ZiLOG Z80-RIO Operating System User's Manual page 192

7.5 THE BARE DISK CONTROLLER
The bare disk controller is analogous to the subroutine
FLOPPY in floppy-disk based systems. It accepts a
standard RIO vector and reads or writes a sector of
the disk. It accepts only a very limited set of
requests, and requires a supplemental parameter vector
specifying the disk address of the sector and the
disk drive to be accessed. The drive is specified in
the most significant three bits of the 3-byte
disk address. The sectors are addressed by
3-byte integers. Sector 0 is cylinder 0, surface 0,
sector
o.
Since disks with various numbers of sectors
per track, various numbers of surfaces, and various
numbers of cylinders can all be used with the same interface,
it is difficult to say on what cylinder, surface, and
sector any given sector address would lie, or what the
highest sector address is. However, with increasing
sector address, the sector number varies most rapidly,
then the surface, then the cylinder.
On drives which have one or more fixed platters and one
removable platter, the removable platter is considered
to be a different physical drive than, and thus to have
a different sector address space from, the fixed
platter or platters. If there are four drives on a
controller, the fixed platters on them will be
referenced as drives 0, 2, 4, and 6. It is not
necessary that any drive have a removable platter, but
if it does, the removable platter is referenced as
drive n+l, where n is the drive by which the fixed
platter is referenced. It is also not necessary that
multiple drives be of the same configuration; i.e.,
the drives need not be in fixed-removable platter pairs.
The typical installation will have a single drive with
one fixed platter, one removable platter, 12 sectors
per track and 406 tracks, and will be connected as
drive
o.
This means that drive 0, sector 0 will address
sector 0, surface 0, and cylinder 0 of the fixed
platter. Sector 1 will access sector 1, surface 0,
cylinder
o.
Sector 12 will access sector 0, surface 1,
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