Date   : Sat, 04 Dec 2004 14:43:06 +0100 (GMT)
From   : Johan Heuseveldt <johan@...>
Subject: Re: ...at line ___

Hi,

On Thu 02 Dec, Sprow wrote:
> In article <Marcel-1.53-1202201828-06cxSBG@...>,
>    Johan Heuseveldt <johan@...> wrote:
> 
> > > The next section of code is at &79FE using 50 bytes giving &7A30.
> >
> > Ah, that could be a clue. The code is wrapping a 256 bytes page border.
> > Some opcodes take that into account, some don't.
> > So you need to do some checking on that. 
> 
> I think you're thinking of JMP(indirect) which on the NMOS 6502 takes a low
> byte from one page and the high byte from the 0th byte of the same page the
> opcode was in rather than the next when assembled over a page boundary.
> 
> JMP absolute shouldn't be an issue,

I vagely remember a problem using zero page addressing, where two zero
page locations contain the abs addr I want to refer to.

Then using zp addressing with ADC or SBC or maybe something else, and using
one of the X/Y indexes, the abslolute address wrapped back to the start of
the same page instead of the next page. Can't remember much of it, but it
happened when dealing with calculations on 16 bits/2 bytes numbers. There
were sets of these, and calculations were done on them, where the locations
of many different sets could be anywhere in RAM. (6502 second processor)

I hope this gives a better clue.



-- 
Johan Heuseveldt <johan@...              >
  aka  waarland

  The best place is a Riscy place
 
The law of the excluded middle either rules or does not rule OK.