ehs@jumbo.dec.com (Ed Satterthwaite) (09/09/87)
Several recent postings have raised questions about CPU upgrades for the 8
bit Ataris. The obvious candidates are the 65C02, a CMOS version of the
6502 with some minor extensions, and the 65802, a version of the 65816
(Apple IIgs CPU) that is pin compatible with the 6502. One source of both
is Jameco Electronics; see any issue of Byte, etc., for their ads. The
most recent prices I've seen are
65C02: $8.95
65C802 $19.95
To keep things in perspective, Jameco's quantity-one prices for the 68000
range from $11.95 (8 MHz) to $17.95 (12 MHz); i.e., neither 6502
replacement is much of a bargain in terms of processing power, especially
considering the relative amount of support available.
For a while, I thought that upgrading my 800 with one of these would be
really nifty. I think I was wrong. Maybe I'm overlooking something, but
here's how I see things:
(1) Software
The 65C02 includes a few new opcodes that plug a number of annoying holes
in the 6502 instruction set. The MAC/65 assembler for the Atari supports
these extensions, so anything for which I have assembler source could be
(re)written to take advantage of them. But most of the software I care
about was either bought on the commercial market or written in Action. No
gain there (or for Basic either).
To my knowledge, the only commercial assemblers that support the
65802/65816 extensions run on the Apple II line of machines. So the
situation there is even bleaker.
In either case, I couldn't give or sell my software to owners of vanilla
Ataris -- or to Apple IIc/e/gs owners either.
(2) Speed
I just glanced through the Rockwell data book (only one at hand covering
the 6500). The tabulated cycles/instruction are essentially identical for
the R6502 and R65C02. In fact, their 65C02 is a cycle *slower* on decimal
arithemetic (as in Basic floating point). I think I recall reading that
the WDC (= NCR?) 65C02 did shave a cycle here and there, but that the
speedups were removed in the WDC 65802/65816 running in compatibility mode,
presumably so that old timing loops would work. No significant gain here
without changing the clock speed.
But changing the clock speed will almost certainly cause major problems
with everything else. The 1.79 MHz. crystal was chosen to match a color
TV standard, not because the processor won't go faster. In fact, at least
some 800's were built with 6502B's, which are rated to run at 3MHz.
(3) Power
The new CMOS chips do use less, but I don't believe that the old NMOS
6502 is consuming any significant amount of the power used by an Atari
8-bitter.
(4) Wider Bus
The 65816 brings out more address lines and thus visions of big memories
without bank switching, etc. Someone who chooses to stuff all this in an
old Atari box might have a lot of fun and learn a lot, but he ought to
understand that he is basically engineering a whole computer system, and
probably much of its software, from scratch.
I'd appreciate comments and counterarguments; I'd still love an excuse to
try the upgrades.
Ed Satterthwaite
Arpa: ehs@src.DEC.COM
UUCP: ...!decwrl!ehsrrwood@lotus.waterloo.edu (Roy Wood) (03/27/90)
So, what gives on the subject of hardware mods/ CPU upgrades? I've heard a lot of hype about the T-816, and a lot of other conflicting information. What's the opinion of people who own such a beast? Is there software available to really take advantage of the investment, or should I just give up on the 8-bit and get new machine? -Roy Wood (rrwood@lotus.waterloo.edu)