; *** Input Validation ***
      ; ************************
      jz msg_index ;If acc=0 we have an acceptable value.
      cjne a,#3,eval_acc ;We have a non-zero value is other than 3?
      jmp msg_index ;No it is 3, a valid value.
      ; *** It's not either a 0 or 3 value ***
      ; *** Test for an invalid value ***
      eval_acc:
      jnc input_err ;If we didn't carry the value exceeds that allowable.