Commodore Amiga A500 - Coprocessor Expansion and 86 Pin Signals

Coprocessor Expansion and 86 Pin Signals
INTRODUCTION
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
A1 000 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.
Changes from Previous
We've kept the 86 pin specification on the B2000 as similar to those
Documents
available on the A2000, A1 000 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 A1 000'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 A1 000 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 pintslot 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 to-
tal 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
d~vir~~.