector, get from DDB bytes 02h-03h.
                02h     sectors per cluster, get from (DDB byte 4) + 1
                03h-04h reserved sectors, get from DDB bytes 06h-07h
                05h     number of FATs, get from DDB byte 08h
                06h-07h number of root dir entries, get from DDB bytes 09h-0Ah
                08h-09h total number of sectors, get from:
                        ((DDB bytes 0Dh-0Eh) - 1) * (sectors per cluster (BPB
                        byte 2)) + (DDB bytes 0Bh-0Ch)
                0Ah     media descriptor byte, get from DDB byte 16h
                0Bh-0Ch number of sectors per FAT, get from DDB byte 0Fh
return  unknown


Function  54h   Get Verify Setting
                Get verify flag status
entry   AH      54h
return  AL      00h if flag off
                01h if flag on
note    Flag can be set with function 2Eh


Function  55h   "Used Internally by DOS" - Create "Child" PSP
 *              Create PSP: similar to function 26h (which creates a new Program
                Segment Prefix at segment in DX) except creates a "child" PSP
                rather than copying the existing one.
entry   AH      55h
        DX      segment number at which to create new PSP.
return  unknown
note 1) This call is similar to call 26h which creates a PSP except that unlike
        call 26h the segment address of the parent process is obtained from the
        current process ID rather than from the CS value on the stack (from the
        INT 21h call). DX has the new PSP value and SI contains the value to be
        placed into PSP:2 (top of memory).
     2) Function 55 is merely a substitute for function 26h. It will copy the
        current PSP to the segment address DX with the addition that SI is
        assumed to hold the new memory top segment. This means that function
        26h sets SI to the segment found in the current PSP and then calls
        function 55h.


Function  56h   Rename a File
entry   AH      56h
        DS:DX   pointer to ASCIIZ old pathname
        ES:DI   pointer to ASCIIZ new pathname
return  CF      clear   successful rename
                set     AX      error code (2, 3, 5, 11h)
note 1) Works with files in same drive only
     2) Global characters not allowed in filename
     3) The name of a file is its full pathname. The file's full pathname can
        be changed, while leaving the actual FILENAME.EXT unchanged. Changing
        the pathname allows the file to be "moved" from subdirectory to
        subdirectory on a logical drive without actually copying the file.
     4) DOS 3.x allows renaming of directories