DOS Reference chapter 6

       **  Programmer's Technical Reference for MSDOS and the IBM PC **
                USA copyright TXG 392-616  ALL RIGHTS RESERVED
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ DOSREF (tm) ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
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                             C H A P T E R   S I X

                       DOS CONTROL BLOCKS AND WORK AREAS


 Contrary to popular belief, DOS is not limited to 640k of work space. This
constraint is enforced by the mapping of ROM and video RAM into the default 1
megabyte CPU address space. Some MSDOS compatible machines, such as the Sanyo
55x series, can have as much as 768k of contiguous DOS workspace with the
appropriate option boards. Since DOS has no real memory management, it cannot
deal with a fragmented workspace. Fragmented RAM (such as RAM mapped into the
option ROM address space) can be dealt with as a RAMdisk or other storage area
by using a device driver or other software.

 The 80386 CPU and appropriate control software can create a DOS workspace of
more than one megabyte. Certain add-on boards can also add more than a
megabyte of workspace, but only for specially written software. Since these
are all proprietary schemes, little information is availible at present.

 When DOS loads a program, it first sets aside a section of memory for the
program called the program segment, or code segment. Then it constructs a
control block called the program segment prefix, or PSP, in the first 256
(100h) bytes. Usually, the program is loaded directly after the PSP at 100h.
 The PSP contains various information used by DOS to help run the program. 
The PSP is always located at offset 0 within the code segment. When a program
recieves control certain registers are set to point to the PSP. For a COM
file, all registers are set to point to the beginning of the PSP and the
program begins at 100h. For the more complex EXE file structures, only DS and
ES registers are set to point to the PSP. The linker passes the settings for
the DS, IP, SS, and SP registers and may set the starting location in CS:IP to
a location other than 100h.

 IBMBIO provides an IRET instruction at absolute address 847h for use as a
dummy routine for interrupts that are not used by DOS. This lets the interrupts
do nothing until their vectors are rerouted to their appropriate handlers.

 A storage block is used by DOS to record the amount and location of allocated
memory within the machine's address space.
 A storage block, a Program Segment Prefix, and an environment area are built
by DOS for each program currently resident in the address space. The storage
block is used by DOS to record the address range of memory allocated to a
program. It is used by DOS to find the next availible area to load a program
and to determine if there is enough memory to run that porogram. When a
memory area is in use, it is said to be allocated. Then the program ends, or
releases memory, it is said to be deallocated.
 A storage block contains a pointer to the Program Segment Prefix associated
with each program. This control block is constructed by IBMDOS for the purpose
of providing standardized areas for DOS/program communication. Within the
PSP are areas which  are used to save interrupt vectors, pass parameters to
the program, record disk directory information, and to buffer disk reads and
writes. This control block is 100h bytes in length and is followed by the
program module loaded by DOS.
 The PSP contains a pointer to the environment area for that program. This
area contains a copy of the current DOS SET, PROMPT, COMSPEC, and PATH values
as well as any user-set variables. The program may examine and modify this
information as desired.
 Each storage block is 10h bytes long, although only 5 bytes are currently
used by DOS. The first byte contains 4Dh (a capital M) to indicate that it
contains a pointer to the next storage block. A 5Ah (a capital Z) in the
first byte of a storage block indicatres there are no more storage blocks
following this one (it is the end of the chain). The identifier byte is
followed by a 2 byte segment number for the associated PSP for that program.
The next 2 bytes contain the number of segments what are allocated to the
program. If this is not the last storage block, then another storage block
follows the allocated memory area.
 When the storage block contains zero for the number of allocated segments,
then no storage is allocated to this block and the next storage block
immediately follows this one. This can happen when memory is allocated and
then deallocated repeatedly.
 IBMDOS constructs a storage block and PSP before loading the command
interpreter (default is COMMAND.COM).

 If the copy of COMMAND.COM is a secondary copy, it will lack an environment
address at PSP+2Ch.



THE DISK TRANSFER AREA (DTA)ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

 DOS uses an area in memory to contain the data for all file reads and writes
that are performed with FCB function calls. This are is known as the disk
transfer area. This disk transfer area (DTA) is sometimes called a buffer.
It can be located anywhere in the data area of your application program and
should be set by your program.

