x=0 do addition ,x=1 do substraction ,x=2 do multiplication ,x=3 do division

org 00h
mov r0,#40h
mov a,@r0
mov r1,a
inc r0
mov a,@r0
mov b,a
inc r0
mov a,@r0 ; second number is in a
cjne r1,#00,n1
 add a,b
 jnc sk
 mov b,#1
sk:mov b,#0h
sjmp las
n1:cjne r1,#01,n2
 add a,b
 sjmp las
n2:cjne r1,#02,n3
mul ab
 sjmp las
n3:cjne r1,#03,n4
 div ab
 sjmp las
n4:mov a,#00h
 mov b,#00h
las:mov 43h,a
    mov 44h,b
sjmp $
end