Path: chuka.playstation.co.uk!news
From: "Craig Graham" <c_graham@hinge.mistral.co.uk>
Newsgroups: scee.yaroze.freetalk.english
Subject: Re: S*ga Dreamc*st
Date: 6 Aug 1998 15:36:39 GMT
Organization: PlayStation Net Yaroze (SCEE)
Lines: 43
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James Russell <James_Russell@scee.sony.co.uk> wrote in article
<35C9787F.3CD3A9AA@scee.sony.co.uk>...
> > Again, it's a matter of OS support. An MMU is useful if you've got the
> > support for it already there. There's no reason why a game has to be a
single
> > process, and in fact, there's no reason why the OS has to be a single
process
> > either.
> 
> Why do you need an MMU for multi-tasking? The Amiga didn't have one, and
neither did the Mac. You
> need an MMU for memory _protection_, and <opinion> I don't believe that's
important enough to
> developers </opinion> to warrant adding the extra silicon in for it.

The Atari ST didn't have one either, and that has two versions of Unix
available on it (very stable as well).
MMU's are used to do 3 things (generally):
1) Process isolation. Stop one process stomping on another process' memory.
Not a truly good thing
in a game where you want performance over technical elegance (overheads
incurred by task swapping
are quite large if you're swapping fast enough to load balance correctly).
2) Virtual memory. Page table management for VM, not really applicable
without a fast writable storage
device.
3) Make a non-contiguous address space appear as a linear space. Not
obvious, but it actually helps
make hardware cheaper if you can just take a single address line as a
device select to a single chip.
You'll get a funny memory map though, so you can use the MMU to make it
look linear to the programmer.

I only mention 3 because I was asked the question in a job interview and
didn't know 3 (the guy interviewing
was a real Uni. Prof type egghead, who lectured me about it afterwards).

> James

Craig.