Commodore Amiga A500 Technical Reference Manual page 28

DESIGN GUIDELINES
FOR PICs
Auto Configuration
General Description of
Auto Configuration
All PICs implement the auto-configuration protocol. The auto config
protocol is designed so that system auto-config software can inter-
rogate the PICs ID locations, build a system table of the installed
PICs, and place the PICs in the 68000 memory space.
If it is difficult to imagine how to implement this protocol while it's
being described, don't worry. The design requires one PAL, one
latch, and one address match circuit. Complete details are given in
the example design.
Upon reset, all PICs come up in the unconfigured state. In the uncon-
figured state, the PIC responds to the 64 kilobyte address space
starting at location E80000, if CONFIGIN* is active to the PIC. If
CONFIGIN* is not active, the PIC does not respond to any bus cycles.
The processor comes out and reads nibbles of ID data on D15-D12
from the PIC. The table of ID data and the locations of control
latches is detailed later in this section. This data includes such things
as size of address space required, manufacturer's product number,
and whether to add the PIC to the free memory pool (if it is a
memory PIC.)
Under normal conditions, the processor determines how much ad-
dress space the PIC requires and then loads the PICs address latch
with an appropriate base address. This permanently relocates the
PIC at its new address (until Reset), and passes CONFIGOUT* out to
the next PIC's CONFIGIN*, whereupon the process is enacted again
until all PICs are configured.
The smallest unit of memory that a PIC can ask for is 64 kilobytes.
The largest is eight megabytes. All PICs should be designed to be
based on boundaries that match their space requirements; for exam-
ple, one megabyte PICs should be designed to reside on one mega-
byte boundaries (match circuit matches A23-A20). There are two ex-
ceptions to this rule, however. Four megabyte PICs must be capable
of being placed on four megabyte boundaries, as well as at hex
200000 and at hex 600000. Eight megabyte PICs should be capable
of being placed on eight meg boundaries and at hex 200000. This
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