3b1 add-on boards **REPOST**

[Brian D. Botton]

In article <1991Mar16.211250.7736 at texrex.uucp> rmfowler%texrex at cirr.com writes:
>This article never showed up on my machine at work so I am reposting it. 
>Sorry if you already saw it.
>

  It showed up here, I just didn't have time to answer the first time.

>
>Hardware hackers,
>
>Why don't you have to set Interrupt Request vectors on boards
>for the 3b1 like you do on boards added to PCs.  Is this just 
>one of the pains in the PC architecture, or does the 3b1 know
>how to avoid interrupt conflicts?  I've been wondering if maybe 
>this has something to do with certain boards not working in some 
>3b1s but working ok in others.
>
>Any thoughts?

  If you happen to have a copy, get out your S4bus - AT&T UNIX PC Expansion
Bus Specifications document.  On page 34 they explain the interrupt protocol.

  Basically, of the 7 possible interrupt levels on a 680x0, level 1 and 5 are
shared with all expansion cards, with level 5 being the highest.  The philsophy
of the 3B1 expansion bus is to never have to set jumpers, all boards are self
configuring.  This is what they (AT&T) does with their switching equipment.
Because of this, each board has an ID, which the kernel reads and matches
up with its device driver.  In the land of crappy architectures, you know what
I'm talking about, ;-), the boards are not autoconfigurable, so you have to
set things like interrupt vector, I/O address, memory address, and DMA channel.
All of these things are taken care of for you by the 3B1.  Now, you could
argue that you loose some flexibility because you are forced to live with some
engineer's view of the system's resources.  But if you compare the limits the
3B1 places on you vs. the PC, the 3B1 very similar limits.

  As for why some boards work and others don't, I'll have to pass on that one.

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