 Only one DTA can be in effect at a time, so your program must tell DOS what
memory location to use before using any disk read or write functions. Use
function call 1Ah (Set Disk Transfer Address) to set the disk transfer address.
Use function call 2Fh (Get Disk Transfer Address) to get the disk transfer
address. Once set, DOS continues to use that area for all disk operations until
another function call 1Ah is issued to define a new DTA. When a program is given
control by COMMAND.COM, a default DTA large enough to hold 128 bytes is
established at 80h into the program's Program Segment Prefix.

 For file reads and writes that are performed with the extended function calls,
there is no need to set a DTA address. Instead, specify a buffer address when
you issue the read or write call.


DOS PROGRAM SEGMENTÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

 When you enter an external command or call a program through the EXEC function
call, DOS determines the lowest availible address space to use as the start of
available memory for the program being started. This area is called the Program
Segment.
 At offset 0 within the program segment, DOS builds the Program Segment Prefix
control block. EXEC loads the program after the Program Segment Prefix (at
offset 100h) and gives it control.
 The program returns from EXEC by a jump to offset 0 in the Program Segment
Prefix, by issuing an int 20h, or by issuing an int 21h with register AH=00h or
4Ch, or by calling location 50h in the PSP with AH=00h or 4Ch.
 It is the responsibility of all programs to ensure that the CS register
contains the segment address of the Program Segment Prefix when terminating by
any of these methods except call 4Ch.

 All of these methods result in returning to the program that issued the EXEC.
During this returning process, interrupt vectors 22h, 23h, and 24h (Terminate,
Ctrl-Break, and Critical Error Exit addresses) are restored from the values
saved in the PSP of the terminating program. Control is then given to the
terminate address.


When a program receives control, the following conditions are in effect:

For all programs:

1) The segment address of the passed environment is contained at offset 2Ch in
   the Program Segment Prefix.

2) The environment is a series of ASCII strings totalling less than 32k bytes
   in the form:       NAME=value      The default environment is 160 bytes.
    Each string is a maximum of 127 bytes terminated by a byte of zeroes for a
   total of 128 bytes, and the entire set of strings is terminated by another
   byte of zeroes. Following the byte of zeroes that terminates the set of
   environment string is a set of initial arguments passed to a program that
   contains a word count followed by an ASCIIZ string. The ASCIIZ string
   contains the drive, path, and filename.ext of the executable program.
   Programs may use this area to determine where the program was loaded from.
   The environment built by the command processor (and passed to all programs
   it invokes) contains a COMSPEC=string at a minimum (the parameter on COMSPEC
   is the path used by DOS to locate COMMAND.COM on disk). The last PATH and
   PROMPT commands issued will also be in the environment, along with any
   environment strings entered through the SET command.
    The environment that you are passed is actually a copy of the invoking
   process's environment. If your application terminates and stays resident
   through int 27h, you should be aware that the copy of the environment passed
   to you is static. That is, it will not change even if subsequent PATH,
   PROMPT, or SET commands are issued.
    The size of the environment may be changed from its default of 160 bytes
   by using the SHELL= command in the config.sys from in DOS version 3.1 up,
   or COMMAND.COM may be patched in earlier versions.

   The environment can be used to transfer information between processes or to
   store strings for later use by application programs. The environment is
   always located on a paragraph boundary. This is its format:
        byte    ASCIIZ string 1
        byte    ASCIIZ string 2
            ....
        byte    ASCIIZ string n
        byte    of zeros (0)
   Typically the environment strings have the form:
        NAME = VALUE
   The length of NAME or VALUE can be anything desired as long as it still fits
   into the 123 byte space (4 bytes are used by "SET ").
   Following the byte of zeros in the environment, a WORD indicates the number
   of other strings following.

   If the environment is part of an EXECed command interpreter, it is followed
   by a copy of the DS:DX filename passed to the child process. A zero value
   causes the newly created process to inherit the parent's environment.

3) Offset 80h in the PSP contains code to invoke the DOS function dispatcher.
   Thus, by placing the desired function number in AH, a program can issue a
   long call to PSP+50h to invoke a DOS function rather than issuing an int 21h.

