Re: IIc SD/MMC RAM

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

kaos116 <kaos116@comcast.net> wrote:

> Yes there is a memory expanision header inside. from what I have read and
> remember. All of the ][c's had it except the very first revisions. I think
> you could get up to 1 meg inside.

The memory expansion slot was added in the third generation of the IIc,
but the second generation (with UniDisk 3.5 support) had basically the
same logic board as the first generation (differed only in default ROM
size and serial port clocking).

The expansion slot is functionally equivalent to a RamWorks card in a
IIe: it has a single byte bank address which allows auxiliary memory to
be switched between up to 256 banks of 64KB, so the theoretical limit is
16 MB, but the hardware interface might not provide all of the bits of
the higher address bus.

The memory on the card is expected to be dynamic RAM able to support 2
MHz access speed (less than 500 ns per read or write cycle) with
seperate row and column address strobes, so interfacing flash memory to
it could be tricky.

I'm not sure what happens to the built-in auxiliary memory in a IIc: the
hardware may be designed so that bank 0 uses the logic board RAM while
banks 1 and higher are supplied by the memory expansion card, or it may
replace the logic board memory, in which case you must have at least 64
KB of RAM on there.

There is no provision to have any firmware associated with the memory
expansion slot, other than what is already present in the main IIc
firmware, so doing anything fancy like a RamFactor equivalent would
require revising the main IIc firmware.

> My idea is to have a very small unit that attaches directly to the back of
> the ][c and has a pass through for adding additional floppy drives. I want
> it to be completely Prodos compatible. Meaning no special drivers. You plug
> it in an Prodos sees it and knows what to do with it.

That means it has to implement the SmartPort protocol.

> This would also make it compatible with the ][e/gss'.

It would only work on a IIc with "UniDisk" or later firmware, or in a
IIe with a "Liron" or "Superdrive" card, or in any IIgs.

> From what I have been reading in the Prodos Tech Ref Manual and Beneath
> Prodos. Their doesn't seem to be limit to the size of a volume.

ProDOS is limited to 32 MB per volume. It only has a 16-bit block number
and the block size is fixed at 512 bytes.

SmartPort has a higher limit: standard SmartPort (used by the IIc and
Liron card) has 24 bit block numbers (8 GB per unit), extended SmartPort
(used by the IIgs) has 32 bit block numbers (2 TB per unit).

ProDOS-8 cannot make use of these increased limits. An external
SmartPort device would have to appear as multiple 32 MB volumes in order
to allow more space to be used by ProDOS, and ProDOS is also limited to
a maximum of 14 logical volumes (some of which are not available for
SmartPort).

You can guarantee getting two units per slot, four for slot 5 with
ProDOS-8 1.2 or later (assuming slot 2 doesn't contain a disk controller
or equivalent) and with ProDOS-8 2.0 or later all slots which don't have
disk controllers can have their unit numbers allocated to other slots
with SmartPort firmware (except slot 3, which is needed for the internal
/RAM volume).

With a IIgs and GS/OS it would be possible in theory to have an external
SmartPort device larger than 32 MB, but you would have to use a file
system other than ProDOS. The only read/write file system supported by
GS/OS is HFS, and you can't boot a IIgs from HFS.

> Just a limit to the number of files per directory.

Only the root directory has a hard limit: 51 files, assuming four blocks
in the directory.

> If there is a limit to volume size(32meg) which I have read in the docs for
> the CF adapters for the ][e/gs , I could add a small dip switch on the side
> to select different volumes. 4x32meg volumes on a 128meg SD card. Not sure
> about this yet.

That should be possible as long as it is handled carefully and the
change is done at a point when nothing has any files open or cached
contents of the volume. The safest option is to tell the computer it is
a removable drive and ensure it knows the drive has been "ejected" when
the selector is changed, then it becomes available again on the next
access.

-- 
David Empson
dempson@actrix.gen.nz