Computer busses....

From: Richard Erlacher <>
Date: Wed Mar 31 00:42:21 1999

-----Original Message-----
From: Tony Duell <>
To: Discussion re-collecting of classic computers
Date: Tuesday, March 30, 1999 4:33 PM
Subject: Re: Computer busses....

>> You've hit most of the important signals. One I'd add, however, is a
>> bus disable, and perhaps an address bus disable as well. This would
allow a
>> front panel or other bus mastering device to steal cycles under certain
>> circumstances.
>He's got BUSreq/BUSack. That's all you really need for a DMA device (at
>least if those signals have similar definitions to the Z80 ones of that
>name). When the Z80 gets a Busreq, it tri-states the internal
>address/data buffers IIRC, and you can use BUSack to tri-state the
>external ones that you should have added.

While it's true that's all you'd need, it's not all you might want, and
while I agree that you can and probably should do that, I've actually seen
it done more by using the processor to do much of the work by jamming a jump
to a front-panel-or diagnostic-card-resident monitor. I doubt, however,
that I've seen this stuff more than a dozen times altogether. I've seen
plenty of front-panels which were connected only to make the lights blink.

>A frontpanel can easily be implemented as a DMA device using those signals.
Received on Wed Mar 31 1999 - 00:42:21 BST

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