4) The disk transfer address (DTA) is set to 80h (default DTA in PSP).

5) File Control Blocks 5Ch and 6Ch are formatted from the first two parameters
   entered when the command was invoked. Note that if either parameter contained
   a path name, then the corresponding FCB will contain only a valid drive
   number. The filename field will not be valid.

6) An unformatted parameter area at 81h contains all the characters entered
   after the command name (including leading and imbedded delimiters), with 80h
   set to the number of characters. If the <, >, or | parameters were entered
   on the command line, they (and the filenames associated with them) will not
   appear in this area, because redirection of standard input and output is
   transparent to applications.

(For EXE files only)
7) DS and ES registers are set to point to the PSP.

8) CS, IP, SS, and SP registers are set to the values passed by the linker.

(For COM files only)
9) For COM files, offset 6 (one word) contains the number of bytes availible in
   the segment.

10) Register AX reflects the validity of drive specifiers entered with the
    first two parameters as follows:
        AL=0FFh is the first parameter contained an invalid drive specifier,
                otherwise AL=00h.
        AL=0FFh if the second parameter contained an invalid drive specifier,
                otherwise AL=00h.

11) All four segment registers contain the segment address of the inital
    allocation block, that starts within the PSP control block. All of user
    memory is allocated to the program. If the program needs to invoke another
    program through the EXEC function call (4Bh), it must first free some memory
    through the SETBLOCK function call to provide space for the program being
    invoked.

12) The Instruction Pointer (IP) is set to 100h.

13) The SP register is set to the end of the program's segment. The segment size
    at offset 6 is rounded down to the paragraph size.

14) A word of zeroes is placed on top of the stack.


 The PSP (with offsets in hexadecimal) is formatted as follows:
 (* = undocumented)

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³     P  R  O  G  R  A  M       S  E  G  M  E  N  T       P  R  E  F  I  X     ³
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³ offset³   size   ³                     C O N T E N T S                       ³
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³ 0000h ³ 2 bytes  ³ int 20h                                                   ³
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³ 0002h ³ 2 bytes  ³ segment address, end of allocation block                  ³
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³ 0004h ³ 1 byte   ³ reserved, normally 0                                      ³
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³ 0005h ³ 5 bytes  ³ FAR call to MSDOS function dispatcher (int 21h)           ³
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³ 000Ah ³ 4 bytes  ³ previous termination handler interrupt vector (int 22h)   ³
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³ 000Eh ³ 4 bytes  ³ previous contents of ctrl-C interrupt vector (int 23h)    ³
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³ 0012h ³ 4 bytes  ³ prev. critical error handler interrupt vector (int 24h)   ³
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³ 0016h ³ 22 bytes ³ reserved for DOS                                          ³
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      * ³ 2 bytes  ³ (16) parent process' PSP                                  ³
      * ³ 20 bytes ³ (18) "handle table" used for redirection of files         ³
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³ 002Ch ³ 2 bytes  ³ segment address of the program's environment block        ³
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³ 002Eh ³ 34 bytes ³ reserved, DOS work area                                   ³
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      * ³  4 bytes ³ (2E) stores the calling process's stack pointer when      ³
        ³          ³      switching to DOS's internal stack.                   ³
      * ³          ³ (32) DOS 3.x max open files                               ³
      * ³  2 bytes ³ (3A) size of handle table   |these functions are in here  ³
      * ³  4 bytes ³ (3C) handle table address   |but reported addresses vary  ³
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³ 0050h ³  3 bytes ³ int 21h, RETF instruction                                 ³
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³ 0053h ³  2 bytes ³ reserved - unused?                                        ³
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³ 0055h ³  7 bytes ³ reserved, or FCB#1 extension                              ³
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³ 005Ch ³ 16 bytes ³ default unopened File Control Block #1                    ³
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³ 006Ch ³ 16 bytes ³ default unopened FCB #2 (overlaid if FCB #1 opened)       ³
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³ 0080h ³  1 byte  ³ parameter length (number of chars entered after filename) ³
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³ 0081h ³   ...    ³ parameters                                                ³
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³ 00FFh ³ 128 bytes³ command tail and default Disk Transfer Area (DTA)         ³
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1. The first segment of availible memory is in segment (paragraph) form. For
   example, 1000h would respresent 64k.

