Date   : Mon, 09 May 2005 22:13:32 +0000 (GMT)
From   : "Thomas Harte" <thomasharte@...>
Subject: Re: Jack Attack & undocumented opcode

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> I've just been trying to work out why Jack Attack by Bug Byte doesn't work
> in 6502Em [corrupted graphics].
> 
> I've tracked it down to the fact that I run a hybrid processor - a
> compromise between 6502 in the BBC and 65C102 in the Master 128.
> 
> Jack Attack appears to rely on opcode &1A doing nothing. [it is described as
> NOP in 64doc.txt and 6502Em was treating it as INA (increment A)]
> 
> The odd thing is that Jack Attack appears to have code that changes the
> opcode to/from &1A to &EA (NOP).
> 
> So the question is: does opcode &1A actually do anything different to &EA on
> a 6502?

ElectrEm (my Electron emulator) emulates a vanilla 6502 and treats &ea and
&1a identically. It runs Jack Attack without glitch, and also passes every test
in the 6502 Test Suite. The emulator has been living on Sourceforge for a bit
now (which is embarassing since my work on it has always been sporadic) and
the CPU source can be seen at http://cvs.sourceforge.net/viewcvs.py/electrem/SRC/6502core.cpp?rev=1.1&view=markup
should you be interested in ElectrEm's implementation of any other operations,
not that I can think why you would be.

At an absolute guess, maybe Jack Attack is using that address not just as an
opcode but as a flag for something else?

-Thomas

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