Let us determine the behavior of the instruction pushq $\%$rsp for an x$86-64$ processor. We could try reading the Intel documentation on this instruction, but a simpler approach is to conduct an experiment on an actual machine. The C compiler would not normally generate this instruction, so we must use hand$-$generated assembly code for this task. Here is a test function we have written $($Web Aside asm:easm on page $178$ describes how to write programs that combine C code with handwritten assembly code$)$:

$1.$text

$2.$globl pushtest

$3$ pushtest:

$4$ movq $\%$rsp$,\%$rax Copy stack pointer

$5$ pushq $\%$rsp Push stack pointer

$6$ Popd $\%$rdx Pop it back

$7$ subq $\%$rdx$,\%$rax Return $0$ or $4$

$8$ ret

In our experiments, we find that function pushtest always returns $0.$ What does this imply about the behavior of the instruction pushq $\%$rsp under x$86-64?$