[CCTECH] Interesting tidbit on 6502

From: Richard Erlacher <edick_at_idcomm.com>
Date: Thu Jun 6 21:43:32 2002

One of the things that kept the 6502 cheaper than the average microprocessor
was that there weren't any such changes of which I was made aware until the
CMOS parts were released. These had several different instruction sets, for
some odd reason, mostly thanks to Rockwell, I think, but the unimplemented
opcodes were, IIRC, implemented as NOP's. New features included such things
as waits on write cycles, and no bogus reads on indexed instructions. This
fouled up some hardware/software interactions.

Dick

----- Original Message -----
From: "Ben Franchuk" <bfranchuk_at_jetnet.ab.ca>
To: <cctalk_at_classiccmp.org>
Sent: Thursday, June 06, 2002 7:54 PM
Subject: Re: [CCTECH] Interesting tidbit on 6502


> Richard Erlacher wrote:
> >
> > One of the more interesting features of the 6502 is that when you're
looking
> > at the data bus, it shows you what last was on the bus in those cases
where
> > there's nothing present to drive the data-in bus. This will give you
> > information useful in figuring out what is going inside the chip, and,
that's
> > what gave me the clues that convinced me that the reason the 6502 is so
cheap
> > on silicon is that it doesn't use counters for its registers, but, rather,
> > uses simple gated latches and uses the ALU to operate on the addresses
during
> > phase-2 while operating on the data during phase-1.
> >
> > If you look at what's required to build a synchronous counter large enough
to
> > support the simple register set in the 6502 you'll see that the saved
gates
> > are sufficient to warrant its design in exactly that way, and that it
would
> > yield a significant savings in silicon. It allows you to use a relatively
> > complex ALU, together with a register set that's essentially a small RAM
array
> > with an instruction set that never operates on two registers in a single
> > cycle. If you build the PC, the address bus registers, the SP, the two
index
> > registers, and the accumulator as 8-bit registers, it's easy to see why
one
> > would do things that way. I'm not sure anybody has ever taken a really
close
> > look at what happens when each possible opcode is fed to the 6502 as the
first
> > instruction after a reset and then recorded what the CPU does with it
right up
> > to the next SYNC, signalling that a new opcode is being fetched, but it
might
> > be a useful extension on what's been done. Of course, the 6502 is of
little
> > interest to persons planning any practical endeavors, so this fits
squarely
> > under the aegis of this forum.
>
> I think that the 6100 (PDP-8 on a chip) fit this model better.
> 8 bit cpu's often used random logic thus don't cares and unimplimented
> opcodes
> could change actions between cpu mask revisions. Other than the 6800
> HCF
> instruction ( Halt and Catch on Fire ) most undefined instructions
> are mostly harmess.
>
> --
> Ben Franchuk - Dawn * 12/24 bit cpu *
> www.jetnet.ab.ca/users/bfranchuk/index.html
>
Received on Thu Jun 06 2002 - 21:43:32 BST