3D Monster Maze - Technology

Technology

The graphical view, animated in real time at around 6 frames per second, is composed of 8×8 pixel black-and-white characters, so the view is roughly square, taking a 25×24 area on the 32×24 text screen. Sub-character resolution of the ZX81 pseudo-graphics character set makes the resolution doubled in each dimension (making the view consist of 50×48 "larger pixels"). Using the 6 pseudo-graphics with a dithering pattern also made it possible for the game author to incorporate a third colour (grey) in the black-and-white picture. Part of the screen is reserved for the score count, and a one-line status message is occasionally overlaid at the bottom of the graphical view. The player always runs along the corridor's centreline, and looks in the current direction only, which simplifies the rendering task.

The game's 3D engine and the random maze creation code is written in Z80 machine code, produced with an assembler. This is augmented by several dozen BASIC lines for less critical tasks, such as the initial greetings and the game legend animation inter-line delay. The machine code subroutines block is embedded into the BASIC line 0, beginning with a REM (BASIC comment) statement, making the interpreter step over it. If, by accident, one tries to edit the line via the BASIC line editor, the changes will not be accepted since 0 is an invalid line number. Such code is typically created by first creating a line with a valid number, and then modifying the number field in the BASIC program area using direct memory manipulation, such as POKE. No copy protection is embedded into the game; moreover, the magnetic tapes of the time being unreliable, one could reuse the save entry point in the BASIC code (that was used by the original developers to have the game auto-run upon being loaded by the user) in order to save another program copy to the tape (for archival/backup purposes).

The game is controlled by three of the keyboard cursor control keys (left, forward and right, respectively 5, 7 and 8 on the ZX81 keyboard). The game speed can also be controlled — according to the original cassette inlay, the BASIC line 370 has a hardwired constant determining a busy waiting loop delay. The constant initially is set to 5; by varying it from 0 to 9 the game can be adjusted from faster to slower tempo. The 6 frame/s figure corresponds to the delay set to zero, while holding the "forward" key to run straight. A somewhat jerky motion is perceived at this slow framerate which gives a feeling of being jerked along with each step of a desperate heavy run.