Ohio Scientific Challenger 1P and Emulator

January 22, 2022

Photograph of the Challenger 1P and cassettes.

  • I've begun digitising and uploading OSI C1P software from my grandfather's old audio cassettes.
  • My C1P suffered a video fault - replacing U40 (one of the two video RAM chips) fixed it.

November 14, 2021

Two bits of unexpected news!

  • I've managed to buy a beautiful, unmodified 8K OSI C1P in an online aution. It's actually this one, formerly owned by collector Terry Stewart. It's here with me now, and works perfectly. Oh my gosh.
  • At the end of 2020, my uncle found twenty-ish audio cassettes filled with C1P software in his garage. (Actually, I noticed them in his trailer headed for the tip! The timing was pretty amazing — I don't visit all that often!)

April 21, 2020

So, I've been working on an emulator for the OSI C1P, UK101, etc.

In case I get popped by hedgehogs or fall into a vat of glue, here's a first release of the source code. It's written in plain C and requires SDL (well, not really, but if you want graphics…). Here's a YouTube video showing what sort of state it's in.

There's lots to improve, but the main annoyance is that writing/saving files isn't yet implemented.

Note: The data tarball contains three subdirectories: fun, machines, and software. If you copy those directly into the source code's main directory, osi65-1.0, things should "Just Work". The emulator is GPL'd, but the ROMs are of "uncertain" status — hence the separate tarballs.

July 2017

I've started working on an emulator for simple 6502 based machines like the O.S.I. and Compukit microcomputers. The emulator is written in C, and I'll release it under the GNU GPL version 3 once it's a little more complete. It already runs pretty well and is able to load O.S.I. Superboard II / C1P (and UK101) software, including the O.S.I. Invaders game that prompted me to start this page (see below). At the end of this page you'll find links to other O.S.I./Compukit emulation projects and related information.

Screenshot of the C1P's initial BASIC prompt Screenshot of O.S.I. Invaders Screenshot of Le Passe-Temps

September 2016

Clearing out the garage, I found a cassette tape from 1978 containing O.S.I. Invaders for the Ohio Scientific Inc. Challenger 1P. I spent hours playing this when I was a kid. Best Space Invaders clone ever! (Okay, not a clone exactly, considering the C1P's character-based graphics, but the play is amazingly good, and fun.)

Photograph of the audio original cassette from which the recording was made

I dug out a dusty tape deck, plugged it into my PC, et voilĂ ! Digitized! Now all I need is a C1P. (Actually, Google turns up some emulators (links below), but a non-proprietary one would be best… No, wait, an actual C1P would be best.)


Here are the notes my grandfather wrote on the paper liner:

Start recorder
Press L
  (Takes 12 minutes)
  Puts in machine language and then lines of BASIC
Self starting

  ?FRE(U) = 224

  LOADER       000 - 011
  MACHINE CODE 011 - 140
  BASIC        140 - 175

42910 by Tim, level 15, 15/12/81
28250 by Tim, level 5, 2/3/81
18830 by Tim, level 4, 30/11/81



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