Commodore Amiga A500 Technical Reference Manual page 93

ADDRESSING AND
CONTROL SIGNALS
Read-Write (R/W)
Address Bus (A1-A23)
Address Strobe (/AS)
DataBus(DO-D15)
These signals are various items used for the addressing of resources
on a coprocessor card by the 68000 and any DMA devices, and for
24 by 16 bit addressing of other system resources by a coprocessor
device (which may easily have more potential). Most of these signals
are directly in common with 68000 signals.
The 68000's R/W output. When driven high it indicates a read or in-
ternal cycle, when driven low it indicates a write cycle. When the co-
processor takes over it drives this line; the 68000's output will tri-
state. Pin 68.
This directly connects to the 68000's address bus, providing 16 me-
gabytes of address space with 23 bits of address for a 16 bit data
bus. The 68000 is capable of driving only this much address space.
Thus, any resources on a coprocessor board must map somewhere
into the 68000 memory space. The best thing to do with any such
memory is allow it to be autoconfigured by the 1.2 OS; this will
place it somewhere in the 8 megabyte space starting at $200000
(the A2000 doesn't support autoconfiguration from the Coprocessor
Slot, the B2000 does). Any resources intended specifically for the
coprocessor only can be located above the 68000's 16 megabyte
space if the coprocessor hardware permits that extended addressing.
All board and Expansion bus resources will normally map into the
first 16 megabytes of the address space of a coprocessor board. See
p. 98 for pin list
The falling edge of this strobe indicates that addresses are valid, the
rising edge signals the end of the memory cycle. This is in common
with the 68000 /AS signal. The coprocessor drives this signal when
it takes over; the 68000's will tri-state. Found on pin 74.
This is directly connected to the 68000's data bus, providing 16 bits
of data accessible by word or either byte. Any coprocessor handling
words larger than 16 bits must either step down to 16 bits on its
own or provide circuitry to convert the 16 bit word size of the main
board and Expansion Bus to the natural size of such a coprocessor,
when accessing main board resources. See p. 98 for pin list.
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