Control of the disk operations IS distributed among several registers
in
the system.
Among the control types are
o Selection, motor control, sensing
o Disk DMA channel control,
DMA
enable.
o Disk
data
read/write.
o Disk format control.
o Interrupts generated.
DISK
SELECTION,
CONTROL,
AND
SENSING
The
disk subsystem uses two 8520 ports plus one FLAGS interrupt port.
The
specific
bits used are detailed in table 8-6.
CIA A ($BFEOOl), port
A,
has four input bits allocated
to
the disk subsystem. CIA B
($BFDOOO),
port B, is entirely dedicated to
output
bits for the disk.
Table
8-6·
Disk Subsystem
Port
Pin
Name
Function
CIA A
PA5
DSKRDY*
Disk ready (active low).
CIA A PA4
DSKTRACKO* Disk heads currently positioned
over track zero (active low).
CIA A PA3
DSKPROT* Disk
is
write protected (active low).
CIA A PA2
DSKCHANGE*
Disk has been removed from
the drive.
The
drives
that
support this
signal latch
it
until the next time the
heads are stepped (active low).
228 Interface Hardware
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