If you'd have told me 25 years ago that I'd be playing 3D Monster Maze on the bus on a device powered by batteries, I'd have said you were mad...

DS81 is a basic emulation of the Sinclair ZX81. It only supports text modes games (basically anything that uses the bog-standard ZX81 display file for the technically minded), so hi-res games will not work. This is being worked on.

It includes a few games to try out. Note that I did not write these games, or own the copyright to them. They are included in good faith (as far as I know the authors do not mind this work being distributed), and if any of them want their work removing I'll gladly oblige:

ZX81 ROM © 1981 Nine Tiles Networks Ltd
Mazogs © 1981 Don Priestley
City Patrol © 1982 Don Priestley
Sabotage © 1982 Don Priestley
3D Monster Maze © 1983 Malcom E. Evans

DS81 also supports the loading of external files from a FAT formatted device. To do this the internal FAT routines of libnds are used, so if you're device isn't supported then I'm afraid you're stuck with the built-in games. Don't worry, you're not alone -- I wrote the damn thing and can't use external files on my flash device!