Interrupt 21h Function 71AAh Minor Code 1h 

Terminate the association between a path and a drive letter.

mov ax, 71aah            ; SUBST

mov bh, 1                ; Terminate Subst

mov bl, DriveNum         ; see below

int 21h

jc error

Parameters

DriveNum

Drive to terminate SUBST. This parameter can 1 for A, 2 for B, and so on. Note that DriveNum cannot be 0 to indicate the default drive.

Return Value

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