Commodore Amiga A500 Technical Reference Manual page 96

ADDRESSING AND
CONTROL SIGNALS
Read-Write (W)
These signals are various items used for the addressing of resources
on a coprocessor card by the 68000 and any DMA devices, and for
2 4 by 1 6 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 RJW 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.
Address Bus (AI -A23) This directly connects to the 68000's address bus. providing 1 6 me-
gabytes of address space with 23 bits of address for a 1 6 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 t o 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 co-
processor only can be located above the 68000's 1 6 megabyte space
if the coprocessor hardware permits that extended addressing. All
board and Expansion bus resources will normally map into the first
1 6 megabytes of the address space of a coprocessor board. See
p. 98 for pin list.
Address Strobe (/AS)
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.
Data Bus (DO-D 1 5)
This is directly connected t o the 68000's data bus, providing 16 bits
of data accessible by word or either byte. Any coprocessor handling
words larger than 1 6 bits must either step down t o 1 6 bits on its
own or provide circuitry t o convert the 1 6 bit word size of the main
board and Expansion Bus t o the natural size of such a coprocessor,
when accessing main board resources. See p. 98 for pin list.