Floppy Disk Variants - Acorn Electron, BBC Micro, and Acorn Archimedes

Acorn Electron, BBC Micro, and Acorn Archimedes

The British company Acorn Computers used non-standard disk formats in their 8-bit BBC Micro and Acorn Electron, and their successor the 32-bit Acorn Archimedes. Acorn however, used standard disk controllers: initially FM, though they quickly transitioned to MFM. The original disk implementation for the BBC Micro stored 100 KB (40 track) or 200 KB (80 track) per side on 5¼-inch disks in a custom format using the Disc Filing System (DFS).

Due to the incompatibility between 40 and 80 track drives, much software was distributed on combined 40/80 track discs. These worked by writing the same data in pairs of consecutive tracks in 80 track format, and including a small loader program on track 1 (which is in the same physical position in either format). The loader program detected which type of drive was in use, and loaded the main software program straight from disc bypassing the DFS, double-stepping for 80 track drives and single-stepping for 40 track. This effectively achieved downgraded capacity to 100 KB from either disk format, but enabled distributed software to be effectively compatible with either drive.

For their Electron floppy disk add-on added, Acorn picked 3½-inch disks and developed the Advanced Disc Filing System (ADFS). It used double-density recording and added the ability to treat both sides of the disk as a single drive. This offered three formats:

• S (small): 160 KB, 40-track single-sided;
• M (medium): 320 KB, 80-track single-sided;
• L (large): 640 KB, 80-track double-sided.

ADFS provided hierarchical directory structure, rather than the flat model of DFS. ADFS also stored some metadata about each file, notably a load address, an execution address, owner and public privileges, and a "lock" bit. Even on the eight-bit machines, load addresses were stored in 32-bit format, since those machines supported 16 and 32-bit coprocessors.

The ADFS format was later adopted into the BBC line upon release of the BBC Master. The BBC Master Compact marked the move to 3½-inch disks, using the same ADFS formats.

The Acorn Archimedes added D format, which increased the number of objects per directory from 44 to 77 and increased the storage space to 800 KB. The extra space was obtained by using 1024 byte sectors instead of the usual 512 bytes, thus reducing the space needed for inter-sector gaps. As a further enhancement, successive tracks were offset by a sector, giving time for the head to advance to the next track without missing the first sector, thus increasing bulk throughput. The Archimedes used special values in the ADFS load/execute address metadata to store a 12-bit filetype field and a 40-bit timestamp.

RISC OS 2 introduced E format, which retained the same physical layout as D format, but supported file fragmentation and auto-compaction. Post-1991 machines including the A5000 and Risc PC added support for high-density disks with F format, storing 1600 KB. However, the PC combo IO chips used were unable to format disks with sector skew, losing some performance. ADFS and the PC controllers also support extended-density disks as G format, storing 3200 KB, but ED drives were never fitted to production machines.

With RISC OS 3, the Archimedes could also read and write disk formats from other machines, for example the Atari ST and the IBM PC. With third party software it could even read the BBC Micro's original single density 5¼-inch DFS disks. The Amiga's disks could not be read as they used unusual sector gap markers.

The Acorn filesystem design was interesting because all ADFS-based storage devices connected to a module called FileCore which provided almost all the features required to implement an ADFS-compatible filesystem. Because of this modular design, it was easy in RISC OS 3 to add support for so-called image filing systems. These were used to implement completely transparent support for IBM PC format floppy disks, including the slightly different Atari ST format. Computer Concepts released a package that implemented an image filing system to allow access to high density Macintosh format disks.