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Re: 6502 illegal opcodes questions



mdj wrote:
John Selck wrote:


Best case: 21 vs 11 clock cycles (almost 2x speed with illegal)
Worst case: 25 vs 15 clock cycles (still 1.67x speed)


Nice... It would be amusing to see a game title on the Apple II+
running faster than on the IIe :-)

Of course, the point remains though that your would never do this on
the Apple II series, as you'd limit your target market to the earliest
machine, rather than just the lowest common denominator of all machines
(64k, 6502 code that's documented and unbuggy)

If I were coding on a different 6502 platform, I might consider using
these.

But then, I'm very much a 65C02 coder these days, as pretty much
everything I write is intended for a IIe/IIc and wouldn't run on an
older Apple II anyway.

Michael will probably shoot me down for saying this, but when I mean
BRA, I write BRA. It's clearer code, and I really wish that anyone who
writes code specifically for the earlier processors would comment their
unconditional branches accordingly.

No shooting from here.  ;-)

I agree that "unconditional" branches should always be commented so,
even when they are written as conditional.  I have found only a very
small fraction of unconditional jumps that cannot be perfectly safely
with the right conditional jump.

-michael

Parallel computing for 8-bit Apple II's!
Home page:  http://members.aol.com/MJMahon/

"The wastebasket is our most important design
tool--and it is seriously underused."