Re: GS ram expansion

[dempson@actrix.gen.nz (David Empson)]

Greg Andrzejewski <pressyourluck85@yahoo.com> wrote:

> I'm looking to add more ram to my ROM 1 gs. Right now I have the standard
> 1mb Apple IIgs ram expansion card 670-2500-A. What cards are available for
> upgrading ram above 1mb?

There are a lot of third party memory expansion cards for the Apple IIgs
which support more than 1 MB of RAM.  Apple never made a card larger
than 1 MB.

The memory expansion slot was designed to allow simple implementation of
a memory expansion card which uses 256Kx1, 256Kx4, 1Mx1 or 1Mx4 RAM
chips, with refreshing and most of the address decoding handled by the
motherboard.  Following the defined standards, a card may provide 1, 2
or 4 "rows" of RAM.

If the card uses 256Kx1 or 256Kx4 chips, it may offer a total of 256 KB,
512 KB or 1 MB of RAM.  (The Apple card falls into this camp - it uses
256Kx1 RAM chips.)

If the card uses 1Mx1 or 1Mx4 chips, it may offer a total of 1 MB, 2 MB
or 4 MB of RAM.

If the card uses 256Kx1 or 1Mx1 chips, they must be installed in groups
of eight.  If the card uses 256Kx4 or 1Mx4 chips, they must be installed
in pairs.

A card which follows one of the designs described above will be fully
compatible with the IIgs, including being able to support direct memory
access (DMA) to/from I/O cards such as SCSI cards.

Some cards go beyond the defined standards, using extra circuitry to
decode the address further and allow more options.  One such mechanism
allows the card to offer almost 8 MB of RAM, but this introduces
compatibility problems, notably that DMA doesn't work reliably.  (The
RamFast SCSI card does some special tricks to work around this, allowing
it to support DMA with some 8 MB RAM cards).

The absolute limit is 8 MB of RAM, including the "fast" RAM on the
motherboard (128 KB for ROM 00 and ROM 01, 1 MB for ROM 3) and any RAM
on the memory expansion card.  The motherboard also has 128 KB of "slow"
RAM.

In my ROM 3 IIgs, I have an Applied Ingenuity card - I forget its model
name.  It supports 1, 2 or 4 MB using 1Mx1 DRAM chips, installled 8 at a
time.  (I have 4 MB in it.)

In my ROM 1 IIgs, I have an OctoRAM card (at least I would if I could
find the computer - I haven't seen it for several years, which annoys me
because it is my original "Woz" machine).  It supports 1, 2, 4 or 8
30-pin SIMMs, which may be either 256 KB or 1 MB (all must be the same 
size).  If all eight slots are used, the card isn't DMA compatible, but
it is if four or fewer are installed.

I've also seen another RAM card (from Quantum Computing?) which supports
1, 2 or 4 30-pin 1 MB SIMMs.

> Could the chips on the card I have now be used to
> fill an unpopulated card?

Unlikely - the memory expansion slot design doesn't allow chip sizes to
be mixed.  A card which supports more than 1 MB will have to use 1
megabit RAM chips.

There is one card from Applied Engineering which supports piggybacking
another memory expansion card, but there doesn't seem much point.

There are also some cards which allow nonstandard mixtures of RAM, but
they are generally incompatible with DMA.

-- 
David Empson
dempson@actrix.gen.nz