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5.5 Disc Organisation
The BIOS supports three different disc formats: SYSTEM format, DATA ONLY
format and IBM format. Under AMSDOS the format of a disc is automatically
detected each time a disc with no open files is accessed. To permit this automatic
detection each format has unique sector numbers.
3 inch discs are double sided, but only one side may be accessed at a time depending
on which way round the user inserts the disc. There may be different formats on the
two sides.
COMMON TO ALL FORMATS
Single sided (the two sides of a 3 inch disc are treated separately).
512 byte physical sector size.
40 tracks numbered 0 to 39.
1024 byte CP/M block size.
64 directory entries.
SYSTEM FORMAT
9 sectors per track numbered #41 to #49.
2 reserved tracks.
The system format is the main format supported, since CP/M can only be loaded (cold
and warm boot) from a system format disc. The reserved tracks are used as follows:
Track 0 sector #41: boot sector.
Track 0 sector #42: configuration sector.
Track 0 sectors #43.. #47 unused.
Track 0 sectors #48.. #49 and track 1 sectors #41.. #49: CCP and BDOS.
Note: 'VENDOR' format is a special version of system format which does not contain
any software on the two reserved tracks. It is intended for use in software distribution.
DATA ONLY FORMAT
9 sectors per track numbered #Cl.. # C9.
0 reserved tracks.
This format is intended for future enhancement, it is not recommended for use with
CP/M since it is not possible to 'warm boot' from it. However, if only AMSDOS is to
be used then there is a little more disc space available.
IBM FORMAT
8 sectors per track numbered 1..8.
1 reserved track.
This format is logically the same as the single-sided format used by CP/M on the IBM
PC. It is intended for specialist use and is not otherwise recommended.
Chapter 5. 5 AMSTRAD Disc Drive & Interface DDI-1 Manual