INT 21 - S/DOS 1.0+ & PTS-DOS 6.51+ - GET OEM REVISION

	AH = 20h
	AL = which OEM code to retrieve
	    00h get PTS OEM code
		Return: CF cleared
			AL = OEM code (currently always 00h)
	    01h get PTS OEM revision code
		Return: CF cleared
			AL = revision code (default is 00h, but may vary)
Desc:	S/DOS is a reduced and modified PTS-DOS 6.51 release with source code
	  on CD
Note:	This function is provided by S/DOS 1.0 (and is most probably also
	  supported with PTS-DOS 6.51+, maybe even earlier).  It clears the
	  CF flag for all other AL arguments, too. However, AL = 00h and 01h
	  are handled separately. The revision code can be individually defined
	  in the MAKEFILE at compilation time.
	The PTS OEM code does not correlate with the DOS OEM code returned
	  by INT 21/AH=30h, likewise the PTS OEM revision does not correlate
	  with the DOS revision returned by INT 21/AX=3306h.
SeeAlso: INT 21/AH=30h,INT 21/AX=3306h