Re: IIGS Memory Card Question

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

Jeff Blakeney <jeff.blakeney@a2central.com.remove-cjf-this> wrote:

>   To: JCarlson
> On Sat, 15 Sep 2007 06:32:52 +0000, JCarlson wrote:
> 
> > I have an opportunity to purchase one of the following memory crads
> > for my IIGS Rom01. Which would you say is the best choice?
> > 
> > - Applied Engineering GSRamPlus 6-meg card (w/4-meg installed)
> > - Applied Engineering GSRamUltra 4-meg memory card
> > - Sequential Systems RAM-GS 4-meg memory card (rev 1)
> > 
> > Any input would be greatly appreciated!
> 
> I have no experience with the Applied Engineering cards but I have read
> lots of things about them.  Specifically that the RamPlus may not be DMA
> compatible (but it is debatable whether that is a major problem or not)
> and that at least some of the AE cards have different versions for or
> need to be set up for ROM 1 or ROM 3 machines.

DMA compatibility is a major issue if you want to use an Apple
High-Speed SCSI card. If your memory card is not compatible with DMA,
you have to disable it on the SCSI card, which causes a huge performance
hit.

The same will go for almost any other slot card which uses DMA outside
of IIgs main RAM (which is extremely rare outside of the SCSI cards).

There is one exception: the RAMFast SCSI card can do DMA even with most
memory cards which are not DMA compatible. It was never clear exactly
how it achieved this, but my theory at the time was that its driver gets
the CPU to do a memory access in the appropriate bank, which is followed
by a burst of DMA accesses to that bank. The CPU's access to the bank is
sufficient to allow non-DMA memory cards to latch the correct bank.

A rule of thumb when considering memory cards is that any which have up
to four "rows" of the same size (either 256 MB or 1 MB), i.e. maximum
total size of 1 MB or 4 MB, should be DMA compatible

Anything larger, or with an unusual number of memory rows (such as six),
or which lets you use a mixture of row sizes, or which uses memory chips
with an address bus width greater than 1M (such as 4Mx1), probably
isn't.

Some cards (such as the OctoRAM) support eight rows and are not DMA
compatible if set to operate with more than four rows, but are OK with
DMA if set to operate with only four or fewer rows.

Another thing to watch out for is cards which require PAL/GAL changes or
different versions of the card to work in a ROM 1 or ROM 3 IIgs. This
almost certainly means they are decoding the bank directly and not using
the DMA-compatible row select pins.

-- 
David Empson
dempson@actrix.gen.nz