_{RETROCOMPUTING} Gaming on the EDSAC and PDP- 1 _{By Tom Owad}

From the earliest computers came the first computer games.

Electronic Delay Storage

Automatic Calculator (EDSAC).

DEC Programmed Data Processor- 1.

There’s something in the names of these pioneering computers that suggests they may not be the greatest gaming platforms. Nonetheless, in between searching for prime numbers and analyzing elliptic curves, A.S. Douglas found time to write what may have been the first computer game. OXO was a tic-tac-toe game Douglas wrote as a demonstration for his doctoral thesis in 1952. The board was displayed on a

35× 16 pixel CRT and allowed players to match wits with one of the earliest digital computers. Today, if you don’t have the EDSAC in your garage, you can download Edsac Simulator ( www.dcs.warwick.ac. uk/~edsac), which includes OXO.

The next well-known digital computer game came in 1962, when MIT’s Stephen “Slug” Russell wrote Spacewar for the PDP- 1. Spacewar was a two-player game in which the objective was to shoot the opposing spaceship and avoid being captured by the gravitational pull of a nearby star. User input was via switches on the front panel. Missiles and fuel were limited. A hyperspace mechanism allowed players to jump out of the paths of incoming missiles, but on the third hyperspace jump, your ship exploded.

Spacewar quickly became very popular and was eventually ported to countless mini and microcom-puters. While the only remaining operational PDP- 1 is at the Computer History Museum, it’s still possible to capture the feel of the original by running on a similar 1970s minicomputer. Fred White has done just that with his Digital Computer Controls (DCC) D- 116 and an old oscilloscope, which he exhibited at the Vintage Computer Festival East in 2004 in Burlington, Mass.

Oscilloscopes were commonly used as minicomputer displays, using a digital-to-analog converter card. White didn’t have one, so he built his own using a Burr-Brown DAC7624. The board connects to the oscilloscope using three wires: X, Y, and intensity. Spacewar loads the X and Y registers with

The Edsac Simulator lets you recreate what may be the first computer game: OXO, a tic-tac-toe game written by a Cambridge University grad student.

the desired position of the spot and then pulses the intensity line to make it light up. This can be performed a few tens of thousand times per second — enough to display several hundred pixels without flicker. If you’d like to read more about White’s

D- 116 projects, visit d116.com.

In 1977, Cinematronics released an arcade version of Spacewar, faithful to the original. The hardware for Cinematronics’ arcade system is emulated in the Multiple Arcade Machine Emulator (MAME), and with the proper ROM it’s possible to play this early version of Spacewar. MAME is capable of creating a very authentic game-playing experience, and many enthusiasts have built custom controllers and arcade cabinets to house their MAME PCs. Original Spacewar arcade machines may be impossible to find, but with some craftsmanship and MAME, it’s possible to create a reproduction with all the tenor of the original.

More information on Cinematronics’ version of Spacewar can be found at mameworld.net/maws/ romset/spacewar.

Source code to the original 1963 version of Spacewar can be found at applefritter.com/spacewar.

See photos of Fred White’s D- 116 at d116.com.

Tom Owad ( owad@applefritter.com) is a Macintosh consultant in York, Pa., and the editor of applefritter.com. He is the author of Apple I Replica Creation (Syngress, 2005).

178 Make: Volume 08