2. Offset 2Ch contains the segment address of the environment.

3. Programs must not alter any part of the PSP below offset 5Ch.


PSP (comments):

offset 00h  contains hex bytes CD 20, the int 20h opcode. A program can end
            by making a jump to this location when the CS points to the PSP.
            For normal cases, int 21, function 4Ch should be used.

offset 02h  contains the segment-paragraph address of the end of memory as
            reported by DOS. (which may not be the same as the real end of RAM).
            Multiply this number by 10h or 16 to get the amount of memory
            availible. ex. 1000h would be 64k.

offset 04h  "reserved or used by DOS" according to Microsoft

offset 05h  contains a long call to the DOS function dispatcher. Programs may
            jump to this address instead of calling int 21 if they wish.
            Used by Basic and other CPM object-code translated programs. It is
            slower than standard int 21h.

offset 0Ah, 0Eh, 12h
            vectors (IP, CS)

offset 16h  PSP:16h is the segment address of the invoking program's PSP, which
        *   will most often be COMMAND.COM but perhaps may be a secondary
            non-permanent COMMAND or a multitasking shell, etc. At any rate,
            the resident shell version of COMMAND.COM has PSP:16H = PSP, which
            indicates "don't look any lower in memory" for the command
            interpreter. To find the beginning of the allocation chain, look
            backwards through the PSP link addresses until the link address is
            equal to the PSP segment address that it resides in. This should
            be COMMAND.COM. To find COMMAND.COM's environment, look at the word
            stored at offset 0BD3h (PC-DOS 3.1 only). This is a segment
            address, so look there at offset 0.

       18h  handle alias table (networking). Also you can make PRN go to CON,
        *   CON go to PRN, ERR go to PRN, etc. 0FFh = availible.

offset 2Ch  is the segment:offset address of the environment for the program
            using this particular PSP. This pointer does not point to
            COMMAND.COM's environment unless it is a second copy of COMMAND.

offset 2Eh  the DWORD at PSP+2Eh is used by DOS to store the calling process's
        *   stack pointer when switching to DOS's own private stack - at the end
            of a DOS function call, SS:SP is restored from this address.

       32h, 34h
        *   table of number of file handles (to 64k of handles!)

offset 40h  2 byte field points to the segment address of COMMAND.COM's PSP in
        *   "weird" EXE files produced by Digital Research RASMPC/LINKPC.
            EXE files created with these tools can cause all sorts of problems
            with standard MSDOS debugging tools.

offset 50h  contains a long call to the DOS int 21 function dispatcher.

offset 5Ch, 65h, 6Ch
            contain FCB information for use with FCB function calls. The first
            FCB may overlay the second if it is an extended call; your program
            should revector these areas to a safe place if you intend to use
            them.

offset 5Ch  16 bytes first command-line argument (formatted as uppercase 11
            character filename)

offset 6Ch  16 bytes second command-line argument (formatted as uppercase 11
            character filename)

offset 7Ch-7Fh
           "reserved or used by DOS"
offset 80h  1 byte number of bytes in command line argument

offset 80h, 81h
            contain the length and value of parameters passed on the command
            line.

offset 81h  97 bytes unformatted command line and/or default DTA

offset 0FFh contains the DTA
         

 The PSP is created by DOS for all programs and contains most of the information
you need to know about a program running. You can change the environment for
the current process, however, but for the parent process, DOS in this case, you
need to literally backtrack to DOS or COMMAND.COM's PSP. In order to get there
you must look at the current PSP. At offset 16h of the current PSP segment,
there a 2 byte segment address to the parent or previous process PSP.
 From there you can manipulate the enviroment by looking at offset 2Ch. As you
know, at offset 2Ch, there is 2 byte segment address to the environment block.

Try this under debug and explore the addresses located at these offsets;

      offset  length                description
     ------------------------------------------------------------
        16h     2       segment address of parent process PSP
        2Ch     2       segment address of environment block.

Remember under debug you will have to backtrack two times.

