Date   : Tue, 10 May 2005 07:16:21 +0100
From   : Michael Borcherds <emulators@...>
Subject: Re: Jack Attack & undocumented opcode

"bill.carr2004" <bbcmicro@...> wrote:

> 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.
> 

Curious - when I save a snapshot out during the game I get:

1A1F 68    PLA
1A20 51 76 EOR (&76),Y
1A22 1A    NOP?
1A23 91    STA (&76),Y

Ah, I've also found:

27B6 LDA #&51
27B8 STA &1A20
27B6 LDA #&76
27B8 STA &1A21

So I guess the opcode &1A is intended as a NOP. [but will mean that Jack
Attack won't work on a Master 128]

cheers,

Mike

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