Guide: Registers Terminator

Navigate to guides/registers-terminator/support/.

Open the registers_terminator.asm file, assemble it, and run it.

The purpose of this guide is to really make you aware how caller-saved registers need to be saved by the caller. It is also important to notice that the rbx and r12-r15 registers are preserved(callee saved).

In the presented assembly file, we set some values to the rax, rcx, rdx, rsi, rdi, r8 and r9 simulating a normal program flow. We print their values and then call the libc printf function and then print their values again. The output can look similar to this one:

rax=1
rcx=2
rdx=3
rsi=4
rdi=404018
r8=6
r9=7
rbx=8
r12=9
r13=a
r14=b
r15=c
Hello, world!
rax=e
rcx=0
rdx=0
rsi=13e5b2a0
rdi=1463ae00
r8=6
r9=7
rbx=8
r12=9
r13=a
r14=b
r15=c

We can easily notice that the values of the rax, rcx, rdx, rsi and rdi registers have been completely altered. If those were real values and we would have needed them after the printf call, the results would be unpredictable but, certainly, wrong. However, we can observe that the callee-saved registers remained untouched.