Decoded Interrupts
Encoded Interrupts
(/I
PLO-/I PL2)
Override
(IOVR)
Two of the
68000
non-encoded interrupt inputs are available at the
Coprocessor slot, on pin 19 for interrupt level 2
(IINT2) and on pin
22 for interrupt level
6
(lINT6). These are the same interrupts used
by the Amiga internal system chips and encoded by the Paula chip.
They can be used by a Coprocessor board by driving them to gener-
ate
68000
interrupts when the
68000
is
in charge, though generally
they don't do much when the Coprocessor is in charge.
The Coprocessor Slot provides the encoded interrupt lines
IIPLO,
IIPLl, and lIPL2 on bus pins 40,42, and
44
respectively, which are
the normal encoded interrupt inputs to the 68000. Nothing on the
Coprocessor slot can drive these lines, but they must be monitored
by any Coprocessor or alternate processor that needs to be able to
respond to any system interrupts when acting as the bus master.
The
IOVR, or Override, signal
is
a special Amiga expansion signal that
can serve two purposes. The signal can basically turn off the on-
board decoding of system memory ranges. As a result of this,
it
can
also turn off internally generated things, like
IDTACK.
The timing in the A500 and B2000, based on the Gary chip (not the
PALS of the older machines) effectively prohibits the use of OVR* for
the area outside of
$200000
to S9FFFFF.
The other use of this signal
is
better supported. Asserting IOVR will
tri-state the internally generated
IDTACK signal, allowing a CO-
processor or Expansion device to create
its
own IDTACK. The same
effect can be achieved for most applications by using XRDY to delay
the motherboard's generation of
IDTACK. Pin
17.
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