Date   : Mon, 09 May 2005 21:47:06 +0100
From   : Michael Borcherds <emulators@...>
Subject: Jack Attack & undocumented opcode

Hi all,

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?

&1A22 is the location that is &1A or &EA

&1D46 STA &1A22 ; &EA

&2B42 STA &1A22 ; &1A

cheers,

Mike

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6502Em version 4 | http://www.borcherds.co.uk/murklesoft/riscos/6502em.html