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README.md

TL;DR

  • What does 0x8(%rbp) mean in movl $0x1, -0x8(%rbp)?

Video Overview

  • Go over C++ code
  • What is the parenthesis operation do?
  • %rbp represents the current function's stack frame
  • Indirect memory addressing in assembly

Helpful LLDB Commands

(lldb) p/x $rbp
# (unsigned long) 0x00007ff7bfefec40

(lldb) x/x $rbp-0x8
# 0x7ff7bfefec38: 0x00000001

(lldb) x/4xd $rbp-0x14
# 0x7ff7bfefec2c: 4
# 0x7ff7bfefec30: 3
# 0x7ff7bfefec34: 2
# 0x7ff7bfefec38: 1

Sample Assembly Code

main`main:
    0x100000f60 <+0>:  pushq  %rbp
    0x100000f61 <+1>:  movq   %rsp, %rbp
    0x100000f64 <+4>:  subq   $0x20, %rsp
    0x100000f68 <+8>:  movl   $0x0, -0x4(%rbp)
->  0x100000f6f <+15>: movl   $0x1, -0x8(%rbp)
    0x100000f76 <+22>: movl   $0x2, -0xc(%rbp)
    0x100000f7d <+29>: movl   $0x3, -0x10(%rbp)
    0x100000f84 <+36>: movl   $0x4, -0x14(%rbp)
    0x100000f8b <+43>: movl   -0x8(%rbp), %edi
    0x100000f8e <+46>: movl   -0xc(%rbp), %esi
    0x100000f91 <+49>: movl   -0x10(%rbp), %edx
    0x100000f94 <+52>: movl   -0x14(%rbp), %ecx
    0x100000f97 <+55>: callq  0x100000f20    ; sum_4 at main.cpp:3
    0x100000f9c <+60>: movl   %eax, -0x18(%rbp)
    0x100000f9f <+63>: xorl   %eax, %eax
    0x100000fa1 <+65>: addq   $0x20, %rsp
    0x100000fa5 <+69>: popq   %rbp
    0x100000fa6 <+70>: retq

Titles

  • What is Indirect Memory Addressing? | C++ Tutorial

  • x86 Assembly Memory Offsets Explained | C++ Tutorial

  • How does C++ store variable data on the stack frame?

  • C++ Deep Dive: Indirect Memory Addressing Explained

  • C++ Deep Dive: Indirect Memory Addressing from Stack Base Pointer

  • How to Read x86 Assembly Memory Offsets? | C++ Tutorial for Beginners

  • Assembly Tutorial: What Does -0x8(%rbp) Mean?

Description

In this C++ tutorial, we go over the parenthesis operation in assembly, which is known as indirect memory addressing. We also learn about the RBP stack frame and access its values using various LLDB debugger commands.

References

  • ChatGPT and Gemini