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Assumptions about the tape format

We need to differentiate between "encoding" and "data format". The former refers to how bits are represented in the media, it can be frequency-keying, bi-phase encoding, etc. The later refers to how bits (once decoded) are organized and what they represent. Reading a tape, therefor, takes two steps: decoding bits and making sense of them.

For LC-81 tape drives, the encoding is mostly drive-specific. In general, it it is assumed that data is grouped into "blocks" separated by physical "gaps". This is generally required in real tapes because the transport will eventually stop in the middle of a file (particularly true for long files). For synchronization purposes, the reading head must stop on a gap, not a data area. The drive must be able to detect the presence of blocks and gaps as they read.

As per data format, the drive must be totally agnostic. This ignorance allows the drive to transparently serve to different file systems and formats. It is the software running at the LC-81 that actually makes sense of the data and the tape data format.

Nevertheless, some assumptions have been made about that format:

* Blocks contain 8-bits bytes.
* There is only one file per tape.
* Software will refer to this file by a "File Identification number" (FID).
* The tape could contain a LABEL with metadata having the FID as part of it.

Tape Drives must be designed to support these assumptions without fully committing to them, that is being able to serve as well in cases where those assumptions do not hold.