Commodore Amiga A500 Technical Reference Manual page 100

The B2000 Coprocessor
Solution
The B2000 hardware has implemented a more sophisticated Co-
processor system that removes these problems. The B2000 Co-
processor Slot has a signal called /CBR (Coprocessor Bus Request) as
a replacement for /BR, a signal called /CBG {Coprocessor Bus Grant)
as a replacement for /BG, and one additional signal, /BOSS, which is
also known as Coprocessor Grant Acknowledge.
Under the B2000 system, there are essentially two ways a
Coprocessor device can receive a Local Bus mastership. Both start in
the same way. To request the bus, the Coprocessor asserts /CBR.
Instead of going directly to the 68000, this signal is prioritized and
latched along with any Expansion Slot /BR signals. The /CBR signal
has the highest DMA priority. Assuming no other DMAs are
currently active, the 68000 issues a Bus Grant via /BG, which will go
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to the priori-tizer and result in /CBG being asserted. At this point,
all other DMA requests will be locked out; no other /BGs of any kind
will be issued. Following the normal 68000 protocol, at this point,
the Coprocessor will assert /BGACK when the 68000 is off the bus,
and will have bus access as before. And as before, it is holding off
any further DMAs from the Expansion Bus (which may be what was
wanted). This type of DMA access is very similar to what a normal
DMA device from the Expansion Bus would achieve.
There is another way to take over the Bus. This starts in the same
manner as before, with a /CBR resulting in a /CBG. Once the Co-
processor has received its Bus Grant, however, it does something
different It asserts the /BOSS signal instead of/BGACK. This has
several immediate effects. First of all, the 68000 sees /BOSS as the
same thing as /BGACK, so it stays off the bus just as if /BGACK had
been asserted. Next, the data direction of /CBR and /CBG change on
the Coprocessor Bus. The /CBR signal is now an output from the bus
control logic, the prioritized and latched combination of all the /BR
signals from the Expansion Bus. The /CBG signal is now an input go-
ing into the bus control logic that will be passed on to the Expansion
Bus in response to an Expansion Bus /BR. The bus control logic also
holds /BR to the 68000 in a low state. The data direction of /CBR
and /CBG changes with a change in /BOSS, so the lines that
alternately drive /CBR and /CBG on a Coprocessor card should be
enabled and disabled with the assertion of /BOSS.
Anyway, what all this means is that, in asserting /BOSS instead of
/BGACK, the Coprocessor has the bus, the 68000 is in tri-state, and
any of the Expansion Slots may initiate a DMA of the Coprocessor at
any time, directly, according to the normal /BR -» /BG -»/BGACK
protocol of the 68000. The Coprocessor can allow the 68000 back
on the bus by negating the /BOSS line. Thus, the Coprocessor can be
a real Coprocessor, functioning as the equivalent of the 68000 for
all things as far as the whole Amiga system is concerned.
'The B2000 system does all of its DMA prioritization via the
"Buster" custom bus controller chip.
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