This page contains very basic information on the x86_64 architecture: the register layout and naming and some basic instructions. For more comprehensive information, see the references listed below.

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 using a 'b' suffix (added registers: r__b): al, bl, r15b

Usage during syscall/function call:

• First six integer and pointer arguments are in rdi, rsi, rdx, rcx, r8d, r9d; remaining arguments are on the stack.
• For syscalls, the syscall number is in rax. For procedure calls, rax should be set to 0 before the procedure is called.
• Return value is in rax (if an integer or pointer).
• The called routine is expected to preserve rsp,rbp, rbx, r12, r13, r14, and r15 but may trample any other registers.

See the ABI documentation for reference.

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
 mov %r10,label   // move date from r10 to the address label
 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
 ret              // routine from subroutine (counterpart to call)
 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; you can think of this as r10 → r11).

For the MOV instruction:

• You can append a suffix indicating the amount of data to be moved to/from memory – e.g., q for quadword (64 bits) (the default), d for doubleword (32 bits), w for word (16 bits), or b for byte (8 bits).

• CPU Instruction Set and Software Developer Manuals
  • AMD: AMD Documentation hub - see the Programmer's Reference section, particularly the AMD64 Architecture Programmer’s Manual Volume 3: General Purpose and System Instructions
  • Intel: Intel® 64 and IA-32 Architectures Software Developer Manuals
• Web sites
  • http://ref.x86asm.net/
  • http://sandpile.org/
• GAS Manual - Using as, The GNU Assembler