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