Commodore Amiga A500 Technical Reference Manual page 90

Coprocessor Expansion and 86 Pin Signals
INTRODUCTION
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This section details the signals found on the various types of 86 pin
expansion connectors on different Amiga computers, especially the
signals found on the B2000 computer's 86 pin Coprocessor Slot, and
how these differ from the similar signals found on A2000 computers
and those of the original A1000 computers. This paper also explains
the Coprocessor Slot's autoconfiguration and DMA protocols and
how they fix the problems introduced in the A2000 Coprocessor
Slot.
We've kept the 86 pin specification on the B2000 as similar to those
available on the A2000, A1000 and A500, wherever possible. How-
ever, some major changes were absolutely required. With the design
of the A2000, the function of the 86 pin slot had shifted from a gen-
eral expansion connector to expansion specifically intended for co-
processors and similar devices. Thus, while the A500's and Al000's 86
pin connectors have to support both some kind of coprocessor
expansion and the normal ZORRO expansion, the A2000 machines
can optimize each slot for its purpose if required (or if necessary,
which is more the case).
The 86 pin connector on the A500 and A1000 becomes something
of an advantage, because of the fact that all expansion must be done
externally. When a coprocessor device, something that needs to com-
pletely replace the 68000 in all forms of bus access and operation
(like a 68020 accelerator card) is added, it can physically sit between
the computer motherboard and the 100 pin expansion box. thus al-
lowing the device to completely replace the action of the mother-
board's processor from the point of view of the expansion box. A
machine with both slots on the motherboard must provide some fa-
cility to logically insert the 86 pin slot in front of the 100 pin slot for
certain applications.
In the A2000, the Coprocessor Slot signals that control DMA can be
used to insert the coprocessor in the place of the normal 68000 via
the standard 68000 DMA request protocol. This, however, isn't a to-
tally transparent replacement; the action of the coprocessor taking
control over the local bus from the 68000, in the A2000, can block
other DMA events coming over from the 100 Expansion Bus. For total
control of the Expansion Bus on the A2000, the 68000 could be
physically removed from the motherboard, but that would result in
the "coprocessor" being a complete "replacement" processor, with
no swapping between the two permissible. The B2000 solves these
problems with a higher-level DMA protocol between the main and
coprocessor devices.
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