Acorn Cassette Filing Systems

There are three cassette storage formats used by Acorn Computers.

The most basic is that used by the Acorn System 1, it is simply a string of bytes preceded by a start and end address, each bit is encoded as either 4 cycles of 1200Hz ('0') or 4 cycles of 2400Hz ('1') resulting in a data rate of 300 bits per second (or baud), there are no filenames and there is no error checking.

In the System 2 Cassette Format the data is formatted into blocks of up to 256 data bytes each with a filename and other header information at the start of each block, the same bit coding is used but this time CRC checksums are added to each block so that data integrity can be checked when read back.

The BBC Computer goes a step further by separating the header and data information into separate blocks each with their own checksum.  As standard it also runs at a higher data rate (1200 bits per second) though maintains backwards compatibility with the 300 baud formats (*TAPE3).

The following diagrams show the details of the three Cassette Formats:

The System 1 Monitor program provides user commands for saving and loading memory to/from cassette in the System 1 Cassette Format.

The System Computers Cassette Operating System or COS provides user commands for saving and loading named files to cassette in System 2 Cassette Format.

 The Atom Operating System is essentially the same as System 2 COS and therefore provides the same cassette storage capabilities though with one small difference - the filename lengths differ.  System COS filenames can be up to 16 characters whilst Atom COS only allows 13 characters.  I'm not sure how this works, what happens if you load a System COS file with a 16 character filename on an Atom ? (something to try sometime).

 The BBC Computer's MOS is also capable of reading and writing in the System 1 and 2 formats though I'm not sure how it handles long filenames as it limits filenames to 10 characters.

 System COS and Atom COS include a set of standard OS calls to routines which enable programs to interface with the filing system:

These OS calls are documented in Chapter 9 of the System 2 Technical Manual.

The BBC Computers MOS provides a more extensive list of OS calls whilst maintaining compatibility with the System COS/Atom COS OS calls.


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