With the exception of the infamous 1K Chess, written by David Horne back in 1983, there are very few memorable releases for the unexpanded Sinclair ZX81, and indeed the technology was nowhere near sophisticated enough to compete with the Atari 400 and 2600, Commodore VIC-20 or even a Commodore PET on the games front without at least a little more RAM. Fortunately, Sinclair's cost-cutting allowed this sleek miniature 8-bit micro to be bundled with a 16K RAM pack, giving it a much needed memory boost and a whole load of software titles to boot, from Sinclair's preferred educational productions through to many dubious and some very good games.

With the aforementioned Chess, most people's knowledge of the ZX81's software would end at 3D Monster Maze, created by New Generation Software, but now Cronosoft is set to change that with three new cassette-based releases, two of which (Lazy Frog and Shapestack) come on one tape and both run on the unexpanded machine.

Lazy Frog - thankfully - has nothing to do with an annoying ringtone for your mobile telephone (I'm sorry if that brought back painful memories) but is instead a basic and cut-down Frogger clone. It's played in a 16 x 11 block of the screen and, as you might expect, uses some of the machine's predefined graphics and characters. The frog is represented by an X, and the task is, in case you were wondering, to guide him to his breeding grounds across a busy road and a presumably polluted river. That latter assumption is based on the implausible story from the original Frogger being that the frog can't swim and must traverse the fast-moving river by hopping across on the passing felled logs. There are then four safe areas for our amphibian hero to copulate. It's simply a case of helping the local frog population as much as possible; one point is scored for each successful journey, and then you're placed back at the bottom of the screen.

There's no time limit to play to, but you'll need to move quickly across the play area. As you would expect, anything like a title screen or instructions are missing too.

Despite its obvious limitations, there's canny gameplay hidden there, requiring quick reactions and impeccable timing. The biggest problem is, of course, the keyboard controls, but at least you won't have to worry about the RAM pack wobble.

Shapestack is a competent implementation of Tetris, with no shape deviations from the original.

In Shapestack, you can rotate each shape (other than the square) anti-clockwise before it's placed on your stack, and there's a 'down' key that will shuttle the piece to the bottom of the play area (or adding it to the stack). My only real complaint is with the key configuration here: 'S' is for rotate and 'D' is for drop, which can easily be confused during play.

For each completed 'line' (that is a whole row formed without a break), a single point is accrued and the row is removed from play, bringing downwards each of the shapes avoid, and although the pace doesn't pick up, it's already adequate for most seasoned Tetris players as well as beginners.

Overall, these games are ultimately limited by the computer hardware on which they run, but there's still enough there for anything from a casual play through to a couple of hours of entertainment. And if the mood takes you, maybe more.

This represents a good, solid release by the Cronosoft label.

An unlikely duo of well-written and surprisingly playable mini-games making a good real-media release on Sir Clive's mighty monochrome beast.

Product: Lazy Frog & Shapestack
Price: £TBA + postage
Developer: Cronosoft
Publisher: Cronosoft - www.cronosoft.co.uk
Platform: 1K Sinclair ZX81 and compatibles