+3 DOS

Notes on the Disk Operating System for the Plus 3

Spectrum +2A

The +2A (not the +2) has the exact same ROM's as the +3, it just has a Cassette tape instead of a 3" floppy drive. As such, +3DOS is available but only provides:

Only drive M: is available (the RAMdisk)
The default drive for filenames is initialised to M: rather than A:
Any attempt to use drives A: or B: will fail with error '22 - Drive not found'
As the sector cache is not required for use with the RAM disk, it is increased to 64K (the whole of pages 1, 3, 4, 6). This will give 62K of data and 2K of directory (64 entries).
The presence of the floppy disk interface can be determined by calling DD_INTERFACE. If the interface were not present, then none of the other low level floppy disk routines (DD... etc.) could be called; the effect of doing so is undefined.

Another method to determine if a machine is a +3 or a +2A is to inspect bit 4 of FLAGS3 (0x5B66). If the bit is set then a disk interface is present, hence a +3. If the bit is reset then no disk is present, hence a +2A or a +3 with no disk interface or the interface disabled.

CP/M File compatibility

+3DOS uses the CP/M file structure, subject to the following restrictions:

Maximum file size of 8 megabytes (CP/M Plus supports a maximum of 32 megabytes).
Maximum drive size of 8 megabytes (CP/M Plus supports a maximum of 128 megabytes).
Directory labels are ignored.
No passwords. XFCBs will be erased, renamed, etc., along with their file(s) but are otherwise ignored.
No date and time stamps. SFCBs are initialised to zero when a file is created, but are otherwise ignored.
The archive file attribute is ignored, i.e. it is unaffected by all routines except DOS SET ATTRIBUTES.