ZiLOG Z80 Handbook page 217

tively simple device that supplied one interface signal, busy or
ready. Although there are a number of devices that interface in such
a simple fashion, there are many other devices that require more
elaborate interfacing. A reel-to-reel magnetic tape has a variety of
functions that may be performed including reading a record, writing
a record, writing an end-of-file, forward or backward skips over
records or files, and so forth. In addition, the tape provides many
status outputs such as end-of-tape, load-point, parity error, end-of-
file, and off-line. Obviously, a software routine for this device has
to be considerably more complicated than just the simple loops de-
scribed. To provide a complete subroutine for servicing the more
sophisticated I/O devices or to provide additional capability for the
simple I/O devices, most large software systems include a software
I/O driver subroutine for each kind of I/O device in the system.
The I/O driver handles all communication with the device type and
acts as a software interface between portions of the system programs
that require I/O service and the I/O devices.
To illustrate this concept, assume that several floppy discs are con-
nected to the Z-80 system. The physical characteristics of each disc
are shown in Fig. 15-1. The basic functions that one would want to
perform with a floppy might be the following:
1. Read track N starting at sector M for J bytes into a specified
buffer.
2. Write from a specified buffer J bytes starting from track N,
sector M.
3. Position head to track N (in preparation for read or write).
• 32 SECTORS
• 77 TRACKS
• 41K BITS OF DATAITRACK
SECTOR 31 SECTOR 0
SECTOR 4 OF
TRACK 20
360 RPM
ONE OF 77 TRACKS
ARRANGED IN
CONCENTRIC
CIRCLES
INNERMOST
TRACK
Fig. 15 -1. Typical floppy-d isc characteristics.
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