Because this function was not defined under MS-DOS version 1, it should always be called with AL = 00H. In an MS-DOS version 1 environment, AL will be returned unchanged.Returns the version number of the host MS-DOS operating system. This function is used by application programs to determine the capabilities of their environment.
Call with:
AH = 30H
AL = 00H
Returns:
If running under MS-DOS version 1
AL = 00H
If running under MS-DOS versions 2.0 or later
AL = major version number (MS-DOS 3.10 = 3, etc.)
AH = minor version number (MS-DOS 3.10 = 0AH, etc.)
BH = Original Equipment Manufacturer's (OEM's) serial number
(OEM-dependent——usually 00H for IBM's PC-DOS, 0FFH or
other values for MS-DOS)
BL:CX = 24-bit user serial number (optional, OEM-dependent)
Notes:
Because this function was not defined under MS-DOS version 1, it should always be called with AL = 00H. In an MS-DOS version 1 environment, AL will be returned unchanged.
Care must be taken not to exit in an unacceptable fashion if an MS-DOS version 1 environment is detected. For example, Int 21H Function 4CH (Terminate Process with Return Code), Int 21H Function 40H (Write to File or Device), and the standard error handle are not available in MS-DOS version 1. In such cases a program should display an error message using Int 21H Function 09H and then terminate with Int 20H or Int 21H Function 00H.
Example:
Get the MS-DOS version number, terminating the current process with an error message if not running under MS-DOS version 2.0 or later.
cr equ 0dh ; ASCII carriage return
lf equ 0ah ; ASCII line feed
msg db cr,lf
db 'Wrong MS-DOS version'
db cr,lf,'$'
.
.
.
mov ax,3000h ; function number
int 21h ; transfer to MS-DOS
cmp al,2 ; version 2 or later?
jae label1 ; yes, jump
; display error message
mov ah,09 ; function number
mov dx,offset msg ; message address
int 21h ; transfer to MS-DOS
; terminate process
mov ah,0 ; function number
int 21h ; transfer to MS-DOS
label1: .
.
.