Re: DOS 3.3 on 3.5 drive

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

In article <gk15140@pro-palmtree.cts.com>,
James Poore <jpoore@pro-palmtree.cts.com> wrote:
> Prosel 8, availiable from Charlie's Appleseeds in San Diego has a program
> called  Uniformatter (or something close) that formats a 3.5 disk for DOS
> 3.3.  It does limit the capacitity of the disk tho.  There is a technical
> explanation for the limit on the dox that come with the program.

I don't know what is said by this program's documentation, but the
absolute maximum limit on the size of a DOS 3.3 disk is 400k.

This limit is due to the size of the VTOC (Volume Table Of Contents),
which is used by DOS 3.3 to identify which parts of the disk are used
and which parts are free.  The VTOC only occupies one sector, and it
has enough space to hold usage information for 50 tracks of 32
sectors.  50 times 32 times 256 bytes (1/4K) equals 400 kilobytes.

All "DOS 3.3 on a 3.5 disk" utilities treat the single 800k disk as
two 400k volumes.  I believe there are at least two ways in which they
arrange the data.


By comparison, a ProDOS volume is limited to 32 megabytes minus 512
bytes (65535 blocks of 512 bytes each).

This seemed like a reasonable limit back in 1980 when SOS was
designed (SOS and ProDOS use the same file system), but now it is a
real nuisance with large hard drives.


The HFS file system used on the Macintosh is limited to 2 gigabytes
(recent implementations can use 4 gigabytes) but is rather inefficient
on large volumes, requiring very large allocation units (64k at a time
with a 4 gigabyte volume).
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand