README.md

hello-world-64

The real programmers can write hello world in 64 bytes of machine code!

This repository contains code in assembly language targeting Windows x86_64.

Here is a list of things that are used to achieve this size:

• undocumented PEB structure - https://en.wikipedia.org/wiki/Process_Environment_Block
• syscall ABI on Windows
• shadow space in stacks
• a lot of hacks to reduce code size
  • push & pop compiles to 3 bytes of machine code (val < 128)

    ; reg = val
    push val ; 2 bytes
    pop reg ; 1 byte

  • hack that allows to push string address to the stack and pop it back

    ; reg = address of data, i.e. reg points to "my data goes here"
    call data_label
    data: db 'my data goes here'
    data_label: pop reg

  • xor of 32-bit registers allows us to zero out a 64-bit register in 2 bytes of code

    xor edx, edx ; 31 d2
    xor rdx, rdx ; 48 31 d2
    ; but it's same

  • if you want to set rdx to 0 you can use cdq to do it in 1 byte (bit 63 of rax should be 0)

    push 0x60 ; 6a 60
    pop rax ; 58 (rax = 0x60, bit 63 of rax = 0)
    cdq ; rdx = 0

• see main.asm for better explanation

Building

You need to install NASM and unpack it into this directory or add to env variable PATH