DOS: What it is, where it is, and what it does. 

Copyright © 2000 by Dr. Hank Levinson

Please note: This article is for 100% FREE distribution only!

Feb 21, 2000


Each digital computer runs a fundamental program which calls other user
programs as directed by the user. This fundamental program is kept in a silicon
chip called a "Read Only Memory" (ROM) and is loaded into the computer's main
memory (RAM) at startup, and set running (executing). This fundamental program
is the computer's "Operating System (OS). It allows other programs and data to
be stored in RAM and also facilitates the execution of these other programs.

In order not to have to enter a program by hand from a keyboard each time we
wish to RUN it, we use external (to the computer) devices to store the program
either on magnetic tape or on magnetic disks. Thus a slightly different OS is
needed, a "Disk Operating System", DOS. Such a system includes commands new to
OS such as CATALOG, LOAD, SAVE, DELETE, and INIT.

In order to "READ" information from a disk, the computer must be told something
about what to expect: how many "tracks" (concentric rings of magnetized strips)
to find, and into how many "sectors" (sections) each track is divided. This
information is different for each main kind of DOS. It is kept in a ROM chip
usually on the "controller card" which interfaces the disk drive to the
computer.

Apple DOS was the first disk operating system released to the public. Several
different versions were released. In chronological order (from "Beneath Apple
DOS", by Don Worth and Pieter Lechner (C) 1981 by Quality Software), these were:

DOS 3	        June 29. 1978
DOS 3.1	        July 20, 1978
DOS 3.2	        Feb. 16, 1979
DOS 3.2.1	    July 31, 1979
DOS 3.2.1 Plus  July 21, 1979
DOS 3.3	        Aug. 25, 1980
DOS 3.3	        Jan. 01, 1983

ProDOS versions ......................... 

All versions of DOS prior to DOS 3.3 used 13 sectors. DOS 3.3 and all subsequent
versions of Apple DOS used 16 sectors per track. DOS occupied the first two
tracks on a disk, numbered 0 and 1.

In order for machines operating under the new 16 sector DOS to read a 13 sector
older disk, the 16 sector version of DOS had to be modified to read 13 sectors.

In the beginning, Apple had a strict policy of "backwards compatibility" This is
to say any newer system had to be compatible insofar as was possible with
previous older systems.) This was accomplished for DOS by two methods: "soft"
and "hard". A "PreBoot" disk was issued which modified the copy of 16-sector DOS
in RAM to 13-sector DOS (and then one would boot a 13-sector disk normally).

Otherwise one could use an "old" disk controller card having the original
13-sector ROM's. Some controller cards (like those made for the Franklin clone
of the Apple) had ROM's containing both versions, 16 and 13 sector DOS. The user
would switch between the two by using a toggle switch or resetting a jumper on
the card. There was a "daughter-card" also available which had both ROM's and
plugged into a standard Apple disk controller card, allowing choice of 13 or 16
sector DOS via a toggle switch.

Finally, there was a third-party modification of DOS 3.3 called "DOUBLE DOS"
which allowed one to switch between the two versions via a simple keyboard
command ("&"). (There were many third-party modifications of DOS. My favorite is
UniDOS by MicroSparc, of Nibble Magazine fame. This allowed DOS 3.3 to be
extended to 3.5" disks and also relocated the image of DOS in RAM normally
located at addresses beginning with $8000) to an address above $9000 (in hex)
thus freeing more RAM above $8000, for program use.

One may ask why one should bother with older versions of Apple DOS? Much early
software was written for 13-sector DOS. I have had voice recognition cards (for
Apple ][ and ][+) whose software "didn't work"! It worked perfectly treated as
13-sector disks. The same occurred with software for a math coprocessor card
called the "APU" (Arithmetic Processor Unit), and a whole host of MECC disks
(Michigan Educational Consortium Committee). The software as well as most
unprotected 13-sector software, can be easily converted to 16-sector software
via a program provided by Apple on its DOS 3.3 Master Diskettes named "Muffin".

So next time you try to boot an unknown disk, and your disk drive makes strange
noises or just sits and spins, consider the possibility it contains some old
DOS, and you may have a valuable antique in your possession.