A 1990 homebrew conversion by Pygmy that requires 64K of RAM (which technically makes it a Plus 4 release). Driller is an impressive achievement on the 6502, even if it does run slowly.

All 8-bit versions of Driller run slowly, but players who know what they’re doing can (kind of) counter the slowness by changing the angle and step parameters to make quicker turns and longer strides.

This conversion of Driller seems to be complete and intact and is nicely playable. There are two versions around too. The second one (labelled ‘Driller Plus‘) has some bugfixes, and also uses different colours in some sectors.

More: Driller on Wikipedia