Re: mips and concurrency/simultaneous running tasks

From: Robert Monsen (rcsurname_at_comcast.net)
Date: 01/19/05 

Date: Wed, 19 Jan 2005 12:13:45 -0800

Scott Keogh wrote:
> Hi
> I was wondering if anyone can help me as im new to the mips architecture.
> I'm told mips is able to run (or run more efficiently than a conventional
> processor) simultaneous tasks because of its architecture. Is there anyone
> out there who can tell me why this is or point me in the direction of some
> further reading that explains it.
>
> thanks
>
> Scott
>
>

This is wrong, in my opinion. I've written several interrupt and tasking
systems for MIPS chips, and maintained several others, and I can tell
you that it's more painful than most, due to the strange coprocessor
architecture and minimal instruction set. I found writing interrupt
level debuggers 'challenging', for various arcane reasons. The
coprocessor register scheme is novel, to say the least.

You would think they would have gotten this right, and made it easy,
since the chip was basically designed to run UNIX. It's not like they
didn't have good models to crib from, like, for example, the 68000
architecture. Processor guys seem to have their own logic, based on a
nightmarish view of efficiency, which is usually at odds with the
programmer model. MIPS was also the first RISC chip to be generally
available, so there are oddities associated with that premere status.

One famous perversion with MIPS is the 'branch delay slot', which is a
scheme in which the instruction *after* a branch is executed regardless
of whether the branch is taken. This has to do with the instruction
pipeline. It probably saved 10k transistors, and may have speeded up
branch processing, but caused a world of hurt to future programmers.

One thing that is useful is the ability to profile code and to optimize
branch statements based on the most likely outcome of the branch. Many
tools (including GCC) are designed to take advantage of this.

The best book I've found (actually, this may be out of date, I've been
out of touch with MIPS for about 4 years now) is "MIPS RISC
Architecture" by Gerry Kane and Joe Heinrich. 

-- 
Regards,
   Robert Monsen
"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.

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