Re: How complex is the 6502?

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

In article <2btdaqINNa6a@iraun1.ira.uka.de> borchert@ira.uka.de writes:
> In article <CGBr4p.4I4@actrix.gen.nz>, dempson@swell.actrix.gen.nz (David Empson) writes:
> |> 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.
> 
> Must be much less. The 68000 was called 68000 because it contained
> about 68000 transistors... The Z80 has, to my knowledge, 8000, and
> has much more registers, etc. 2500 seems to fit better. That would
> evaluate to about 800 gate equivalents, and should fit onto three
> Doppeleuropa cards (160x233.4 mm�).

I've found some information about gate equivalents for Motorola
microprocessors: the 6800 has 1,367 gates; the 68000 has 12,667.

I wasn't able to locate anything for the Z80 or 6502.  My educated
guess is that the 6502 is comparable to the 6800, or would have
slightly fewer gates (a few less registers, and a similar number and
complexity of instructions, but with more complicated addressing modes).

I think these numbers include the microcode ROM.

> |> 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!  :-)
> 
> Much faster. Say, about 28 MHz (for easy sync with the Apple's video
> circuitry).

I found some information in "Apple Thesaurus" by Aaron Filler, which
says that one of the reasons that microprocessors went to NMOS was that
it was a major improvement in speed over small scale TTL (at the time).
The 8008 used TTL, but the 8080 and 6800 used NMOS.  Apparently the
8008 was about ten times slower than the equivalent TTL logic chips
(probably medium scale TTL logic, such as single-chip ALUs, registers,
etc.).

With modern TTL or CMOS, I don't see a problem making it at least as
fast as a real 6502.

How "low level" do you want to go?  Would you accept using a
single-chip ALU, or would you want to build everything from logic gates?
-- 
David Empson                                                               
dempson@swell.actrix.gen.nz                                                
Snail mail: P.O. Box 27-103, Wellington, New Zealand