; cycle 0 1 2 3 4 5 6 7 8 9 10
irmovq $5, %rdx ; F E D M W
irmovq $0x100, %rsp ; F E D M W
rmmovq %rdx,0(%rsp) ; F E D M W
popq %rsp ; F E D M W
nop ; F E D M W
nop ; F E D M W
rrmovq %rsp, %rax ; F E D M W
If the two cases were reversed, then the register %rax will be set to the incremented program counter rather than the value read from memory.