Question

Say R1 = 5, R2 = 6. Push or pop the following registers to the stack and see if they are equal. Specify the top and bottom of the stack.

push {r2, r1}

pop{r2, r1}

AND

push {r1, r2}

pop{r2}

pop{r1}

AND

Push {r1}

Push{r2}

Pop {r1, r2}

Are these operations equivalent?

Answer 1

1)

push {r2, r1}

So the stack contains

r2     6

r1     5

The top of the stack contains 6 and the bottomof the stack contains 5

pop{r2, r1}

First 6 is popped and then 5 is popped

2)

push {r1, r2}

So the stack contains

r1     5

r2     6

The top of the stack contains 5 and the bottom of the stack contains 6

pop{r2, r1}

r2 cannot be popped because the top of the stack contains r1

3)

push {r1}

So the stack contains

r1     5

push {r2}

So the stack contains

r2     6

r1     5

The top of the stack contains 6 and the bottom of the stack contains 5

pop{r1, r2}

r1 cannot be popped because the top of the stack contains r2

So these operations are not equivalent