[Coco] "Reading" non-readable bytes with PEEK vs ZBUG

[jdaggett at gate.net]

On 22 Jan 2008 at 17:13, Robert Gault wrote:

> jdaggett at gate.net wrote:
> > On 22 Jan 2008 at 8:17, Darren A. wrote:
> > 
> > 
> >>Changing the JMP instruction at $B754 to an equivalent LBRA will
> >>cause PEEK to return a different value for address $FFA0.
> > 
> > 
> > has this been verified?
> > 
> > I can't verify a mechanism as to why the ACCB would change inside
> > the 6809 just because the next opcode byte changes. There  has to be
> > another reason or mechanism. If this were true then it should happen
> > in other places in the code and would result in erronious exection.
> > 
> > james
> > 
> 
> Don't forget this depends on floating bits. There are very few bytes
> that float this way on a Coco.
> 
Floating or stable the databuss is latched on the falling edge of the E clock . 
So what ever the state of the data buss is 10nS after the falling edge is 
irrelavent. It can change 10 or 15 times and that will not afftect what is 
loaded into the accumaltor.

james