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DOSPLUS disc formats

This page describes the FAT16-compatible disc format supported by various versions of Digital Research's DOSPLUS operating system. CP/M-compatible formats are described elsewhere.

Standard formats

The standard formats are those created by contemporary PC-DOS versions:

BBC Master 512k

The Master 512 supports a couple of unique disc formats with nonstandard sector sizes. They are DOS filesystems, but the low-level format is from Acorn's ADFS (Advanced Disc Filing System). The 640k version can contain a tiny ADFS filesystem in the first track, to boot from.

Since there is no boot sector, there is no BPB. The following values are therefore deduced:

BPB field	640k	800k
Sector size	256	1024
Sectors/cluster	8	2
Reserved sectors	16	0
FAT copies	1	1
Root dir entries	112	176
Total sectors	2560	800
Media descriptor	n/a	n/a
Sectors / FAT	2	2
Sectors / track	16	5
Heads	2	2

It is possible to read disks in these formats under Linux, by converting them to a flat file, adding a false boot sector, and mounting them with "-o loop". Note that the false boot sector would have to have a total number of sectors that included itself.

In addition to these two, the BBC Master version of DISK.CMD can also generate a number of standard and nonstandard DOS formats. DISK.CMD does not contain complete BPBs so some fields have been guessed. The first (Nimbus 720k) appears to match the standard PCDOS 720k format:

BPB field	Nimbus 720k	Tandy 720k	Olivetti 720k
Sector size	512	512	512
Sectors/cluster	2	4?	2?
Reserved sectors	1	1	1
FAT copies	2	2	2
Root dir entries	112	112	144
Total sectors	1440	1440	1440
Media descriptor	0xF9	0xFD	0xFD
Sectors / FAT	3	2	3
Sectors / track	9	9	9
Heads	2	2	2

It is not known whether all these formats are supported by the BBC Micro version of DOSPLUS itself.

DISK.CMD - Amstrad / Jasmin

The DISK.CMD utility in the versions of DOSPLUS supplied for the Amstrad PC1512 and Jasmin TRAN contains definitions for three nonstandard formats. These definitions are never used, and have no descriptive text attached. The matching versions of DOSPLUS itself do not support the formats.

315k Apricot

This format uses 70 tracks on a single-sided floppy disc. It appears to be the native floppy format used by the Apricot PC / Xi computers.

720k :YES

This appears to be the native floppy format used by the Philips :YES, as described in this newsgroup posting. The posting also describes an 800k :YES format which is not listed here.

720k Tandy

This format appears to be the same as the "Tandy 720k one" used in the BBC Micro version of DISK.CMD.

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