Re: How complex is the 6502?

[zcaa1122@rpool9.rus.uni-stuttgart.de (Wulf Hofbauer)]

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).

>
>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).

Something like a 74381 should probably do.

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

Is the 6502 microcoded? This would be a surprise to me, though I frankly
don't know.

- Wulf

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 ______________________________________________________
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| Wulf Hofbauer | zcaa1122@rpool1.rus.uni-stuttgart.de |
|                                                      |