        Programs        Parent
      --------------------------
        command.com     none
        debug.com       command.com
        program         debug.com




MEMORY CONTROL BLOCKSÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

 DOS keeps track of allocated and availible memory blocks, and provides four
function calls for application programs to communicate their memory needs to
DOS. These calls are:
              48h --- allocate memory                 (MALLOC)
              49h --- free allocated memory
              4Ah --- modify allocated memory blocks  (SETBLOCK)
              4Bh --- load or execute program         (EXEC)

DOS manages memory as follows:

 DOS build a control block for each block of memory, whether free or allocated.
For example, if a program issues an "allocate" (48h), DOS locates a block of
free memory that satisfies the request, and then "carves" the requested memory
out of that block. The requesting program is passed the location of the first
byte of the block that was allocated for it - a memory management control block,
describing the allocated block, has been built for the allocated block and a
second memory management control block describes the amount of space left in the
original free block of memory. When you do a SETBLOCK to shrink an allocated
block, DOS builds a memory management control block for the area being freed and
adds it to the chain of control blocks. Thus, any program that changed memory
that is not allocated to it stands a chance of destroying a DOS memory
management control block. This causes unpredictable results that don't show up
until an activity is performed where DOS uses its chain of control blocks. The
normal result is a memory allocation error, which means a system reset will be
required.

 When a program (command or application program) is to be loaded, DOS uses the
EXEC function call 4Bh to perform the loading.

 This is the same function call that is availible to applications programs for
loading other programs. This function call has two options:

      Function 00h, to load and execute a program (this is what the command
                    processor uses to load and execute external commands)

      Function 03h, to load an overlay (program) without executing it.

 Although both functions perform their loading in the same way (relocation is
performed for EXE files) their handling of memory management is different.

FUNCTION 0: For function 0 to load and execute a program, EXEC first allocates
the largest availible block of memory (the new program's PSP will be at offset
0 in that block). Then EXEC loads the program. Thus, in most cases, the new
program owns all the memory from its PSP to the end of memory, including memory
occupied by the transient parent of COMMAND.COM. If the program were to issue
its own EXEC function call to load and execute another program, the request
would fail because no availible memory exists to load the new program into.

NOTE: For EXE programs, the amount of memory allocated is the size of the
      program's memory image plus the value in the MAX_ALLOC field of the file's
      header (offset 0Ch, if that much memory is availible. If not, EXEC
      allocates the size of the program's memory image plus the value in the
      MIN_ALLOC field in the header (offset 0Ah). These fields are set by the
      Linker).

 A well-behaved program uses the SETBLOCK function call when it receives
control, to shrink its allocated memory block down to the size it really needs.
A COM program should remember to set up its own stack before doing the SETBLOCK,
since it is likely that the default stack supplied by DOS lies in the area of
memory being used. This frees unneeded memory, which can be used for loading
other programs.

 If the program requires additional memory during processing, it can obtain
the memory using the allocate function call and later free it using the free
memory function call.

 When a program is loaded using EXEC function call 00h exits, its initial
allocation block (the block beginning with its PSP) is automatically freed
before the calling program regains control. It is the responsibility of all
programs to free any memory they allocate before exiting to the calling
program.

 FUNCTION 3: For function 3, to load an overlay, no PSP is built and EXEC
assumes the calling program has already allocated memory to load the new program
into - it will NOT allocate memory for it. Thus the calling program should
either allow for the loading of overlays when it determines the amount of memory
to keep when issuing the SETBLOCK call, or should initially free as much memory
as possible. The calling program should then allocate a block (based on the size
of the program to be loaded) to hold the program that will be loaded using the
"load overlay" call. Note that "load overlay" does not check to see if the
calling program actually owns the memory block it has been instructed to load
into - it assumes the calling program has followed the rules. If the calling
program does not own the memory into which the overlay is being loaded, there is
a chance the program being loaded will overlay one of the control blocks that
DOS uses to keep track of memory blocks.

 Programs loaded using function 3 should not issue any SETBLOCK calls since
they don't own the memory they are operating in. (This memory is owned by the
calling program)

 Because programs loaded using function 3 are given control directly by (and
return contrrol directly to) the calling program, no memory is automatically
freed when the called program exits. It is up to the calling program to
determine the disposition of the memory that had been occupied by the exiting
program. Note that if the exiting program had itself allocated any memory, it
is responsible for freeing that memory before exiting.

