Re: Smartport Limitation

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

Bryan Parkoff <BParkoff@satx.rr.com> wrote:

>     Remember that I mentioned in a previous newsgroups about ProDOS Kernel
> Rewritten to handle 8GB blocks.  David is right that it has to do with
> firmware.  I did check to find out about Smartport firmware.  Smartport
> firmware can handle up to 8GB using $FFFFFF (24 bit) blocks.

With you so far.  24 bits allows block numbers as high as 16777215, so
one block less than 8 GB is the maximum volume size which can be
accessed through a standard SmartPort firmware interface.

> With $FFFFFF blocks of 8GB will work with any format such as ProDOS, HFS,
> and others.

Except that the ProDOS file system is limited to a volume size of 32 MB,
due to its use of 16 bit physical block numbers in the file system (at
512 bytes per physical block).  You would have to design a new file
system (which wouldn't be "ProDOS" any more) to increase these limits.

HFS is limited to a volume size of 2 TB, due to its use of 16 bit
allocation block numbers and at most 65535 physical blocks per
allocation block.  It gets wasteful above about 1 GB, due to needing
ridiculously large allocation blocks (32 KB per allocation block at 2
GB, 64 KB per allocation block at 4 GB).

HFS+ potentially has a huge maximum volume size, as it uses 32-bit
allocation block numbers, typically with 4 KB per allocation block
(though that can be changed, generally getting larger for volume sizes
in the order of hundreds of gigabytes so that the total number of
allocaiton blocks doesn't become unmanageable).

>     I don't know if there is a newr version of Smartport that will handle
> $FFFFFFFF (32 bit) blocks that will go up to 2TB.

This is provided by the "extended SmartPort" calls, which are supported
by Apple's High Speed SCSI card, but they are only available if the card
is used in a IIgs.  (I don't recall offhand whether these calls are also
available on the original Apple SCSI card.)

> I don't know if Macintosh still uses Smartport that has 24 bit or 32 bit
> blocks.

The Macintosh doesn't use SmartPort at all (neither the firmware
interface, nor the actual protocol used to communicate with intelligent
drives like the UniDisk 3.5).

The Mac's device driver interfaces are potentially able to support very
large volume sizes - at least 31 bits for the physical block number, and
possibly more.  SCSI itself is limited to 48 bits for the block number,
assuming the command set hasn't been extended even further since I last
checked, and the latest versions of the ATA standard also allow 48 bit
block numbers.  (48 bits * 512 bytes = 128 exabytes = 131072 terabytes
~= 134 million GB.)

The previous limit on the ATA interface was 28 bits for the physical
block number, which works out to 128 GB.

-- 
David Empson
dempson@actrix.gen.nz