push and pop: How RSP Changes

• push rax is equivalent to:

sub  rsp, 8       ; make room (stack grows down)
mov  [rsp], rax   ; store value at new top

• pop rax is equivalent to:

mov  rax, [rsp]   ; read value from top
add  rsp, 8       ; release the slot

• Each slot on a 64-bit stack is 8 bytes.
• push decrements RSP; pop increments it.