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From: dahl@SRC.Honeywell.COM (Peter Dahl)
Newsgroups: comp.arch
Subject: Re: Coprocessors - Business?
Keywords: coprocessor, OS, interface
Message-ID: <1991Apr4.191019.23006@src.honeywell.com>
Date: 4 Apr 91 19:10:19 GMT
References: <2892@megatek.megatek.uucp>
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Organization: Honeywell Systems & Research Center
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In article <2892@megatek.megatek.uucp> mark@megatek.UUCP (mark thompson) writes:
>Has anyone tried to do something wonderful with one of the undedicated
>coprocessors, examples of wonderful being I/O channel controllers or a
>business instruction set?
>-mark

Well I don't know about wonderful, but...

For my master's thesis I studied this interface with the intent of using it
for an operating system coprocessor. The idea is that a coprocessor would
take over running the OS kernel. It would have an independent instruction 
memory and would basically offload user processes to the main processor.
Coprocessor instructions would be analogous to system calls in the user code.
The thesis explores issues relating to interrupts, passing code back and forth,
control, context switching, etc. ("An Operating System Coprocessor Interface",
Peter Dahl, University of Minnesota, 1988)

The main problem with this particular interface is that there is no way for
the coprocessor to interrupt the main CPU using the interface, the coprocessor
must generate an external interrupt. The overhead becomes significant. I
tried variants on the same theme like making the main CPU run the OS and the
coprocessor run user tasks, but the same problem keeps occurring. What needs
to be done is expand the interface so that the coprocessor can efficiently
interrupt the main CPU. This is a problem even if you have a FPU, the 
difference is that you don't have real-time constraints so you can afford to
catch the problem later.

peter dahl@src.honeywell.com
I had fun once, and it wasn't anything like this