On Monday, August 29, 2005, at 06:56 PM, Bill Garber wrote:
: This may sound like a dumb question, but is 8 megabytes
: the *actual* RAM ceiling for a IIgs?
Yes, with 24 bit addressing, 16 MB is all you can literally
get out of it, but, there are only 16 address lines going to
and coming from the memory expansion slot. However, A10-A15
are read only as you may have seen by now, although they can
be made to read as well, with the proper decoding methods,
which would make for a mighty big card. The first 4 MB is
directly addressable, as the IIgs provides internal decoding,
and 8 MB can be had once the memory manager is applied. Any
amount over that would require logic that would fill 2 cards,
unless a large CPLD or FPGA was used in place of TTL logic.
: I only ask because I recall reading somewhere that you
: could actually get 12 megabytes of RAM on a IIgs.
Similar to the 4 MB simm problem, to get 8 MB, /CAS and /RAS
are used to define the columns and rows in the ram, where in
the 4 MB only /CAS is needed because the IIgs has logic on the
slot which defines the rows. Columns are defined by the simms
internally. In the 8 MB configuration, each row of 2 MB, one
in the first 4 MB half, and one in the second are defined at
the same time, while the column is split between the 2. So,
that makes DMA in an 8 MB card nearly impossible. Imagine an
attempt at this with 3 x 4MB chunks. :o)
: If 8MB is the limit, then I must ask: what happens to the IIgs's RAM
on
: the motherboard when you install an 8MB card? You would have more
than
: 8MB of RAM total, regardless of what revision IIgs you have. Does it
: just get ignored?
As in the case of the Sirius ram card, the 8th simm is removed
in the 8 MB configuration for ROM03. Otherwise, the ram onboard
is disabled.
I hope I answered all of your questions satisfactorily.
Bill Garber
Yes, thank you.
This is precisely what I wanted to know.
-Nat
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