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From: rbt@sftig.UUCP (R.Thomas)
Newsgroups: net.micro.apple,net.micro.68k
Subject: Re: 68000 vs 6502 (!) (65816 <-> 6502 compatibility)
Message-ID: <617@sftig.UUCP>
Date: Mon, 4-Nov-85 00:28:09 EST
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Posted: Mon Nov  4 00:28:09 1985
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>                I was thinking of putting a 65816 on a board and
> running it at 4Mhz, or faster if possible and using the 6502 as an
> i/o processor.  I would like to be able to let the 65816 handle the
> video display, because it's got such neat tricks built in, like move
> a block of memory, etc, but I can't think of a neat way to do this without
> slowing it down to 1Mhz.  Perhaps a wait state while the 65816 was
> addressing low memory would do the trick (slowing it to only 2Mhz).  
> The disk drives would have to be handled by the 6502 (it would be 
> preferable, too!).
> 
> ==================
> David L. Smith

How about putting a cache memory on the board too.  (about 256K should
do the trick, I would think (:-), but seriously, at today's memory
prices, why not!?)  There are boards available that do just that with
a 65C02 running at about 6MHz (They claim a 3 to 1 speedup, anyway)
But they use high-speed static ram (about 8K, I think I remember
reading) for the cache.  And indeed, they do slow down when accessing
the video memory and the IO block between C000 and CFFF.  Still, why
not put a *big* chunk of memory at addresses above (say) 2MB, that
can be accessed fast by the 65816 and only make it slow down to
access the low order 128K that it shares with the 6502.  The logic of
putting it above 2Meg is to get it out of the way of things like
RAMWorks or Slinkey, if some way can be worked out to allow the 65816 to
access that sort of memory directly.  (Probably not, probably have to poke
soft switches just like everybody else.  But it's worth a try! I say.)
Does anybody know enough about the IIe MMU chip to say one way or
another?

I think it has possibilities!

Check the latest issue (or maybe the one before) of Call A.P.P.L.E
for an article about the speedup card.  I'll bet the people who make
that card could be talked into bringing out a 65816 version with only
a little prodding.  (Unfortunately, I forget the name of the people
who make the card, and I cant find the article right now.  I'll post
that info though, when I can find the article.)

Rick Thomas
ihnp4!attunix!rbt