CODEWITHSUNDEEP
assemblyx86-64cpu-registersatt
user15874403
user15874403

Reputation:

Why changing %rbp doesn't crash Assembly?

I was testing the following assembly code:

.data
msg1: .ascii "HOW YOOOU DOOIN?"
msg2: .ascii "JOEY DOESN'T SHARE FOOD!"
msg1_len: .quad msg2-msg1
msg2_len: .quad msg1_len-msg2
all_msg_len: .quad msg1_len-msg1

.text
.global main

main:

  mov $msg1, %rsi
  mov $1, %rdi
  mov $1, %rdx
  mov $1, %rax
  mov $0, %rbx
  mov all_msg_len, %r9
  call exm


exm:
  cmp %rbx, %r9
  je end
  addb $0x20, (%rsi)
  test $1, %rbx
  jnz skip
  syscall

skip:
  inc %rsi
  inc %rbx
  call exm

end:
  ret

I changed the 2 usages of register r9 with register rbp and my code worked fine. why is that?

Isn't rbp a pointer to the base of the stack's frame? I expected that changing its value would cause a catastrophic but it seems my expectations were wrong. why is that?

Upvotes: 0

Views: 423

Answers (1)

Sep Roland
Sep Roland

Reputation: 39166

Isn't rbp a pointer to the base of the stack's frame?

The %rbp register only becomes special once you start using it for function prologue/epilogue. Your exm doesn't use it that way and so the %rbp register is no different from %r9.

Upvotes: 1

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