Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!zaphod.mps.ohio-state.edu!van-bc!rsoft!mindlink!a344
From: Tom_Klok@mindlink.UUCP (Tom Klok)
Newsgroups: comp.sys.atari.8bit
Subject: Re: 65C02?
Message-ID: <5042@mindlink.UUCP>
Date: 7 Mar 91 03:29:54 GMT
Organization: MIND LINK! - British Columbia, Canada
Lines: 28

> gdtltr@brahms.udel.edu writes:
> 
>    Yes, I believe that this is usually possible. Note, however, that
> the Atari 8-bits use a modified 6502, the 6502C. I think it does
> something strange with interrupts to facilitate the use of gadgets
> like the Antic chip. Therefore, I don't believe you can just pop a
> 65C02 into an Atari and have it work properly.

Unless you have a 400 or 800 with one of the old, old CPU boards, that is.
Atari originally used the normal 6502, but soon moved some of the TTL logic
onto the CPU chip (since they were having them custom produced anyways, I
believe).  If you compare an older and newer CPU board side by side, the
difference is obvious... at least two less TTL chips on the newer ones, over on
the side where the colour-clock delay trimmer is.

I swapped an older CPU card into my 400, and now I've got a 3MHz R65C02 in
there.  Running at only 1.79MHz, of course, but it stays nice and cool.

There are some minor disadvantages, basically because of the bug fixes.  The
JMP ($xxFF) bug is fixed, illegal instructions now map to either new
instructions or NOPs (which can throw off some programs expecting nasty tricks
to work), and I believe a few instructions execute a clock cycle quicker, which
can throw off super-critical timing loops.  The added instructions are quite
nice, but since using them makes your code nontransportable, the thrill wears
off quickly.  There's not a lot of point to the upgrade, really.

Tom Klok
a344@mindlink.UUCP