Interrupt 21h Function 71AAh Minor Code 0h 

Associates a path with a drive letter.

mov ax, 71aah            ; SUBST

mov bh, 0                ; Create Subst

mov bl, DriveNum         ; see below

mov dx, seg PathName     ; see below

mov ds, dx

mov dx, offset PathName

int 21h

jc error

Parameters

DriveNum

Drive to SUBST. This parameter can be 0 for the default drive, 1 for A, 2 for B, and so on.

PathName

Address of path to associate the drive with.

Return Value

Clears the carry flag if successful. Otherwise the function sets the carry flag and returns an error value in the AX register