Re: 6502 illegal opcodes questions

["Michael J. Mahon" <mjmahon@aol.com>]

Bruce Tomlin wrote:
> In article <1147397570.212608.111710@i40g2000cwc.googlegroups.com>,
>  "mdj" <mdj.mdj@gmail.com> wrote:
>
>
> > John Selck wrote:
> >
> >
> > > Some illegals are quite useful, like SHX/SHY which in some cases can be
> > > used as STX absolute,Y / STY absolute,X if you use the right. Also you
> > > can do a CPU test and use different code on different CPUs. The demo
> > > "Oneder/Oxyron" does this. It works on 6502 with illegal opcodes, and on
> > > 65816 it uses a slower but illegal-free code.
> >
> > This is reasonable on the C64 where all machines were fitted with
> > 6502's, and in the case of a 65816 it's accelerated, so a slower code
> > section is acceptable.
> >
> > The Apple II series is essentially 3 platforms, and 3 CPU generations,
> > all backwards compatible.
>
>
> And of course you've neglected to mention the other major 6502 computer 
> series of the era, Atari.  They never used anything but a standard 6502 
> core (but with an extra pin added to make DMA easier), so the 65C02 and 
> 65816 were never an issue.  In fact, Atari's 6502C with the extra signal 
> meant that you _couldn't_ just drop a 65C02 into the same socket.

That was DMA for video RAM acces, right?  If so, they just didn't get
how trivial it is to share RAM with a 6502--it only accesses RAM during
half the clock signal!

> This was even less of an issue on the 2600, which always used a 6507, 
> which is a standard 6502 die in a reduced pin count package.  Quite a 
> few games used illegal opcodes to reduce cycle count so that they could 
> do things that were otherwise impossible with its clock-locked 1-D 
> graphics chip.

It's kind of a self-fulfilling prophecy.  If enough people write
applications that depend on undocumented behavior, then that behavior
becomes a "feature" that cannot be changed in future versions--usually
preventing useful improvements.

(Not that I think of the 65C02 as much of an improvement.  All the
changes made in the 65C02 were what is known technically in the
computer architecture community as "mouse nuts".  ;-)

> Oddly enough, the Apple II is where I heard most about people using 
> illegal opcodes and side effects like JSR ($xxFF) back in the day, as a 
> way to obfuscate copy-protection code.  Like I cared, because I had a 
> TRS-80 (then a CoCo, and a 128K Mac), and most of the Z-80 undocumented 
> opcodes were obvious, even when they weren't useful.  And none of the 
> 6809 undocumented opcodes were useful at all, except there was one that 
> would do an SWI (equivalent to 6502 BRK) through the FIRQ vector and 
> another that would jump through the RESET vector.

There were a few cases of using undocumented behavior to obfuscate, but
it was never much more than a curiosity on the Apple II platform.

-michael

Music synthesis for 8-bit Apple II's!
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