Re: How complex is the 6502?

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

In article <2bt96o$ns0@info2.rus.uni-stuttgart.de> zcaa1122@rpool9.rus.uni-stuttgart.de (Wulf Hofbauer) writes:
> In article <CGBr4p.4I4@actrix.gen.nz>,
> David Empson <dempson@swell.actrix.gen.nz> wrote:
> >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.
> 
> The 6502 is a dynamic design, so it doesn't use latches or flip-flops
> for its registers. (This is the reason why cards using DMA must grant
> some cycles to the 6502 frequently so it doesn't loose its register
> contents).

Good point.  The 65C02 does have static registers, and I'd think you
would want to use them in a discrete design.  Can you get a one byte
dynamic memory?  Hey - build them from capacitors and transistors!  :-)

> Something like a 74381 should probably do.

You might need to do something unusual to cope with the 6502's
upside-down carry flag in subtraction.  I don't have a full TTL data
book handy (only pinout diagrams), so I don't know what the 74381 can
do (all my sheet says is "8 functions").
-- 
David Empson                                                               
dempson@swell.actrix.gen.nz                                                
Snail mail: P.O. Box 27-103, Wellington, New Zealand