INT 2eH: Perform DOS Command

                                                           Compatibility: ?? 
 Expects: DS:SI offset of an Lstring of a DOS command to perform
                (note: memory must be available when loading programs)
 Returns: ??    (it's not apparent how to obtain status of request)
          SS:SP are destroyed ◄════

    Info: This is ◄undocumented► through DOS 6.0.  There are too many
          drawbacks to make this a really useful function (see notes).

          INT 2eH performs a DOS command, as if it had been typed at the
          DOS prompt.

          DS:SI must point to a string in which the first byte is the
          length of the string, followed by the command text.  After the
          command, a byte of 0dH (CR) must be appended (but don't count it
          in the first byte).  For instance:

            DB  0fH, "set PROMPT=$p$g", 0dH  ;text is 0fH (15) bytes long

          This example illustrates the main potential use -- it is able to
          Set values into the DOS master (root) environment.  In all other
          cases, fn 4bH is preferred, (e.g., EXEC COMMAND.COM, passing it
          the line "/c dos_cmd") since the EXEC fn is fully documented.

   Notes: ■ Don't use this while a batch file is running.  This would cause
            a fatal reentrancy problem (COMMAND.COM uses this to execute
            batch commands).  Since a program cannot control whether a
            batch is in progress, this is NOT a reliable function.

          ■ Be sure to free up some memory to execute programs and
            remember to relocate your local stack into the shrunken memory

          ■ Provide plenty of stack space -- DOS really eats it up.  SS and
            SP must be saved before the call and restored directly after.

See Also: INT 2fH aexxH (hook COMMAND.COM command line)
          fn 4b00H (load and execute a program)
          DOS Environment