Date   : Mon, 09 May 2005 23:32:38 +0100
From   : "bill.carr2004" <bbcmicro@...>
Subject: Re: Jack Attack & undocumented opcode

<snip>
>
> 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
>

>From the brief look at the code

at &1D46 - &1A20, &1A21 and &1A22 are all changed to &EA for NOP
at &24B2 - &1A20, &1A21 and &1A22 are changed to &4C, &92, &1A to make JMP 
&1A92

So I don't think that it matters that &1A22 becomes &1A, because it is part 
of the JMP instruction.