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Format

The tape content is encoded in ASCII. In the case of non-textual content (such as binary files or programs object code), it is coded in Hexadecimal which in turn is encoded in ASCII.

The entire media is treated as a single sequence of bytes broken into Blocks being this the transfer unit. VTFS reads or writes tapes one block at the time into internal buffers. Applications read or write one byte at the time from VTFS buffers. VTFS takes care of the synchronization between application requests and buffers content.

In real tapes, blocks are physically separated by gaps. This is required for tapes containing long files because they will eventually stop in the middle of a file; tapes must stop with the head positioned over a gap, not a data zone.

The size of the block is fixed for a given tape but it can vary from tape to tape. Some types of media such as Floppy have a pr-established block size but this parameter is generally user-defined and ranges from 256 to 4096 bytes. The maximum number of blocks a file can contain is 256, hence the maximun size for a VTFS file is 1 MB.

The block format for real tapes is as following:

    gap
    02H     ASCII Start-Of-Transmission (STX)
    MSN     Block Number (Hex less signif. nibble)
    LSN     Block Number (Hex most signif. nibble)
    .
    .
    data
    .
    .                                .
    03H        ASCII End-Of-Transmission (ETX)
    gap

In virtual tapes, blocks contain just the data.

LC-81 Homebrew Minicomputer -- this software is based on Help Books running at melissa