DOS Environment

  DOS maintains an area of memory that holds a set of ASCIIZ strings that
  can be used to obtain certain system-level information and to pass data to
  other programs.

  ■ The environment is a series of strings in the form: name=value.
  ■ The Set command is a general purpose interface to the environment.  The
    Path and Prompt commands also affect its contents.
  ■ In DOS 3.0+, you can obtain the name by which your program was started
    (and therefore, your startup directory) by examining the end of the
  ■ In DOS 3.3+ batch files, any text surrounded by percent marks is
    replaced with the environment value that matches the name.  Examples:
    │  Path c:\my_dir;%PATH%
    │  If (%USER%) == (dan) GOTO dan_label
    │  Echo %SET_CMPRS% > %CUR_LPT%

  ■ In DOS 6.0+, you can use SET in CONFIG.SYS.

  The environment is limited to an arbitrary size, but a larger DOS
  environment can be created with the SHELL= directive in the CONFIG.SYS
  file or the command 'COMMAND /e:nnnn' which starts a secondary copy of the
  command interpreter, providing nnnn bytes of environment space.

     Layout:                       Example using ASM mnemonics:
     ▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀             ▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀
     name_1=value_10               db  'COMSPEC=c:\',0
     name_2=value_20               db  'PROMPT=$p$g',0
          :                        db  'WP=c:\wordproc',0
     name_N=value_N0               db  'PATH=d:\;c:\dos;c:\utils',0
     0                             db  0
  ┌─ xxxx ───────────┐   DOS    ┌─ dw  1  ───────────────────────┐
  │  EXEC_string_10  ╞═► 3.0+ ◄═╡  db  'C:\ACCTNG\AR001.EXE',0   │
  │    :             │   only   └─ db  0  ───────────────────────┘
  │  EXEC_string_NN0 │
  └─ 0 ──────────────┘

  where 0    is the ASCII NUL character (00H)
        xxxx is a 16-bit binary value (usually 0001H)

█▌Using the Environment▐█
  The environment is less than 32K bytes and it starts on a paragraph
  boundary.  Offset 2cH of the PSP of the current program contains the
  paragraph number of the environment for the active program.

  Alas, there are no DOS services to help access the environment.  You can
  find a selected name by using a series of ASCIIZ string comparisons, until
  you come to a null string (zero length), indicating the end of the
  environment.  Normally, the name portion of each string in the environment
  is in uppercase, but this is not enforced.

  One typical operation with the environment is used by shell-type programs
  which execute a secondary copy of COMMAND.COM.  Shells typically search
  for the name of "COMSPEC" and use the value as the drive, path, and
  filespec of the DOS command interpreter--the program to EXEC via fn 4bH

  Some programs (including some DOS commands) request the operator to store
  application-specific information in the environment via the SET command.
  The application can use that information each time it is executed.  For
  instance, a word processor may search the environment for a name of
  "DICTIONARY" and use the text of the value as the filespec for a
  dictionary data file.

  See Predefined Environment Variables for related info.

█▌Locating Your Load Path▐█
  Starting with DOS 3.0, DOS stores an additional string after the end of
  the formal environment that can be used to locate the drive and directory
  from which a program was loaded.  Applications can use this to help locate
  program overlays and data files.

  This becaem important with DOS 3.0, when DOS began allowing an operator to
  invoke a program by prefixing a pathname before the program filename.

  Following the final string of the environment is a byte of 0, indicating
  the end of the formal environment.  The next two bytes are a 16-bit binary
  count of additional strings (this value is normally 0001H).  Following
  this binary value, you can expect to find an ASCIIZ string of the filespec
  (including the drive, path, and extension) which was used by DOS Fn 4bH
  (EXEC) to load and execute the program.

  Even if the program file was not in the default directory and DOS or
  Windows needed to use the command search path to locate the program file,
  the correct full filespec (d:\path\filename.ext,0) will be stored after
  the end of the environment.

█▌The "Master" Environment▐█
  Each program receives a static copy of the initial environment.  Thus,
  any changes you make to an environment will be in effect for all your
  child processes (if any), but will be lost when control is returned to
  your parent process.  See Program Startup & Exit.

  The DOS Set command affects the master environment of the active copy of
  COMMAND.COM, but there is no simple or documented way to do so in your own
  programs.  The undocumented way to locate the "root" or "master"
  environment is:

    ■ Examine the word at offset 16H of your PSP; that's the PSP of your
      parent (undocumented).

    ■ Examine the same offset in that "parent PSP".  Keep working backward
      up the chain until the "parent PSP" is the same as the PSP
      itself--that will be the PSP of COMMAND.COM.

    ■ The word at offset 2cH in that PSP is the segment of the environment
      for COMMAND.COM (that's the "master" environment).

    ■ The size of the master environment can be obtained by examining the
      Memory Control Block (MCB) which is in the paragraph just before the
      environment.  The word at offset 03H is the size of the memory block,
      in 16-byte paragraphs.

  If you want to change anything in the master environment, you'll need to
  be careful to avoid overwriting the end and to leave it in good shape.
  Also remember that when running multiple DOS sessions (e.g., under
  Windows), this will affect the environment of ONLY the current session.

See Also: Predefined Environment Variables
          Program Startup & Exit
          DOS Functions