IBM AT 5170 Technical Reference page 439

for a High Capacity Diskette Drive and 20 milliseconds will
be enforced for a Double Sided Diskette Drive.
If
a
parameter block contains a motor start wait parameter of less
than 1 second for a write or format operation of 625
milliseconds for a read or verify operation, Diskette BIOS
will enforce those times listed above.
• The following procedure is used to determine the type of
media inserted in the High Capacity Diskette Drive:
1. Read Track 0, Head 0, Sector 1 to allow diskette BIOS
to establish the media/drive combination. If this is
successful, continue with the next step.
2. Read Track 0, Sector 15. If an error occurs, a double
sided diskette is in the drive.
Note: Refer to the DOS Technical Reference
manual for the File Allocation Table (FAT)
parameters for single- and double-sided
diskettes.
If a successful read occurs, a high capacity diskette is in
the drive.
3.
If Step 1 fails, issue the reset function (AH=O) to
diskette BIOS and retry. If a successful read cannot be
done, the media needs to be formatted or is defective.
ROM BIOS and DOS do not provide for all functions. The
following are the allowable I/O operations with which IBM will
maintain compatibility in future systems.
• Control of the sound, using port hex 61, and the sound
channel of the timer/counter. A program can control
timer/counter channels 0 and 2, ports hex 40,42, and 43. A
program must not change the value in port hex 41, because
this port controls the dynamic-memory refresh. Channel 0
provides the time-of-day interrupt, and can also be used for
timing short intervals. Channel 2 of the timer/counter is the
output for the speaker and cassette ports. This channel may
also be used for timing short intervals, although it cannot
interrupt at the end of the period.
Compatibility 9-13