Short Note for Registers

Open Table Of Contents

Intro
General Purpose Registers (64-bit)
A Mini Example
Other versions
Benefits!
Outro

Intro

There are a lot of resources about registers so I assume you might already know them. This note is not a deep dive but a recap of how registers actually work.

Think of registers as super-fast little drawers inside the CPU. You use them to hold data temporarily while your program is running. They’re faster than RAM or disk. If you’re doing math or moving stuff around, the CPU wants it in a register.

Ready to take a peek into the registers on the x86_64 (Intel) architecture? :)

General Purpose Registers (64-bit)

Name	What It’s Commonly Used For
rax	Return value from functions, math operations
rbx	Base pointer (can be general-purpose too)
rcx	Loop counter, string ops
rdx	Function arguments, I/O ops
rsi	Source index for string/memory ops
rdi	Destination index for string/memory ops
rsp	Stack pointer (points to top of the stack)
rbp	Base pointer (points to base of the stack)
r8 - r15	Extra general-purpose registers (yay, 64-bit!)

You might not want to use rsp and rbp registers because of their functions. (They control how stack works). But you can perform arithmetic operations on them directly, which make them general purpose.

A Mini Example

section .text
    global _start

_start:
    mov rax, 5        ; Put 5 into rax
    mov rbx, 10       ; Put 10 into rbx
    add rax, rbx      ; rax = rax + rbx → 5 + 10 = 15

After running this, rax holds 15, just like a calculator using registers! Isn’t it awesome?

Other versions

rax, rbx, etc. are 64-bit. Want smaller versions? You can use:

eax → for 32-bit version of rax
ax → for 16-bit version of rax
al →for 8-bit version of rax

Same goes for rbx → ebx, bx, bl, etc.

mov al, 0xFF        ; 8-bit
mov ax, 0xFFFF      ; 16-bit
mov eax, 0xDEADBEEF ; 32-bit

My dumb way of noting is, a single ‘F’ in ‘0xFF’ is 4 bit long.
So ‘F + F = 4 + 4 = 8 (0xFF is 8 bit)’.

Benefits!

What You’ll Learn from playing with registers:

How the CPU processes and transfers data
The power of using the right register at the right time (Very important!)
Why “low-level” = high control

Outro

Registers are your best friends when speaking assembly. They can give you the ability to control the CPU like a wizard with a wand. They’re fast, flexible, and essential.
Next time if you’re debugging or writing a shellcode, you’ll see these register names everywhere.
Happy Coding!