0xFF: indicates MS-DOS, only if OEM vendor chooses to identify
0x00: indicates PC-DOS
BL:CX
24bit OEM Serial number if BH = 0xFF
Notes & Comments
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