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

["Redskull" <redskulldc@yahoo.com.au>]

"Dosius" <steve@dosius.zzn.com> wrote in message
9307085f.0210211337.68cea360@posting.google.com">news:9307085f.0210211337.68cea360@posting.google.com...
> 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. :)
>
> - The Usotsuki http://sourceforge.net/projects/dapple

Assume that you mean on a 6502/65C02 ?

Have a look at the 6502 hardware manual to be found at:
http://highgate.comm.sfu.ca/~rcini/classiccmp/my_docs.htm

Both the Rockwell and CSG (Commodore) manuals are
there.

6502 data lines are tri-state devices with no internal pull-ups,
which means if nothing on the bus responds to a chip enable
signal, the inputs will be undefined. = Random Garbage.

In reality, other devices even though they may not be enabled
will load the data bus in one direction or the other.

Someone mentioned the video data, and also the data bank register
on a 65816 being "phantom read".

On a Z80 which has internal pull-ups on it's data lines, you will
get $FF on any memory or I/O address which has no device
connected.

Cheers,
Red