X86 64 Register and Instruction Quick Start
This page contains very basic information on the x86_64 architecture: the register layout and naming and the some basic instructions.
The 64-bit versions of the 'original' x86 registers are named:
- rax - register a extended
- rbx - register b extended
- rcx - register c extended
- rdx - register d extended
- rbp - register base pointer (start of stack)
- rsp - register stack pointer (current location in stack, growing downwards)
- rsi - register source index (source for data copies)
- rdi - register destination index (destination for data copies)
The registers added for 64-bit mode are named:
- r8 - register 8
- r9 - register 9
- r10 - register 10
- r11 - register 11
- r12 - register 12
- r13 - register 13
- r14 - register 14
- r15 - register 15
These may be accessed as:
- 64-bit registers using the 'r' prefix: rax, r15
- 32-bit registers using the 'e' prefix (original registers: e_x) or 'd' suffix (added registers: r__d): eax, r15d
- 16-bit registers using no prefix (original registers: _x) or a 'w' suffix (added registers: r__w): ax, r15w
- 8-bit registers using 'h' ("high byte" of 16 bits) suffix (original registers - bits 8-15: _h): ah, bh
- 8-bit registers using 'l' ("low byte" of 16 bits) suffix (original registers - bits 0-7: _l) or 'b' suffix (added registers: r__b): al, bl, r15b
Usage during syscall/function call:
- First six arguments are in rdi, rsi, rdx, rcx, r8d, r9d; remaining arguments are on the stack.
- For syscalls, the syscall number is in rax.
- Return value is in rax.
- The called routine is expected to preserve rsp,rbp, rbx, r12, r13, r14, and r15 but may trample any other registers.
Floating-Point and SIMD Registers
x86_64 also defines a set of large registers for floating-point and single-instruction/multiple-data (SIMD) operations. For details, refer to the Intel or AMD documentation.
These instructions are sufficient to complete the SPO600 Assembler Lab (GAS syntax):
add %r10,%r11 // add r10 and r11, put result in r11 add $5,%r10 // add 5 to r10, put result in r10 call label // call a subroutine / function / procedure cmp %r10,%r11 // compare register r10 with register r11. The comparison sets flags in the processor status register which affect conditional jumps. cmp $99,%r11 // compare the number 99 with register r11. The comparison sets flags in the processor status register which affect conditional jumps. div %r10 // divide rax by the given register (r10), places quotient into rax and remainder into rdx (rdx must be zero before this instruction) inc %r10 // increment r10 jmp label // jump to label je label // jump to label if equal jne label // jump to label if not equal jl label // jump to label if less jg label // jump to label if greater mov %r10,%r11 // move data from r10 to r11 mov $99,%r10 // put the immediate value 99 into r10 mov %r10,(%r11) // move data from r10 to address pointed to by r11 mov (%r10),%r11 // move data from address pointed to by r10 to r10 mul %r10 // multiplies rax by r10, places result in rax and overflow in rdx push %r10 // push r10 onto the stack pop %r10 // pop r10 off the stack syscall // invoke a syscall (in 32-bit mode, use "int $0x80" instead)
Note the syntax:
- Register names are prefixed by %
- Immediate values are prefixed by $
- Indirect memory access is indicated by (parenthesis).
- Hexadecimal values are indicated by a 0x prefix.
- Character values are indicated by quotation marks. Escapes (such as '\n') are permitted.
- Data sources are given as the first argument (mov %r10,%r11 moves FROM r10 INTO r11).
For the MOV instruction:
- You can append a suffix indicating the amount of data to be moved -- e.g., q for quadword (64 bits), d for doubleword (32 bits), w for word (16 bits), or b for byte (8 bits).
- CPU Instruction Set and Software Developer Manuals
- AMD: https://developer.amd.com/resources/developer-guides-manuals/ (see the AMD64 Architecture section, particularly the AMD64 Architecture Programmer’s Manual Volume 3: General Purpose and System Instructions)
- Intel: http://www.intel.com/content/www/us/en/processors/architectures-software-developer-manuals.html
- Web sites
- GAS Manual - Using as, The GNU Assembler: https://sourceware.org/binutils/docs/as/