Re: [stella] "Illegal" opcodes

Subject: Re: [stella] "Illegal" opcodes
From: "Andrew Davie" <adavie@xxxxxxxxxxxxx>
Date: Mon, 19 Jun 2000 15:17:08 +1000
I imagine they designed the instruction set (and a lovely one it is, too)
many moons before actual hardware was fabricated.  Probably to a rather
formal specifications document.  And when the design came along, they
encoded bit fields in the op-code to do certain things.  And it may just
have turned out that because of this choice, the hardware was capable of
doing "illegal" things, too.
But, their design spec says nothing about these other things - they designed
the hardware to the specs, and it meets the specs.  I expect books/manuals
were printed long before actual hardware was available.  And with the
possibility of other manufacturers making chips to those specifications, you
can't guarantee they will implement things on a hardware level the same way.
They COULD, for example, have made all "illegal" ops work like NOPs, and
still meet the specs.  Undefined behaviour is undefined behaviour.  So
there's no way you "should" use those op-codes" - its just a lucky
coincidence that a majority of 6502 variants work the same way.
I believe the NES (please correct me if I'm wrong, someone) did NOT work
with these op-codes - I am certain I would have used them (particularly TXY)
had they been available.

----- Original Message -----
From: "Russ Perry Jr" <slapdash@xxxxxxxxxxxx>
To: <stella@xxxxxxxxxxx>
Sent: Monday, June 19, 2000 2:55 PM
Subject: Re: [stella] "Illegal" opcodes

> At 12:19 AM -0400 6/19/00, Erik Mooney wrote:
> >>I've always wondered why they didn't just tie off any used opcodes as
> >>NOPs to actually MAKE them unuseable.
> >As I said, because tying them off as NOPs would increase complexity and
> >cost for an actual _reduction_ in chip functionality, which is rather a
> >bad idea on the engineering level.
> True, you said that; sorry I didn't recall that while responding.  That
> does make some sense.  BUT!
> >I'd still maintain that the chip designers knew exactly that those
> >operations would result from those opcodes and intentionally left it
> >that way.
> Then why not document them?
> I imagine they may have planned to announce new opcodes in the next
> release?  :-)
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