Commodore Amiga A500 Technical Reference Manual page 98

Processor Status
(FCO-FCZ)
Bus Error (BERR)
These signals are the 68000 Processor Status outputs, which can be
used by bus devices t o determine the internal state of the 68000 any
time /AS is asserted. When a coprocessor is in charge, it must drive
these pins in a way compatible with how the 68000 does it. The dif-
ferent 68000 status codes can be found in any 68000 spec sheet.
Pins 31. 33, 35.
This is an input that goes directly t o the 68000. Its used t o indicate
the occurrence of some kind of bus error. Any Expansion Card capa-
ble of detecting a bus error relating directly to that card can assert
IBERR when that bus error condition is detected. At other times, the
card must monitor IBERR and be prepared to tri-state all of its on-
bus output buffers whenever this signal is asserted. The Coprocessor
card won't have to tri-state on IBERR, but it must note it and pro-
vide some way of handling the occurrence (the 68000 under normal
Amiga OS control merely signals a Guru Error based on the Bus Er-
ror Exception). Since any number of devices may assert IBERR, and
nearly everything in the system must monitor it, any device that
drives IBERR must drive with an open collector o r similar device ca-
pable of sinking at least 12ma, and any device that monitors IBERR
should place as little load on it as possible (1 "F" type load or less,
per board, is suggested). This signal is connected t o a low valued on-
board pullup resistor, and shouldn't need any more pulling up. Pin
46.
System Reset (IRST)
Pin 53 of the bus contains the IRST signal which is in common with
the original 68000 reset signal. The IRST signal is bidirectional, and
the 68000 tri-states it when the coprocessor takes over. It is only
necessary for the processor t o output this signal if it needs t o reset
the system under program control. The IRST signal is connected t o a
medium valued on-board pullup resistor and shouldn't need any
more pulling up. The coprocessor must monitor this signal and re-
spond t o it appropriately; this may mean a complete reset, but it
doesn't have to. The Coprocessor can also assert this line if a system
reset is desired.
System Halt (IHLT)
This is the 68000's processor halt signal. tied directly t o the 68000.
It is connected t o a medium valued on-board pullup resistor and
shouldn't need any more pulling up. This signal, when asserted, will
halt and tri-state the 68000 a t the end of the current bus cycle. If
driven by the
68000,
it indicates detection of a double bus fault. For
a complete system reset, the 68000 looks for both the IRST and
IHLT lines t o be asserted. The Coprocessor should handle this signal
in a similar fashion. Pin 55.