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registers

x86-64 Registers You Can Use in NASM (User-mode)​

General-Purpose (GPRs)​

64-bit → 32-bit → 16-bit → 8-bit

  • RAX → EAX → AX → AH/AL (avoid AH in 64-bit; see note)
  • RBX → EBX → BX → BH/BL (avoid BH in 64-bit)
  • RCX → ECX → CX → CH/CL (avoid CH in 64-bit)
  • RDX → EDX → DX → DH/DL (avoid DH in 64-bit)
  • RSI → ESI → SI → SIL
  • RDI → EDI → DI → DIL
  • RBP → EBP → BP → BPL
  • RSP → ESP → SP → SPL
  • R8–R15 → R8D–R15D → R8W–R15W → R8B–R15B

AH/BH/CH/DH caveat: in x86-64, whenever an instruction uses a REX prefix (most 64-bit forms), the high-byte regs AH/BH/CH/DH can’t be used. Prefer AL/BL/CL/DL or R8B–R15B.

Instruction Pointer & Flags​

  • RIP — 64-bit instruction pointer (read via RIP-relative addressing, not directly writable).
  • RFLAGS/EFLAGS — status/controls (common flags: CF, PF, AF, ZF, SF, OF, DF, IF).

Segment Registers (rare in user-mode)​

  • CS, DS, ES, SS, FS, GS
    • In 64-bit user code, FS/GS are the only ones you might touch indirectly (e.g., TLS via fs:/gs:). Others are effectively fixed.

SIMD / Vector (baseline x86-64 has SSE2)​

  • XMM0–XMM15 — 128-bit (SSE/SSE2/…).
  • YMM0–YMM15 — 256-bit (AVX/AVX2).
  • ZMM0–ZMM31 & K0–K7 — 512-bit + masks (AVX-512). (Only if the CPU/OS enables them.)
  • MXCSR — SSE control/status (use ldmxcsr/stmxcsr).

Legacy (usually avoid)​

  • x87 FPU: ST(0)–ST(7) (use SSE2+ instead).
  • MMX: MM0–MM7 (obsolete with SSE2).
  • Control/Debug: CR0–CR8, DR0–DR7 (privileged; not for user-mode).

Call-preserved vs Call-clobbered (handy when mixing C/asm)​

Windows x64 (MS ABI)​

  • Args: RCX, RDX, R8, R9 (then stack).
  • Caller-saved (clobbered): RAX, RCX, RDX, R8–R11, XMM0–XMM5.
  • Callee-saved (preserved): RBX, RBP, RSI, RDI, R12–R15, XMM6–XMM15.
  • Stack: 32-byte shadow space; 16-byte alignment at each call.

System V AMD64 (Linux/macOS)​

  • Args: RDI, RSI, RDX, RCX, R8, R9 (then stack); FP args in XMM0–XMM7.
  • Caller-saved: RAX, RCX, RDX, RSI, RDI, R8–R11, XMM0–XMM15.
  • Callee-saved: RBX, RBP, R12–R15; (RSP always preserved).
  • Stack: keep 16-byte alignment at call sites.

NASM notes​

  • Register name ⇒ size (no suffixes needed): mov eax, 1 (32-bit), mov rax, 1 (64-bit).
  • Memory sizes use keywords: byte, word, dword, qword (e.g., mov byte [rdi], al).