Re: How complex is the 6502?

[dempson@swell.actrix.gen.nz (David Empson)]

In article <CG99LM.Enu@IRO.UMontreal.CA> guertinp@IRO.UMontreal.CA (Paul Guertin) writes:
> A question I've been asking myself, lately:
>  
> How complex is the 6502? For example, what would a 6502 emulator made
> out of TTL logic look, how much would it cost, and how much power
> would it need?

The 6502 is probably one of the simplest microprocessors.  I seem to
recall reading somewhere that it contains about 25,000 transistors.

Even a 6502 made out of standard logic chips would be quite big!
Think what you need:

For the registers: 6 read/write 8-bit latches, at least two of which
have an auto-increment facility.  The processor flags require a
further 6 or 7 flip-flops.  There is at least one temporary register,
and a few buffers for the address and data buses.

There are standard TTL chips which could be used to build the ALU (at
least, the adding part - you might need additional hardware to support
subtraction, shifting and other operations).

You need an instruction decoder (probably microcoded, so you could do
it with a bipolar PROM and a few support chips).

There is also a lot of logic required to tie it all together.

Do you want this to work as fast as a real 6502, or just a slow
working model?  Making sure it had the same behaviour for the
unimplemented instructions might be tricky!  :-)

> Are there any books about the internal architecture of the 6502? I'm
> not talking of the assembly programmer's model found on the data
> sheets, but of a real, down-to-the-silicon description.

There are block diagrams of the components, but I can't think of
anything more detailed.
-- 
David Empson                                                               
dempson@swell.actrix.gen.nz                                                
Snail mail: P.O. Box 27-103, Wellington, New Zealand