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
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
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 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
co-
processor 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.
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 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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