mov ax, 3305h ;Get Startup Drive
int 21h
mov StartupDrive, dl ;drive (1 = A, 2 = B, etc.)
Get Startup Drive (Function 3305h) returns a number representing the drive that was used to load MS-DOS.
This function has no parameters.
The DL register contains the number of the startup drive (1 = A, 2 = B, 3 = C, and so on).