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

[CUTjblakeney@sentex.net (Jeff Blakeney)]

On Tue, 22 Oct 2002 15:32:41 GMT, Andy McFadden <fadden@fadden.com>
wrote:

>Dosius <steve@dosius.zzn.com> wrote:
>> 
>> I wonder how DOS detected a sub-48K system then...
>
>Write a byte.  Read it back.  Write a different byte.  Read it back.
>
>Pick the bytes carefully (not $ff or $00).  $55/$AA might be a good combo.
>So long as the results are semi-stable or semi-predictable, you should
>pretty much always get the right answer.

You could also use a sequence of bytes like $00, $01, $02, $03 and so
on.  Write a byte, read it back, increment the byte by one and write
it, read it back and so on.  The chances of random values being
generated by electronic noise in that fashion is quite small and the
more digits in your sequence, the less likely it becomes.

-- 
 Jeff Blakeney - Dean of the Apple II University in the
                 Apple II Community on Syndicomm.com
 CUT the obvious from my address if you want to e-mail me