sixteen bit substraction

org 0h
mov r0,#40h
mov r1,#42h
mov a,@r0
subb a,@r1
mov r2,a
inc r0
inc r1
mov a,@r0
subb a,@r1
inc r1
mov @r1,a
mov a,r2
inc r1
mov @r1,a ;result in 44h onwards
sjmp $