 Memory control blocks, sometimes called "arena headers" after their UNIX
counterpart, are 16 bytes long. Only the first 5 bytes are used. 16 bytes are
used for the memory control block, which always starts at a paragraph boundary.
When DOS call 48h is made to allocate "x" many paragraphs of memory, the amount
used up is actually one more than the figure in the BX register to provide
space for the associated memory control block. The location of the memory
control block is at the paragraph immediately before the segment value returned
in AX by the DOS function 48h call i.e. ((AX-1):0).

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³ Bytes ³                           Function                                   ³
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³   0   ³ ASCII M or Z                                                         ³
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byte 1    will always have the value of 4Dh or 5Ah. The value 5Ah (Z) indicates
          the block is the last in a chain, all memory above it is unused. 4Dh
          (M) means that the block is intermediate in a chain, the memory above
          it belongs to the next program or to DOS.

byte 2,3  hold the PSP segment address of the program that owns the
          corresponding block of memory. A value of 0 means the block is free
          to be claimed, any other value represents a segment address.

byte 3, 4 indicate the size in paragraphs of the memory block. If you know the
          address of the first block, you can find the next block by adding the
          length of the memory block plus 1 to the segment address of the
          control block. Finding the first block can be difficult, as this
          varies according to the DOS version and the configuration.

 The remaining 11 bytes are not currently used by DOS, and may contain "trash"
characters left in memory from previous applications.

 If DOS determines that the allocation chain of memory control blocks has been
corrupted, it will halt the system and display the message "Memory Allocation
Error", and the system will halt, requiring a reboot.

 Each memory block consists of a signature byte (4Dh or 5Ah) then a word which
is the PSP value of the owner of the block (which allocated it), followed by a
word which is the size in paragraphs of the block. The last block has a
signature of 5Ah. All others have 4Dh. If the owner is 0000 then the block is
free.

 Once a memory control block has been created it should only be manipulated
with the appropriate DOS function calls. Accidentally writing over any of the
first 5 bytes of a memory control block can cause a memory allocation error
and cause the system to lock up. If the first byte is overwritten with
something other than an 'M' or a 'Z' then DOS will complain with an error
return code of 7 signifying "Memory Control Blocks destroyed". However, should
you change the ownership or block size bytes, you've had it.

 When a .COM program is first loaded by DOS and given control, the memory
control block immediately preceding the Program Segment Prefix contains the
following data:

          ID    = 'Z'
          Owner = segment address of PSP (= CS register of .COM program)
          Size  = number of available paragraphs in DOS memory pool

  An .EXE file will have the following data in the memory control block for
the program (just prior to the PSP):

          ID    = 'M'
          Owner = segment address of PSP (= DS register of program)
          Size  = the number of paragraphs allocated to the program according
                  to the information in the .EXE program header

 In the case of an .EXE program file the amount of memory allocated depends
on the contents of the program header which informs the DOS loader how much to
allocate for each of the segments in the program. With an .EXE program file
there will always be a 'Z' memory control block created in memory immediately
after the end of the space allocated to the program itself.

 One important fact to remember about DOS memory allocation is that blocks of
RAM allocated by different calls to DOS function 48H will NOT be contiguous. At
the very best, they will be separated by the 16 bytes of the memory control
block, and at worst they could be anywhere in RAM that DOS manages to find a
existing memory control block of sufficient size to accomodate the memory
request.

 DOS treats the memory control blocks as a kind of linked list (term used
loosely). It uses the earlier MCBs to find the later ones by calculating the
location of the next one from the size of the prior one. As such, erasing any
of the MCB data in the chain of MCBs will upset DOS severely, as each call for
a new memory allocation causes DOS to scan the whole chain of MCBs looking for
a free one that is large enough to fulfill the request.

 A separate MCB is created for the DOS environment strings at each program
load, so there will be many copies of the environment strewn through memory
when you have a lot of memory resident programs loaded. The memory control
blocks for the DOS environment strings are not returned to the DOS memory pool
if the program goes resident, as DOS will need to copy this enviroment for the
next program loaded.