x86-64 Architecture Guide — CPU Modes, Registers, and Instruction Set

In this tutorial, you will learn about x86. We cover key concepts, practical examples, and best practices to help you master this topic.

x86-64 (also called x64 or AMD64) is the 64-bit extension of the x86 architecture -- supporting 64-bit registers, 64-bit addressing, and backward compatibility with 32-bit and 16-bit code through multiple operation modes.

CPU Modes

| Mode         | Bits  | Addressing  | OS           |
|--------------|-------|-------------|--------------|
| Real Mode    | 16    | 1 MB        | DOS, BIOS    |
| Protected    | 32    | 4 GB        | Linux, Win   |
| Long Mode    | 64    | Huge        | 64-bit OS    |
| Compatibility| 64    | 4 GB        | 64-bit OS    |

General Purpose Registers

64-bit   32-bit   16-bit   8-bit    Purpose
─────────────────────────────────────────────
RAX      EAX      AX       AL      Accumulator
RBX      EBX      BX       BL      Base
RCX      ECX      CX       CL      Counter
RDX      EDX      DX       DL      Data
RSI      ESI      SI       SIL     Source Index
RDI      EDI      DI       DIL     Destination
RBP      EBP      BP       BPL     Base Pointer
RSP      ESP      SP       SPL     Stack Pointer
R8-R15   R8d-R15d R8w-R15w R8b-R15b Extended

Special Registers

RIP - Instruction Pointer (points to next instruction)
RFLAGS - Status flags (carry, zero, sign, overflow)

Instruction Set Highlights

; Data movement
mov rax, rbx       ; copy register
mov rax, [mem]     ; load from memory
mov [mem], rax     ; store to memory

; Arithmetic
add rax, rbx       ; rax = rax + rbx
sub rax, rbx       ; rax = rax - rbx
imul rax, rbx      ; signed multiply
idiv rbx           ; signed divide

; Stack
push rax           ; push onto stack
pop rax            ; pop from stack

; Control flow
jmp label          ; unconditional jump
je label           ; jump if equal
call label         ; call function
ret                ; return from call

Common Mistakes

1. Using 32-bit registers for addresses

mov eax, [address] zero-extends to 64 bits. Use mov rax, [address] for full 64-bit addresses.

2. Forgetting REX prefixes

Instructions like mov r8, rax need a REX prefix. Most assemblers handle this automatically.

3. Calling conventions

Arguments go in RDI, RSI, RDX, RCX, R8, R9 on Linux (System V). On Windows, it's RCX, RDX, R8, R9.

FAQ

{{< faq question="Why is x86 so complex?" >}} x86 evolved over 40+ years while maintaining backward compatibility. Each generation added features (MMX, SSE, AVX) and widened registers (16->32->64 bit). {{< /faq >}}

{{< faq question="What is long mode?" >}} 64-bit mode where the CPU uses 64-bit registers and addressing. Only available on 64-bit operating systems with x86-64 processors. {{< /faq >}}

{{< faq question="What is the difference between RSP and RBP?" >}} RSP is the stack pointer (top of stack). RBP is the base pointer (frame reference). RBP helps debuggers generate stack traces. {{< /faq >}}

What's Next

Now learn about CPU registers in detail.