This document tries to explain the “why”, “what” and “how” details of the tape format used by all 8-bit Commodore computers. The specifics mentioned below certainly apply to tapes written by the VIC-20, it is possible (or even likely) that other models have subtle differences to what is described here.

There are several “layers” to discuss, for clarity each will be introduced in turn to, ultimately, build up a complete picture. At times there are topics which are related but not essential to the main subject. These will be prefixed with “Diversion”, they can be skipped if preferred. Only the standard Commodore structure and encoding will be discussed, formats used by fastloaders vary and I have not studied them.