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From: hans@umd5.umd.edu (Hans Breitenlohner)
Newsgroups: comp.sys.atari.8bit
Subject: Re: 1200xl and the cpu and OSe
Message-ID: <1935@umd5.umd.edu>
Date: Tue, 15-Sep-87 18:55:37 EDT
Article-I.D.: umd5.1935
Posted: Tue Sep 15 18:55:37 1987
Date-Received: Thu, 17-Sep-87 06:30:42 EDT
References: <870907174221.284781@DOCKMASTER.ARPA>
Reply-To: hans@umd5 (Hans Breitenlohner)
Organization: University of Maryland, College Park
Lines: 25


The XL and XE systems, as well as some 400s (maybe all), have a 6502C cpu 
chip.  Its main distinction is the fact that it can tri-state the address
bus, a feature which none of the standard 6500 family chips have.

I recall reading that there is a chip in the 65c00 family which does have
the tri-state feature (maybe 65c11), but have no information as to whether
its pinout is compatible with the Atari chip.
Being somewhat non-standard, it might also be considerably more expensive
or harder to find.

As far as I know there are no programming differences between the 6502 and
the 6502c.  

As far as souping up the XL with a faster cpu, I have two concerns:
1. coordinating a faster cpu and the standard Antic might be quite a
challenge.  You would need memory which can run at slow and fast cycle 
rate, depending on who is talking to it.  And while there is a fair amount
of slack in memory timing with the current design, a significant speedup
might cause much difficulty.
2. The i/o bandwidth is about 1920 bytes/second, and unless you main goal is
to compute prime numbers, or do Mandelbrot pictures, any speedup is likely
to make the i/o restrictions seem even more painful.  Of course you might
have disks on an MIO or similar device, but then you need to re-visit the
first point I made.