Re: ZipGS question

[Jeff Blakeney <CUTjblakeney@sentex.net>]

On Mon, 03 Mar 2003 11:50:07 GMT, Rubywand <rubywand@swbell.net>
wrote:

>     For sure, the extra 1MB thing is a source of confusion for ROM 3
>owners. I have never owned a ROM 3 and don't know one way or the other.
>However, Supertimer is a good source. If he says adding the extra 1MB
>(going from 7MB to 8MB on the card) is a good idea, it probably is.

I think that has to do with having to have complete banks of memory
installed in the card.  Most memory expansion cards for the IIgs with
four banks of memory can only be set up in three configurations.
There are some 4 MB cards that can only be set to 1 MB, 2 MB or 4 MB.

With a ROM 3 IIgs and an 8 MB memory expansion card, the ROM 3 will
only ever use 7 MB of the memory on the card but the card design
probably requires that 8 MB be installed as 7 MB is an invalid
configuration.  However, I'm just guessing here.

>     In more general terms, I've never liked the idea of GS Expansion
>Memory cards which add more than 4MB. Apple flatly states that 4MB is
>the limit. Sure enough, that is the max which is addressable via the
>regular address lines.

I've been running my main IIgs (previously a ROM 1 and now a ROM 3)
with 8 MB for years without any real problems.  In fact, my main
machine currently has both a RamFAST and an Apple High Speed SCSI card
installed and I don't think I've turned off DMA on either one of those
cards and can't think of any problems I might attribute to having 8 MB
of RAM installed.

>     The method for addressing memory beyond 4MB on the cards is a
>kludge; and, users are constantly experimenting to discover whether
>or not some system problem is related to non-support for the extra
>RAM.

I wouldn't say it is a kludge.  If I remember correctly, Apple said
that the max memory that the IIgs memory expansion slot can handle is
4 MB is because they only put address decoding circuitry on the
motherboard to handle decoding 4 banks of RAM.  This is why, if your
memory card uses those bank select lines, you are limited to 256 KB,
512 KB, 768 KB or 1MB if using 256 Kb memory chips or 1 MB, 2 MB, 3 MB
or 4 MB if using 1 Mb memory chips.

If the memory card doesn't bother using those bank select lines and
instead decodes the memory address directly from the address lines
then there is no reason why the memory expansion connector cannot be
used to support up to 16 MB of memory.  Apple does say that it
supports ROM cards and ROM disks and these are in a range outside of
the area support by the bank select lines so I believe this is a
prefectly legitimate way to do things and therefore is not a kludge.

However, my memory may be way off here and there are probably some
minor details that I've skipped for simplicity so don't take what I've
said as being the absolute truth.  I'm just stating my opinion based
on my experience and what I can remember.

-- 
 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