A file format used to store an image of a Commodore 64 computer (aka C64) true and tangible floppy disk. A typical use of .D64 files is under C64 emulators.

Among the ways to create one, a comfortable and easy one is to use the nice Star Commander program.

d100 means that you roll two d10s, treat a roll of 10 as zero, and take the result as the low two digits of a number from 1 to 100. Therefore, d64 should mean that you roll two d8s, treat a roll of 8 as zero, and take the result as the low two digits of a number from 1 to 100 ... in octal.

Something to keep in mind if you ever decide that your Dungeons & Dragons game isn't geeky enough ....