Commodore Amiga A500 Technical Reference Manual page 103

The B2000 Coprocessor
The B2000 hardware has implemented a more sophisticated CO-
Solution
processor system that removes these problems. The B2000 CO-
processor Slot has a signal called ICBR (Coprocessor Bus Request) as
a replacement for /BR, a signal called ICBG (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 Coproces-
sor 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 /BC, which will go to the priori-
tizer' and result in ICBG being asserted. At this point, all other DMA
requests will be locked out; no other IBGs 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 ICBR resulting in a /CBG. Once the CO-
processor has received its Bus Grant, however, it does something dif-
ferent. It asserts the /BOSS signal instead of IBGACK. This has sever-
al immediate effects. First of all, the 68000 sees /BOSS as the same
thing as IBGACK, so it stays off the bus just as if IBGACK had been
asserted. Next, the data direction of /CBR and ICBG 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 alternate-
ly 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
IBGACK, 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
- ,
/BC
- ,
/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" custnm
bus
controller chip.