ASM

---

The 5 things assembly is made of

1. Mnemonic — the verb: mov, add, cmp, jmp, call, ret
2. Operands — what it acts on: registers (rax), immediates (42), memory ([rbp+8])
3. Registers — the CPU’s built-in variables
4. Labels — names for addresses: loop:
5. Directives — assembler commands: .text, .data, global main

---

Learn these 12 mnemonics first (ignore the rest)

• Data move: mov, lea
• Math/logic: add, sub, and, or, xor
• Compare/branch: cmp, jmp, je, jne
• Calls/returns: call, ret

With those you can read/write most small functions.

---

The registers you actually need at start

General-purpose (64/32/16/8-bit views)

• RAX (return value) → EAX/AX/AL
• RBX → EBX/BX/BL
• RCX → ECX/CX/CL
• RDX → EDX/DX/DL
• RSI → ESI/SI/SIL
• RDI → EDI/DI/DIL
• RBP (frame/base) → EBP/BP/BPL
• RSP (stack ptr) → ESP/SP/SPL
• R8–R15 → R8D–R15D → R8W–R15W → R8B–R15B

Tip: avoid the legacy high bytes AH/BH/CH/DH in 64-bit code.

Flags you’ll see from cmp/ALU

• ZF (zero) — result was 0 → used by je/jne
• SF (sign), CF (carry), OF (overflow) — used by signed/unsigned j*

---

Two calling conventions (pick one target)

Windows x64 (MS ABI)

• Args: RCX, RDX, R8, R9 (then stack)
• Return: RAX
• Caller-saved: RAX, RCX, RDX, R8–R11
• Callee-saved: RBX, RBP, RSI, RDI, R12–R15
• Stack rule: before each call, provide 32 bytes shadow space and keep stack 16-byte aligned

System V (Linux/macOS)

• Args: RDI, RSI, RDX, RCX, R8, R9
• Return: RAX
• Caller-saved: RAX, RCX, RDX, RSI, RDI, R8–R11
• Callee-saved: RBX, RBP, R12–R15
• Stack rule: keep stack 16-byte aligned at call

---

Memory operand pattern (Intel)