Function  30h  Get DOS Version Number 
     Return DOS version and/or user number 
entry   AH      30h 
return  AH      minor version number  (i.e., DOS 2.10 returns AX = 0A02h) 
	AL      major version number 
	BH      OEM ID number 
		00h     IBM 
		16h     DEC    (others not known) 
	BL:CX   24-bit user serial number 
note 1) If AL returns a major version number of zero, the DOS version is  
	below 1.28 for MSDOS and below 2.00 for PCDOS. 
     2) IBM PC-DOS always returns 0000h in BX and CX.  
     3) OS/2 v1.0 Compatibility Box returns a value of 10 for major version. 
     4) Due to the OS/2 return and the fact that some European versions of DOS 
	carry higher version numbers than IBM's DOS, utilities which check 
	for a DOS version should not abort if a higher version than required 
	is found unless some specific problems are known.