Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!dlyons
From: dlyons@Apple.COM (David Lyons)
Newsgroups: comp.sys.apple
Subject: Re: resource and data forks
Message-ID: <31080@apple.Apple.COM>
Date: 19 May 89 05:05:33 GMT
References: <8905161922.AA00955@crash.cts.com> <10173@claris.com>
Organization: Apple Computer Inc, Cupertino, CA
Lines: 57

In article <10173@claris.com> jazzman@claris.com (Sydney R. Polk) writes:
[...]
>It is impossible with ProDOS to acces more than 32 meg of space on a 
>volume because addressed in ProDOS can only address that much.  Period.
>
>GSOS doesn't have this limitation because it has more bits allocated
>for disk blocks internally.

Not exactly.  GS/OS doesn't have a 32 Meg limitation because it supports
file systems other than the ProDOS file system.  The ProDOS file system
is still limited to 32M, even under GS/OS.

To use a volume larger than 32M, you need an FST (File System Translator)
other than the ProDOS FST (PRO.FST in the *:System:FSTs folder).  The only
FSTs available under System Disk 5.0 are for character devices (not relavent
here), ProDOS, High Sierra (for CD-ROM drives), and AppleShare.  The High
Sierra FST supports reading only--you can't write to a CD these days.

So the AppleShare FST is the only one that currently lets you have read/write
access to volumes larger than 32M, and you need an AppleShare server running
on an AppleTalk network to use it.

>32 Meg in 512 K blocks requires 7 bits, which means that the block
>address can be stored in one byte.  The other bit is used for something
>like marking a block as used or some nonesuch.  ProDOS 8 uses only one
>byte for addresses, so it cannot access more than 32 meg.  This is so
>ingrained into the OS that a patch is basically impossible.

(#define blunt TRUE)  This makes almost no sense.  (#undef blunt)

The 32M limit is built into the format of the information on a ProDOS disk.
The most important limit is that 2 bytes (16 bits) are used for block numbers,
and blocks are 512 bytes.  65536 * 512 bytes = 32M.

The bitmap consists of 1 bit for each block, which is 1 block for every 2M on
the disk.

>GS/OS uses more than one byte for block addresses, so doesn't have the 32
>Meg limitation.

GS/OS allows 4 bytes (32 bits) for block numbers, and blocks can be pretty
big, too.  Again, there may be smaller limits imposed by particular device
drivers and File System Translators.

>Syd Polk           | Wherever you go, there you are.
>jazzman@claris.com | Let the music be your light.
>GO 'STROS!         | These opinions are mine.  Any resemblence to other
>GO RICE!           |  opinions, real or fictitious, is purely coincidence.

 --Dave Lyons, Apple Computer, Inc.          |   DAL Systems
   AppleLink--Apple Edition: DAVE.LYONS      |   P.O. Box 875
   AppleLink--Personal Edition: Dave Lyons   |   Cupertino, CA 95015-0875
   GEnie: D.LYONS2 or DAVE.LYONS         CompuServe: 72177,3233
   Internet/BITNET:  dlyons@apple.com    UUCP:  ...!ames!apple!dlyons

   My opinions are my own, not Apple's.