INT 2F U - DOS 5+ - COMMAND.COM INTERFACE                                       
	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 HMAsee High Memory Area; only the primary COMMAND.COM retains this
	  portion
	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 & PCIBM 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/PCIBM PC DOS COMMAND.COM checks the version
	  of the underlaying OS to see if it is their own. Hence, the
	  PCIBM PC DOS 6.1 (and by SETVER version faking also the PCIBM PC DOS 7 and 2000)
	  COMMAND.COM also run on Novell DOS 7 - DR DOS 7.03, which all
	  identify themselves as DOS API(Application Program[ming] Interface) The defined set of calls which a program may make to interact with or request services of the operating system or environment under which it is running.  Because the inputs and outputs of the calls are well-defined, a program using the API can continue using the identical calls even if the internal organization of the program providing the API changes. level 6.0 and IBMInternational Busiuness MachinesInternational Busiuness Machines) A hardware, software and other service technology company founded in 1911. OEM(Original Equipment Manufacturer) a company which purchases components that are resold as part of its own products under the company's own brand name, e.g. a Gateway 2000-branded monitor may actually be a Mag or NEC monitor. (not yet tested
	  as primary shell).  PCIBM PC DOS 5.0 COMMAND.COM should run on DR PalmDOS
	  (untested).
	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
	  SHELL=C:\DRDOS\COMMAND.COM instead. If a MS-DOS CONFIG.SYS file
	  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/PCIBM 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


Format of Microsoft COMMAND.COM file image signature:
Offset	Size	Description	(Table 04100)
 00h  3 BYTEs	E9h xxh xxh (JMP instruction)
 03h	BYTE	version
		bits 7-4: major version
		bits 3-0: minor version code
		( 60h for MS-DOS 6.0 and PC-DOS 6.1;
		  64h for MS-DOS 6.20, Japanese MS-DOS 6.2, Hangeul MS-DOS 6.2;
		  66h for MS-DOS 6.22, PRC (Chinese) MS-DOS 6.22;
		  70h for PCIBM PC DOS 7, PCIBM PC DOS 7 Y2K edition )
Notes:	Since DR DOS 6.0+, MS-DOS 7.0+, PTS-DOS, S/DOS, 4DOS/NDOS use an .EXE
	  style shell file format, this signature is not met by any release of
	  these shells. DR DOS 3.31-5.0 used a different jump (E8h) at offset
	  00h.
	Older releases of MS-DOS/PCIBM PC DOS all had a jump (E9h) at offset 00h, but
	  other values at offset 03h. For reference here is a list of the
	  values for some of these older shells:
	    00h for PCIBM PC DOS 1.10, Olivetti DOS 2.11, MS-DOS 5.0,
		  Russian MS-DOS 5.0
	    1Eh for MS-DOS 4.01
	    BAh for PCIBM PC DOS 3.10, 3.20, MS-DOS 3.30