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 - 20:54:16 BST