Category: shells/command interpreters
Flags: Undocumented function


	AX = 5500h
Return: AX = 0000h if an instance of COMMAND.COM is already running
	DS:SI -> entry point table
Notes:	used to access the shareable portion of COMMAND.COM, which may have
	  been moved into the HMA; only the primary COMMAND.COM retains this
	procedures called from a dispatcher in COMMAND's resident portion;
	  most assume that the segment address of the resident portion is on
	  the stack and are thus not of general use
	DR PalmDOS up to DR-DOS 7.03 COMMAND.COM do not support this call.
	When loading the default command processor (no SHELL= directive in
	  CONFIG.SYS), MS-DOS 6.0-6.22 IO.SYS & PC DOS 6.1-2000 IBMBIO.COM
	  check a signature (see #04099) in the COMMAND.COM file image to test
	  if it is actually their own shell and has the correct version, before
	  they will launch it. If the signature is not found, the message
	  "Invalid COMMAND.COM" will be displayed. This test seems to have
	  vanished with MS-DOS 7+, as it uses an .EXE style file format.
	In addition to this, the MS-DOS/PC DOS COMMAND.COM checks the version
	  of the underlaying OS to see if it is their own. Hence, the
	  PC DOS 6.1 (and by SETVER version faking also the PC DOS 7 and 2000)
	  COMMAND.COM also run on Novell DOS 7 - DR DOS 7.03, which all
	  identify themselves as DOS API level 6.0 and IBM OEM (not yet tested
	  as primary shell).  PC DOS 5.0 COMMAND.COM should run on DR PalmDOS
	The MS-DOS 7+ COMMAND.COM seems to no longer perform this kind of
	  version check any more, which can cause a serious deadlock situation
	  on a multi-boot system with DR-DOS installed, when the MS-DOS 7+
	  COMMAND.COM is placed in C:\. When IBMBIO.COM attempts to load the
	  MS-DOS 7+ COMMAND.COM as a primary shell under DR-OpenDOS 7.02 to
	  DR-DOS 7.03 no error message will be displayed by COMMAND.COM, but
	  the machine will just hang. To defuse this situation, DR-DOS 7.02+
	  IBMBIO.COM was changed to still scan for a SHELL= directive in
	  [D]CONFIG.SYS even in F5-mode. For maximum safety, the MS-DOS 7
	  COMMAND.COM should be moved from C:\ to C:\WINDOWS\COMMAND\
	  and a DCONFIG.SYS file should be created containing for example
	  exists, it should contain a SHELL=C:\WINDOWS\COMMAND\COMMAND.COM
	  directive. (If no shell can be found at all, DR-DOS IBMBIO.COM would
	  display a prompt to enter the proper path to COMMAND.COM).
	The DR-OpenDOS 7.02+ COMMAND.COM is designed to also run on 3rd party
	  operating systems like MS-DOS/PC DOS 3.31+, Windows 9x, and in DOS
	  boxes of OS/2, Windows NT and 2000, and will for example also take
	  advantage of long filenames.
SeeAlso: AX=5501h