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Assembly & Low-Level Programming — Overview

Overview​

Assembly language is a human-readable, (mostly) one-to-one text representation of machine code — the literal instructions an ISA defines. Learning to read it — not necessarily write large programs in it — is one of the highest-leverage skills for understanding what your compiled code actually does: how function calls work, why undefined behavior in C/C++ can do "anything," and what a debugger is actually stepping through.

Core Concepts​

TermMeaning
MnemonicThe human-readable name for an instruction (mov, add, jmp) that assembles to a specific opcode.
RegisterA named CPU storage location referenced directly in assembly (rax, rbx on x86-64; x0-x30 on ARM64).
Stack frameThe region of the call stack allocated for one function invocation's local variables and return address.
Calling conventionThe agreed-upon rules for how arguments are passed and results returned between caller and callee (which registers, what order, who cleans up the stack).
DisassemblyConverting compiled machine code back into assembly text for inspection — what tools like objdump and debuggers do.

From Source to Machine Code​

# See the assembly your compiler generates for a C function
gcc -S -O2 -masm=intel example.c -o example.s

# Disassemble an already-compiled binary
objdump -d --no-show-raw-insn a.out

In This Section​

  • x86-64 Registers and Instructions — the general-purpose registers, the historical rax/eax/ax/al sub-register naming, common instruction categories, and addressing modes.
  • Calling Conventions & the Stack — the System V AMD64 ABI's argument-register order, stack-frame anatomy, and a real gcc -S -O0 worked example.
  • Reading Disassembly — a hands-on walkthrough of compiling, disassembling with objdump, and recognizing loops, if/else, and function calls in real output.

Why It Matters​

  • Instruction Set Architecture: assembly is the concrete text syntax for the ISA concepts covered there.
  • Operating Systems: system calls are, at the lowest level, a specific assembly instruction (e.g., syscall on x86-64) that transfers control to the kernel.
  • ABI (C++): calling conventions are one piece of the broader binary-compatibility contract compiled C/C++ code relies on.
  • Debugging a crash often means reading a stack trace and a few lines of disassembly around the faulting instruction — this is where that skill applies directly.