Re: What do you get when you access RAM that isn't there?

[david@uow.edu.au (David Wilson)]

Tilghman Lesher <tilghman@nospam.the-tilghman.com> writes:

>Andy McFadden wrote:

>> Dosius <steve@dosius.zzn.com> wrote:
>>> Let's say you have a 32K system and you access byte $8000 of RAM.
>>> Obviously there's nothing there, so what happens?  I'm just curious,
>>> but perhaps I can see what happens when one uses a 32K, 16K, or even
>>> 4K Apple ][, just for the hack value. :)
>> 
>> If memory serves, it was a pattern of largely 00 and ff bytes, with
>> some semi-random other junk.  I can't remember if the results were
>> stable.

>On the IIgs, if a bank of memory doesn't exist, the bank number is
>repeated in every memory location.  For example, bank 40 doesn't
>exist on my 2MB ROM 01, so listing 40/0000.0010:

>40/0000:40 40 40 40 40 40 40 40-@@@@@@@@
>40/0008:40 40 40 40 40 40 40 40-@@@@@@@@

That makes sense, the 65816 drives the bank byte on the data bus during
the start of the cycle, then nothing is selected to drive the data bus
while the 65816 reads from it so because of capacitance the data bus
acts like a one byte dynamic memory and you read the bank number.
--
David Wilson  School of IT & CS, Uni of Wollongong, Australia