Assembly registers in 64-bit architecture

Following the answer about assembly registers' sizes:

  • First, what sizes are eax, ax, ah and their counterparts, in the 64-bit architecture? How to access a single register's byte and how to access all the 64-bit register's eight bytes?

    I'd love attention for both x86-64 (x64) and Itanium processors.

  • Second, what is the correct way to use the four registers for holding the first four parameters in function calls in the new calling convention?


  • With the old names all registers remain the same size, just like when x86-16 was extended to x86-32. To access 64-bit integer registers you use the new names with R-prefix such as rax, rbx...

    Register names don't change so you just use the byte registers (al, bl, cl, dl, ah, bh, ch, dh) for the LSB and MSB of ax, bx, cx, dx like before. (How do AX, AH, AL map onto EAX?)

    There are also 8 new registers called r8-r15. You can access their LSBs by adding the suffix b (or l if you're using AMD). For example r8b, r9b, r10l, r11l... You can also use the LSB of esi, edi, esp, ebp by the names sil, dil, spl, bpl with the new REX prefix, but you cannot use it at the same time with ah, bh, ch or dh.

    Likewise the new registers' lowest word or double word can be accessed through the suffix w or d. Writing a 32-bit register zero-extends into the full 64-bit register, unlike writing low-8, high-8, or low-16 partial registers where the 8086 / 386 semantics still apply.

    Update: Intel has just introduced a new extension for x86-64 called APX which adds 16 more registers named r16-r31

    So the list of general-purpose registers is like this:

    64-bit register Lower 32 bits Lower 16 bits Lower 8 bits
    rax eax ax al
    rbx ebx bx bl
    rcx ecx cx cl
    rdx edx dx dl
    rsi esi si sil
    rdi edi di dil
    rbp ebp bp bpl
    rsp esp sp spl
    r8 r8d r8w r8b (r8l)
    r9 r9d r9w r9b (r9l)
    r10 r10d r10w r10b (r10l)
    r11 r11d r11w r11b (r11l)
    r12 r12d r12w r12b (r12l)
    r13 r13d r13w r13b (r13l)
    r14 r14d r14w r14b (r14l)
    r15 r15d r15w r15b (r15l)
    r16 (with APX) r16d r16w r16b (r16l)
    r17 (with APX) r17d r17w r17b (r17l)
    ... ... ... ...
    r31 (with APX) r31d r31w r31b (r31l)

    Of course there are also other types of registers like control, debug, flag, floating-point, vector, segment, test... registers. For more details check See also What are the names of the new X86_64 processors registers?

    Calling conventions

    Regarding the calling convention, on each specific system there's only one convention1. Follow the links for details on which integer and vector registers are call-clobbered vs. call-preserved, and additional details like 16-byte alignment of RSP before a call, and quirks for variadic functions.

    • On Windows:

      • RCX, RDX, R8, R9 for the first four arguments if they're integer or pointer
      • XMM0, XMM1, XMM2, XMM3 for floating-point arguments
      • The caller reserves 32 bytes of "shadow space" above the return address. (If there are any stack args, they go above the shadow space.)

      1Since MSVC 2013 there's also a new extended convention on Windows called __vectorcall so the "single convention policy" is not true anymore.

    • On Linux and other systems that follow the System V AMD64 ABI, more arguments can be passed in registers and there's a 128-byte red zone below the stack which may make leaf functions faster.

      • The first six integer or pointer arguments are passed in registers RDI, RSI, RDX, RCX, R8, and R9
      • Floating-point arguments are passed in XMM0 through XMM7

    For more information should read x86-64 and x86-64 calling conventions

    There's also a convention used in Plan 9 where

    • All registers are caller-saved
    • All parameters are passed on the stack
    • Return values are also returned on the stack, in space reserved below (stack-wise; higher addresses on amd64) the arguments.

    Golang follows the Plan 9 calling convention, but since go 1.17+ they're gradually introducing a register-based calling convention for better performance. The calling convention can change in the future, and the compiler can generate stubs to automatically call assembly functions in older conventions. At the moment the ABI specifies that

    • 9 general-purpose registers will be used to pass integer arguments: RAX, RBX, RCX, RDI, RSI, R8, R9, R10, R11
    • 15 registers XMM0-XMM14 are used for floating-point arguments

    In fact Plan 9 was always a weirdo. For example it forces a register to be 0 on RISC architectures without a hardware zero register. x86 register names on it are also consistent across 16, 32 and 64-bit x86 architectures with operand size indicated by mnemonic suffix. That means ax can be a 16, 32 or 64-bit register depending on the instruction suffix. If you're curious about it read

    OTOH Itanium is a completely different architecture and has no relation to x86-64 whatsoever. It's a pure 64-bit architecture so all normal registers are 64-bit, no 32-bit or smaller version is available. There are a lot of registers in it:

    • 128 general-purpose integer registers r0 through r127, each carrying 64 value bits and a trap bit. We'll learn more about the trap bit later.
    • 128 floating point registers f0 through f127.
    • 64 predicate registers p0 through p63.
    • 8 branch registers b0 through b7.
    • An instruction pointer, which the Windows debugging engine for some reason calls iip. (The extra "i" is for "insane"?)
    • 128 special-purpose registers, not all of which have been given meanings. These are called "application registers" (ar) for some reason. I will cover selected register as they arise during the discussion.
    • Other miscellaneous registers we will not cover in this series.

    The Itanium processor, part 1: Warming up

    Read more on What is the difference between x64 and IA-64?