Re: Modern, but fully compatible 6502?

From: groepaz_at_gmx.net
Date: Tue, 06 Sep 2022 14:10:37 +0200
Message-ID: <2223688.2JlShyGXDD_at_rakete>
Am Dienstag, 6. September 2022, 14:00:18 CEST schrieb silverdr_at_srebrnysen.com:
> > On 2022-09-05, at 16:51, Jim Brain <brain_at_jbrain.com> wrote:
> >>> But would that also cover the illegal Opcodes correctly? From what I
> >>> understand, those are side effects of the NMOS implementation.>> 
> >> Were they? I always considered them to be a side-effect of the internal
> >> decoder matrix which did not specifically "NOP" the unused opcodes
> >> (which IIRC was done with the CMOS versions), but just bluntly decoded
> >> the individual parts of the opcode, generating corresponding enable
> >> signals.
> >> 
> >> So stuff happened like e.g. loading the accumulator, but also throwing
> >> the X register onto the internal data bus.> 
> > As I recall, a few incomplete decodes enabled two things to bump data onto
> > a bus, which would then boil down to NMOS behavior with 2 outputs pushing
> > electrons to the input.
> Still, you don't always have to go bottom-up and reimplement NMOS physics to
> get the desired behaviour. Top-down approach is OK sometimes too. You
> know.. if it looks like a duck, walks like a duck and quacks like a duck..
> ;-)

It is actually the easiest (and smallest!) way to implement it (except the 5 
exceptions)... just have the exact same decode ROM/Table with the exact same T 
states, and most of those illegals magically work perfectly. The only thing 
you have to patch into it is the weird behaviour related to BA/DMA line, which 
can not be explained by a pure digital model.

-- 

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Received on 2022-09-06 15:03:17

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