Re: 5.25" Disk

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

<dklugh@iwaynet.net> wrote:

> I'm not sure what "OS overhead" is,but only AppleDOS kept DOS on a certain
> track as part of the disk format (all disks were bootable).

The DOS image on a bootable DOS 3.3 disk occupies three tracks (well,
two tracks plus two sectors, but normal DOS doesn't let you use the rest
of the space).  It is easy to remove the DOS image and free up two
tracks for your own files.  (It is also possible to free up most of the
remaining track, but it requires a patch to DOS.)

In addition to the DOS image, there is the Volume Table Of Contents
(VTOC) and catalog.  On a standard disk, these occupy one track (number
17).  It is possible to set up the disk specially to save all but two of
these sectors, with a reduction in the number of files that can be
stored on the disk.

Therefore a standard DOS 3.3 disk has 140KB minus 16KB (4KB per track)
for a total of 124KB usable space.  Removing DOS gives you 132KB, and if
you try really hard you can get 139.25KB.

> ProDOS allowed you to make non-DOS disks.

Much easier, because PRODOS itself is simply a file on the disk.  The
boot blocks, directory and bitmap occupy seven blocks, leaving you with
136.5KB usable space, assuming no subdirectories.

> What other OSes are there for the Apple rather than the one in ROM?

I wouldn't call the firmware in the ROM an "operating system".  It
doesn't support disk I/O at all, and has limited tape support (on the
IIe and earlier models).

DOS 3.3, ProDOS, Apple II Pascal and CP/M are examples of operating
systems available for 8-bit Apple II machines (CP/M requires a Z80
card).  Of these, only Pascal and CP/M can be classified as a full
operating system - DOS 3.3 and ProDOS are only disk operating systems.

> Well if you're going to store a bunch of little files,remember that each
> file will have a one sector overhead for the FAT.

There is no such thing as a "FAT" in any Apple II file system.  In
MS-DOS, the FAT (File Allocation Table) is a fixed sized data structure
near the start of the disk which contains linked lists of clusters used
by all files on the disk.  A similar concept is used in Apple's original
file system for the Macintosh (only used extensively on 400K 3.5"
disks).

DOS 3.3 has a "Volume Table of Contents" and "Track/Sector lists".  The
VTOC has one bit for each sector on the disk.  The Track/Sector lists
form a linked list of sectors which are used to locate the data sectors
of the file.  There is a minimum of one sector overhead for each file
(more if the file gets larger than about 32KB).

Pascal cannot have fragmented files, and relies on file location and
size information in the directory.

ProDOS has a "bitmap" and "index blocks" (which vary in quantity
according to the storage type of the file).  A small file has no index
blocks.  A file between 512 bytes and 128KB has a single index block.
Larger files have a master index block, plus an index block for every
128KB of data in the file.

CP/M uses information embedded in the directory to keep track of disk
space usage.

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