Question

9. (2 marks) We are working with register X and Y in our program when we call subroutine Do Something that states it receives a value using ACCD and returns a value using ACCA. We want to ensure that X and Y is not changed, should we (explain your answer): a. Push X and Y onto the stack, followed by D before calling the subroutine b. Push D onto the stack, followed by X and Y before calling the subroutine c. Push D onto the stack, and have the subroutine push and pop the X and Y registers d. Do nothing since the subroutine receives ACCD and returns the value in ACCA - there is no mention of X or Y for the subroutine
microprocessors

Answer 1

Below is the answer. If you have any doubts let me know in comments section. Please upvote:)

Answer is option a.

push X and Y on to the stack, followed by D before calling subroutine.

ACCD represents register D.

In the question it is asked that, we want to ensure that X and Y is not changed. But when we call subroutine, data of registers might be changed. So inorder to avoid this, store the registers data in stack. So before calling subroutine, push the registers into the stack. At the end of the subroutine, you can pop the stored data of the stack and then return control to the main program. Therefore registers X and Y will remain unchanged.