INT 21,30 - Get DOS Version Number

	AH = 30h

	on return:
	AL = major version number (2-5)
	AH = minor version number (in hundredths decimal)
	BH = FF  indicates MS-DOS, only if OEM vendor chooses to identify
	   = 00  indicates PC-DOS
	BL:CX = 24 bit OEM serial number if BH is FF

	- for an example DOS version 2.1 returns AL=2 and AH=10
	- DOS versions prior to DOS 2.0 return zero in AH and AL
	- DOS version 4.0 and 4.1 usually return the same value of 4.00
	- the OEM serial number is a rarity, though some older OEM DOS
	  versions implemented this feature
	- the OS/2 compatibility box returns 10.10 for OS/2 1.1, 10.20
	  for OS/2 1.2, etc...
	- when testing for version, a specific test can often cause your
	  code to not work in following versions of DOS.  It is often better
	  to test for a version number greater or equal to the minimum rather
	  than a specific version number where possible
	- see DOS Versions