Grid Lock

How to play Grid Lock

The board is three rows of three squares, with a small number in the corner of each empty square. The numbers run like a phone keypad: 1, 2 and 3 along the top, 4, 5 and 6 in the middle, 7, 8 and 9 along the bottom. Press a number to put your X in that square. If the square is already taken you'll hear a low buzz and can pick again.

The Spectrum then thinks for a moment and places its O. Get three in a row, across, down or diagonally, and the round is yours. If all nine squares fill up with no line, it's a draw. You and the Spectrum take turns to start each new round, so every other round it has the first move.

On a phone or tablet the on-screen Spectrum keyboard opens with the game, so you can tap the number keys. On a computer, use the number keys along the top of your keyboard.

Scoring

Your best score is shown on the title screen and in the side panel. In the Spectrum Arcade it's saved in your browser, so it's still there next time you play.

Tips

• Take the centre. The middle square, 5, is part of four lines, more than any other. When you start a round, it's the strongest first move.
• Corners next. Each corner sits on three lines. Edges only sit on two.
• Always check for two in a row. Before you press anything, look for a line where the Spectrum has two O's and the third square is empty. Block it, or the game ends.
• Set a double threat. The Spectrum can only block one line per move. Make two lines of two X's at once and one of them wins.
• Draws add up. Five points a round is slower than ten, but a careful draw keeps your run alive. Early on the Spectrum often forgets to block, so take the wins while they come.

How Grid Lock was made

Grid Lock was built as the simplest possible test of the Spectrum Arcade's pipeline, from Z80 code to the arcade, the repository and this page. It's written in Z80 assembly and needs no Sinclair ROM routines: it has its own font, its own keyboard reading and its own interrupt handler, so it starts instantly in the Speccy emulator.

The Spectrum's play is short and simple. It takes a winning square if it has one, blocks your line if it notices it, then prefers the centre, and otherwise picks any free square. At first it misses your line about half the time; after three wins, one time in four; after six, one time in eight. On a 128K the AY chip plays a gentle title tune, a calm theme during play and short jingles for a win, a draw and a loss. The 48K has beeper sound effects.

A scripted player tested every build through the real 48K and 128K loaders. It played twelve rounds without losing, then threw a round on purpose to check the game-over screen and the saved best score. New to the Spectrum? Our history of the ZX Spectrum is a good place to start.