Re: Ram Question

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

Frank Carney <frankc@cyberhighway.net> wrote:

> The Apple II uses the second clock phase to refresh RAM.

It actually uses the second clock phase to display video, which has the
side effect of refreshing the RAM.

> Does the IIGS do the same thing for RAM in the RAM expansion slot?

No.

The "slow" RAM on the IIgs motherboard (128 KB), which is under the
control of the "Mega II" chip, is refreshed exactly like in 8-bit
machines.  (In effect, the Mega II contains nearly all the motherboard
components of a IIe, minus the CPU and RAM.)

The "fast" RAM on the IIgs motherboard (128 KB or 1 MB), plus the fast
RAM in the memory expansion slot, is refreshed using explicit refresh
cycles generated by the FPI (Fast Processor Interface) chip.  (With the
1 MB motherboard, a.k.a. ROM 3, this chip is actually labelled "CYA",
which is rumoured to stand for "Control Your Apple".)

The fast RAM nominally operates at 2.8 MHz, but the refreshing causes
the average access speed to drop to more like 2.6 MHz.

It is interesting to note that the FPI can generate fast RAM refresh
cycles without interfering with CPU accesses to slow RAM, I/O or ROM.
Since the ROM runs on the "fast" side of the machine, this means that
ROM accesses can proceed at the full 2.8 MHz.

This is one of the main reasons why a ROM 3 IIgs is slightly faster than
a ROM 1: there is more toolbox code in ROM (particularly the newer
System 5 version of QuickDraw with its improved performance), so
graphics drawing can run as much as 7% faster.


One other point about refreshing the memory expansion slot: this only
works properly if the card is designed to use the row select signals
decoded by the FPI.  If the card decodes access addresses directly from
the bank address provided by the CPU, then it must provide its own
refreshing (and it will also have DMA compatibility problems).

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P O Box 27-103, Wellington, New Zealand