Re: More info A curious small circuit baord from a "]["?

[Jonathan Schattke <wizwom@ripco.com>]

Jeff Blakeney wrote:
> 
> On Wed, 02 Feb 2000 02:39:43 -0600, Jonathan Schattke
> <wizwom@ripco.com> wrote:
> 
> >greg@apple2.com wrote:
> >>
> >> In article <38966FD9.CE4156EC@ripco.com>,
> >> Jonathan Schattke <wizwom@ripco.com> wrote:
> >> >Bart wrote:
> >>
> >> >> I am curious: if one pulled the 64k RAM on the motherboard, and
> >> >> replaced with 256k can it be addressed or is that a waste of time?
> >>
> >> > Nope, it's a waste of time.  The hardware would not be able to access
> >> > any more RAM.
> >>
> >> Would it be a suitable replacement for when the correct RAM chips go bad
> >> and the proper chips can no longer be obtained?  If it can only access a
> >> portion of the RAM, can it still access that portion reliably?  Provided
> >> suitable pinouts or adapters?
> >
> >Yes, the 64K DRAM used 8 Rows and 8 collumns, the 256K used 9 of each.  The
> >extra address line should be grounded, then it will work fine.
> 
> Well, depending on how you want to access the RAM above 64k, it is
> possible to put 256K chips in place of the 64K ones and get more
> memory.  After all, all you need is two more address bits which you
> could handle with the annunciator outputs and some wires.  That way
> you can write a program that could select which of the four 64K banks
> to use at any time.
> 
> Or you could just hooked up a couple switches so that you can manually
> select which bank of 64K to use.  This would be handy if you just want
> 64K of RAM and discover that the low 64K of your 256K chips has bad
> RAM in it.
> 
> However, it would be best if you were to try to emulate the standard
> Apple memory expansion so that programs that support that method could
> access the extra RAM automatically.  This would require extra
> circuitry but it could be done.

It would be nice if that would work, but you'd have to make up a circuit to go
theough each of the 4 banks for refreshing, otherwise you would lose the memory
to charge leak.  Now, you're not talking a very complex circuit, but it's a lot
more difficult than I expect that greg was looking for.

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