MORE INFORMATION

The text in the center column is the required user input, followed by pressing ENTER. The items on the left are what Debug will return (the values to the left of the colon will not be an exact match to the example). The right column contains comments.

Debug           User
Response        Input           Comments
--------        -----           --------

C:\>            debug           ; Start the Debug utility
-               rax             ; Displays and alters contents in AX
AX 0000                         ; Current value in AX
:               3000            ; Set function 30h in AH for Int 21
-               a 100           ; Assemble starting at 100
16A0:0100       int 21          ; General MS-DOS services
16A0:0102       int 20          ; End MS-DOS services
16A0:0104       ENTER
-               p               ; Proceed

After the final step, the true MS-DOS version number will be returned in the AX register, as follows:

AX=0005  BX=FF00  CX=0000  DX=0000  SP=FFEE  BP=0000  SI=0000  DI=0000
DS=16A0  ES=16A0  SS=16A0  CS=16A0  IP=0102   NV UP EI PL NZ NA PO NC
16A0:0102 CD20          INT     20
-               q               ; quit debug

The number in the AX register is in hexadecimal and in reverse order. So, if AX=0A02, it is MS-DOS version 2.1, or if AX=0005, it is MS-DOS version 5.0. The hexadecimal conversion is as follows:

0-9 hexadecimal = 0-9 decimal
A(h)=10
B(h)=11
C(h)=12
D(h)=13
E(h)=14
F(h)=15

Example

If AX=0A02, reverse the pairs to:               02 0A
Convert these hex numbers to decimal numbers:   02 10
The MS-DOS version number is:                   2.10

Reference(s):

"Microsoft MS-DOS User's Guide and Reference," version 5.0, pages 399-431

"Supercharging MS-DOS," Appendix E, Microsoft Press

"The New Peter Norton Programmer's Guide to the PC & PS/2," Appendix B, Microsoft Press