Compatibility: 2.0+
Expects: AX    4b00H
DS:DX address of an ASCIIZ  filespec of the program to execute
ES:BX address of an ExecParamRec
                                                                  
Returns: AX    error code if CF is set to CY
else  All registers are destroyed, including SS and SP
                                                                  
Info: Provides a means for a program (the parent) to execute another
program (the child).  When the child exits, control will return
to the parent.  It prepares a PSP for the child, loads its code,
performs any needed segment fixup, est the registers as expected,
then begins executing the child program.

DS:DX points to an ASCIIZ string in the form...
d:\path\filename.ext 0 
If the drive or path is omitted, defaults are assumed.

ES:BX points to a block of memory prepared as an ExecParamRec that has
been filled with the information needed.

Since the parent program initially receives all of available
memory for its own processing, you must free some memory via
Fn 4aH before performing using this fn.

To EXEC a program:

  Call Fn 4aH with ES=your PSP and the minimum amount of memory
that your program requires (in paragraphs).
  Prepare an ASCIIZ  string of the program file to EXEC and set
DS:DX to point to its first character.
  Prepare an ExecParamRec containing all required fields
  Save the current values of SS, SP, DS, ES, and the DTA in
variables that are relative to CS (CS is the only point of
reference after EXEC returns from the child).
  Issue the EXEC call with AL=0
  Restore SS and SP to local values
  Check CF (the carry flag) to see if EXEC failed with an error
  Restore DS, ES and the local DTA if necessary
  Check the exit code  via Fn 4dH WAIT (if desired)

All open files are duplicated, so the child can process data via
file handle and/or use the standard I/O for input and output.
Each handle's Access Mode is duplicated, but any "file locks"
that are in effect will not belong to the child.  See Fn 5cH.

Upon return from the child, the INT 22H (Terminate), INT 23H
(Ctrl-Break), and INT 24H (Critical Error) vectors are restored
to their previous values.

Notes:   This function uses the loader portion of Command.Com which is
transient in DOS 2.0+ (a program may have overwritten it).
Thus, this call could fail if DOS can't locate the Command.Com
file.  You should make sure that COMSPEC= in the environment is
valid before the call.

  Rather than parse the FCBs (as needed in the ExecParamRec), you
may find it convenient to load and execute a secondary copy of
Command.Com, using the /C option.  For instance, to execute the
FORMAT.COM program, set DS:DX to the address of an ASCIIZ
string:

"c:\dos\command.com",0

and set EPB+2 to the segment and offset of the command line:

12H," /c format a: /s/4",0dH

This secondary command interpreter uses a very small amount of
memory (around 4K).  You can locate the drive and directory
that contains COMMAND.COM by searching the DOS Environment for
the COMSPEC string.

See Also: Fn 4b01H (load program)
Fn 4b03H (load overlay)
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DOS Fn 4b00H: Execute a Program