Xor

Xor is a puzzle game created for the ZX Spectrum in 1990 by Marek Trmač from the group GCC; it was officially released in 1992 by Proxima – Software v.o.s. as part of the Tinny collection. Its source was the original commercial game Xor by Astral Software from 1987, available on a number of other platforms, but Trmač's Spectrum version wasn't a mere port – the author designed thirty entirely new levels for it, twice as many as the British original had, and adapted them to the capabilities and graphical style of the ZX Spectrum.

The gameplay resembles Boulder Dash, but follows its own, more complex rules: the player moves through a maze full of blocks that need to be pushed, destroyed, or bypassed in the right order to reach the goal, and a single wrong move can make a level impossible to finish, forcing the player to start the level over from the beginning. The emphasis on forward planning and patient trial of approaches makes Xor a typical representative of the puzzle-game genre, which was very widespread on the ZX Spectrum due to its low graphical demands and was popular even with players lacking fast reflexes.

Thanks to its expanded number of levels and local distribution, Trmač's version won its own audience among Czechoslovak players independent of the foreign original, and it still appears today in overviews of the most accomplished domestic puzzle games for this platform. It's also worth noting that this is an adaptation of an existing concept rather than an original idea – such reworkings of foreign games were common on the scene of the time, since license fees were unattainable for amateur authors, so they simply reworked interesting mechanics according to their own ideas. For genre fans, Xor remains a challenge even today, because its puzzles depend neither on reflexes nor on graphical execution but purely on the ingenuity of the solution, and it can be played just as well on the original machine as on a modern emulator.