https://github.com/stackoverflowexcept1on/hello-world-64

The real programmers can write hello world in 64 bytes of machine code!
https://github.com/stackoverflowexcept1on/hello-world-64

assembly hello-world low-level syscalls windows x86-64

Last synced: about 1 month ago
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The real programmers can write hello world in 64 bytes of machine code!

• Host: GitHub
• URL: https://github.com/stackoverflowexcept1on/hello-world-64
• Owner: StackOverflowExcept1on
• Created: 2022-09-16T18:51:23.000Z (over 2 years ago)
• Default Branch: master
• Last Pushed: 2023-02-27T23:22:53.000Z (almost 2 years ago)
• Last Synced: 2024-11-12T13:39:44.124Z (3 months ago)
• Topics: assembly, hello-world, low-level, syscalls, windows, x86-64
• Language: Assembly
• Homepage:
• Size: 12.7 KB
• Stars: 0
• Watchers: 2
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md

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README

        
# hello-world-64

[![Build Status](https://github.com/StackOverflowExcept1on/hello-world-64/actions/workflows/build.yml/badge.svg)](https://github.com/StackOverflowExcept1on/hello-world-64/actions/workflows/build.yml)

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

![image](https://i.imgur.com/DqUNBSV.png)

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)

      ```asm

      ; reg = val

      push val ; 2 bytes

      pop reg ; 1 byte

      ```

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

      ```asm

      ; 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

      ```asm

      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)

      ```asm

      push 0x60 ; 6a 60

      pop rax ; 58 (rax = 0x60, bit 63 of rax = 0)

      cdq ; rdx = 0

      ```

- see [main.asm](main.asm) for better explanation

### Building

You need to install [NASM](https://nasm.us) and unpack it into this directory or add to env variable PATH