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