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The following Debug script will allow you to request the MS-DOS version from the MS-DOS kernel (MSDOS.SYS). With some versions of MS-DOS, the VER command and COMMAND.COM's startup banner display a different version number than the kernel.
Programs, including MS-DOS 5 Upgrade SETUP.EXE, use this method to determine the MS-DOS version.
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.
After the final step, the true MS-DOS version number will be returned in the AX register, as follows:
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
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:
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
0-9 hexadecimal = 0-9 decimal
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
"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
Article ID: 75195 - Last Review: October 7, 2013 - Revision: 3.0
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