Commodore Amiga A500 Technical Reference Manual page 99

Decoded Interrupts
Encoded Interrupts
(/I PLO-/I PL2)
Override (IOVR)
Two of the 68000 non-encoded interrupt inputs are available a t the
Coprocessor slot, on pin 1 9 for interrupt level 2 (IINT2) and on pin
2 2 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 t o 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 4 0 , 4 2 , and
44
respectively, which are
the normal encoded interrupt inputs t o the 68000. Nothing on the
Coprocessor slot can drive these lines, but they must be monitored
by any Coprocessor or alternate processor that needs t o be able t o
respond t o 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 t o S9FFFFF.
The other use of this signal
is
better supported. Asserting IOVR will
tri-state the internally generated IDTACK signal, allowing a CO-
processor o r Expansion device t o create its own IDTACK. The same
effect can be achieved for most applications by using XRDY t o delay
the motherboard's generation of IDTACK. Pin 17.