Sudoku86

Sudoku86 is not an old program somebody dug up. It is a sudoku written in 2014 deliberately for DOS, and written so that it will run on a machine with an 8086 processor. The author took that seriously: the game is sixteen-bit, runs in real mode, gets by on fifty kilobytes of conventional memory and a CGA card at 320 by 200. It was built in Turbo C and is free, source code included, under a BSD licence.

The game itself is exactly what you expect. Nine by nine squares, every row, column and box holding the digits one to nine once each, and you filling in whatever can be deduced. A hundred puzzles of varying difficulty come built in, and the program can load external files with more, so you cannot exhaust it.

It is driven by a mouse, and specifically a two-button one, which is not a whim: the second button handles pencil marks, the candidate notes without which the harder grids get considerably more painful. The interface is spare, legible and quick; nothing is explained, nothing is timed, and there is no score to chase.

The obvious question is why play it here when sudoku sits on every phone. The honest answer is that the game is not the reason, the program might be. It is proof that something complete and genuinely usable can still be written for prehistoric hardware, and it is one of the few modern DOS programs that would run better on a genuinely ancient PC than plenty of things from the nineties. As sudoku it is unusually clean; as an experience it is about as thrilling as a finished crossword.

On oldgame.cz it starts directly in the browser, with nothing to install.