Invaders from Odyssey Computing is a high resoution - yes, high resolution on your ZX81 - version of the popular arcade game. Unbelievable? Well, it's true, and no expensive hardware either, just the standard computer with 16K RAM. None of the usual ZX81 characters to represent the alien fleet; in this game, they are the genuine article. Action is fast and smooth too, and the explosion when your ship is hit really is shattering!

After loading - which takes about five minutes - the program runs automatically. As well as the hi-res display, the facilities offered are impressive. The game required is first set up: a one or two player game can be selected (two players can either take turns or play simultaneously); the keyboard mode is selected; and the missile firing rate are chosen. Selecting a high missile firing rate and a low bomb dropping rate gives a relatively easy game in which even the beginner can achieve a high score; increasing the missile firing rate makes the game more difficult for the experienced player.

The game is robust but will crash if, for example, invalid entries are made in the setting up sequence. A small price to pay for an excellent game.

I am afraid your existing library is arcade games for the ZX81 is likely to lose much of this one. No sound or colour, of course, but the hi-res display really does break new ground. An outstanding achievement.

The use of high resolution graphics will almost certainly open up a whole new generation of ZX81 software as other software houses follow suit.

An excellent game.

Invaders costs £4.95, from Odyssey Computing, 28 Bingham Road, Sherwood, Notts NG5 2EP.