The Anitra Computer

A Complete Minimalist Computer System Designed, Built and Programmed at a Low Level of Abstraction
A hobby electronics project by Eirik Bakke (2004).

Read the whole paper here.

Abstract

A classic exercise in computer science courses is programming hypothetical minimalist computers such as the Move Machine or the One Instruction Set Computer (OISC). The goal of this project was to design, build and program a computer/central processing unit (CPU) from scratch using standard 74TTL-series compatible digital logic circuits, while investigating the minimum component usage needed to maintain a certain minimum functionality.

I first defined design requirements and laid out possible CPU architectures. I created a minimized datapath unit, then specified an instruction execution sequence, and designed an associated control unit. The resulting computer, called Anitra, supports 32 kilobytes of memory and two universal instructions: "move and complement" and "add, complement and branch if carry". For testing, I designed a development board and wrote a cross-assembler and an interactive debugger/emulator, before simulating the circuits on CAD software and finally building a working hardware prototype.

Given my requirements, I have shown it possible to construct a computer that comes close to a provable lower component limit, and my investigations suggests that a simpler datapath portion of the CPU is unlikely to exist. The study of minimalist computers casts light upon issues in architecture design that may otherwise go unnoticed; one such corollary is a better explanation of why the concept of a program counter is always present even in the simplest of practical computers; another is the observation that even the OISC computer can be significantly simplified and even still in hardware terms provide another instruction for free.

Watch a demonstration video of the Anitra computer (4.8Mb, Windows Media Video, 2:16 min).
View project poster 1 (Norwegian Contest for Young Scientists) (164Kb, PDF file).
View project poster 2 (European Union Contest for Young Scientists) (1013Kb, PDF file).
View project poster 3 (Intel International Science & Engineering Fair) (917Kb, PDF file).

Links

This project was one of two winning projects in the Norwegian Contest for Young Scientists in Oslo, Norway in April 2004. It participated in the EU Contest for Young Scientists in Dublin, Ireland in September 2004 (no prizes this time!), and received a Third Award in the Intel International Science & Engineering Fair (ISEF) in Phoenix, Arizona from May 8-14, 2005.

Konkurransen Unge Forskere (Norwegian page)
European Union Contest for Young Scientists, Dublin 2004
Intel International Science & Engineering Fair, Phoenix AZ 2005
The Retrocomputing Museum, a directory of other strange computers and programming languages
Homebrew CPU, a similar project on a much more grand scale: it runs Minix 2 (as per November 4, 2007)!