NULLBYTE

Read the code. Find the one byte. Take over the machine.

NULLBYTE is a puzzle about reading someone else's program and the single-byte mistake that lets you bend it to your will. Everything happens on one screen you can hold in your head: a tiny machine of 256 cells, four registers and a stack, with the whole of its memory laid out in front of you. You don't touch real computers — you learn how the machine actually thinks, and then make it do what was never written in its code.

What makes it different

• The whole machine fits on one screen. 256 bytes, and you see every one of them — the code, the stack, your input, all live.
  
• Nothing is told, everything is shown. The listing is a disassembler of memory as it runs. Rewrite a byte and watch the instruction change; bytes that came from your input are lit up so you can follow them.
  
• You can't solve a level with "the right answer." There is no answer key. The only thing that counts is what the machine actually did when it ran.
  
• No textbook, no wall of theory. A card appears the first time a thing shows up on screen and files itself into your Journal — you learn exactly when you need it.
  
• From first principles to real technique. 43 puzzles across six blocks: from "what is an address" to overwriting a return address to building a chain out of borrowed pieces of code.

Its own little machine

Not real x86 — a machine small enough to understand completely. Thirteen instructions, 256 cells, a stack that grows down, byte-level permissions, and a return address that is one visible byte. You learn the ideas behind memory safety and exploitation, not a pile of opcodes.

Features

• 43 hand-made levels in six blocks, each one provably solvable
  
• Leaderboards: the shortest payload wins — come back and shave off a byte
  
• A step-by-step machine you can run, rewind, and poke at
  
• Three-step hints when you want them, and achievements for doing without
  
• English and Russian, including the listings and the comments