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 ....