

                   The Operating Sytem of the Atari ST
                   === ========= ===== == === ===== ==
                      copyright 1987 by John Ogawa

No part of this document  may  be  reproduced without permission from the
author. Permission is granted  to  copy  this document for non-commercial
purposes only.

                            Table of Contents
                            ----- -- --------

1.0 A wide angle view

2.0 The TOS and some history
        2.1 The GEMDOS
        2.2 The BIOS
        2.3 The XBIOS
        2.4 The Line A routines
        2.5 Wrapup of TOS

3.0 GEM
        3.1 The AES
        3.2 The VDI
        3.3 Wrapup of GEM

4.0 A final wrap up

5.0 Where to read

6.0 Where to reach me

Hi there.  This article is  an  attempt  to  describe the workings of the
Atari ST's operating system.  Be forewarned  that  I am not an ST wizard,
and that any information in  here  was  gained  through reading non Atari
documentation, playing with the  ST,  and  talking  to  people.  In other
words, it may not be right! But maybe it is. Who knows? Read on . . .

The Atari ST has  an  extremely  complex  operating  system consisting of
several groups of  routines  that  comprise  several  different levels of
interaction with the machine.  Geez, what  I  meant to say was that there
are a lot of calls, and that  they  can be divided into groups, and these
groups are related to each other in a tree-like way.  The major libraries
are the BIOS, the XBIOS, the  GEMDOS,  the  Line A routines, the AES, and
the VDI.

These groups can in turn  be  put  into  TOS  and  GEM.   How do they all
relate? Well, follow me! In the following  sections I'll take a bottom up
look at the individual libraries, how they relate to each other, and what
they do.  The first section will  talk  about the overall organization of
the OS (operating system).

1.0 A wide angle view
----------------------
The ST's OS  can  be  divided  into  two  large  groups  of routines, TOS
(Tramiel Operating System), and GEM  (Graphics Environment Manager).  TOS
has the underlying routines that make  the  machine  run, and GEM has the
routines that make it talk nice to you.

TOS can be further broken down into  the  BIOS, XBIOS, GEMDOS, and Line A
libraries.  These libraries  control  machine  dependant  functions (like
controlling the peripherals) and  basic  I/O.   The  Line  A routines are
basic graphics routines that  are  used  by  GEM  to create the windowing
environment that the user sees.

GEM contains two libraries, the AES and VDI routines.  GEM is responsible
for the windowing environment and the desktop that make using the ST more
intuitive.  The AES is concerned mostly  with windows and dialogs, things
that the user interacts with.

The VDI routines handle  complex  text  display  and  high level graphics
manipulations.

In the next sections I'll go  into  the  two groups of libraries, TOS and
GEM, and their respective libraries in more detail.  I'll start with TOS.

2.0 The TOS and some history
-----------------------------
TOS is the heart of the ST.  Everything  you do, run, or write on your ST
uses TOS.  These are the routines  that  do  all the lower level tasks of
running programs.  TOS is present and active  in  the ST at all times.  A
little information about the background  and  design  of  TOS may help to
make it's job clear.

In the beginning was CP/M, an OS that  I am very familiar with :-).  CP/M
was designed to run  on  a  large  number  of  computers, while keeping a
standard interface so that people and programs could migrate from machine
to machine without  too  much  pain.   A  good  idea,  one  that was very
popular.

CP/M achieved  its  portability  by  dividing  itself  into  two distinct
sections, the machine independent  and  machine  dependent routines.  The
code for the machine independent  routines  was the same across machines,
and whenever it did  anything  it  called  the machine dependent routines
using standard names.  This worked  well,  and programs very easily moved
from machine to machine, and the CP/M community thrived.

The came IBM.  When IBM/Microsoft designed PC/MS-DOS they kept several of
the  concepts  of  CP/M.   Then  came   DRI.   They  designed  a  machine
independent set of routines called GEMDOS which was based on CP/M and MS-
DOS.  It is GEMDOS that makes  up  the  hardware independent part of TOS.
The file manipulation and task  handling  routines  of TOS are in GEMDOS.
To fill out TOS, Atari added  the  machine dependant routines in the BIOS
and XBIOS.  Finally, to make graphics programming easier, Atari added the
Line A routines.

So TOS is composed of  the  "higher"  level  GEMDOS routines, the machine
dependant BIOS and XBIOS routines, and  the Line A graphics routines.  In
the next sub-sections I'll very briefly describe each of these libraries.

2.1 The GEMDOS
---------------
The GEMDOS, as noted  before,  are  the  machine independent routines for
handling files and tasks  (programs)  and  doing  "high" level I/O.  They
bear a great resemblance to MS-DOS routines.

These routines make it easy  to  manipulate  files by using Fopen, Fread,
and Fseek.   Controlling  the  execution  of  programs  is  managed using
routines such as Pexec and  Pterm.   Easy  I/O  is  done using Cconin and
Cconout.  These routines allow  the  programmer  to  perform most of what
s/he would like to do with little fuss.

The GEMDOS is the highest level of  TOS.   It is built on routines in the
rest of TOS, namely the BIOS and the  XBIOS.   As we look at the BIOS and
XBIOS in the next sub-sections you will find overlap between them and the
GEMDOS.  GEMDOS was  written  to  make  the  programmer's  job  easier by
incorporating the lower level  routines  into  higher level routines that
are more specific.


2.2 The BIOS
-------------
The BIOS is the collection of  routines  that  the ST uses to communicate
with the many devices available to it.   Using BIOS routines you can read
and write to, and get the status of,  the serial port, the midi port, the
parallel port, the disks, and  the screen/terminal.  These routines (like
Bconin and Bconout) are general routines.   With the same routine you can
get information from several sources  by  varying the calling parameters.
You can see how the device specific  routines  in the GEMDOS can be built
from the BIOS routines.

2.3 The XBIOS
--------------
These routines are very much like the BIOS, but involve much more control
over the ST's peripherals.   In  general,  you  read  or  write with BIOS
calls, you *control* with XBIOS calls.   These routines also allow you to
control the sound chip, the floppy disks, the serial port, the mouse, and
the midi port.  GEMDOS is also built from these calls.

2.4 The Line A routines
------------------------
These routines are the graphics primitives in  the ST.  They allow you to
create complex graphics with relatively few, simple commands.  With these
routines you can draw pixels, lines and  boxes as well as control sprites
and the mouse pointer.  The  GEM  routines  are built from these routines
(but more about that later).

2.5 Wrapup of TOS
------------------
That was TOS.  Hope it  was  kind  of  clear.   Sorry if it wasn't. These
routines are the basis for anything  you  will  run  on the ST.  The next
level up is GEM.

3.0 GEM
-------
Now we come to the complex stuff.  GEM, the Graphics Environment Manager,
is a machine independent  (runs  on  the  IBM  too)  set of routines that
enable the programmer to use  windows,  dialogs,  the mouse, buttons, and
sliders to interface with an end  user.   GEM  also allows you to present
complex graphics and output them to a number of different devices.  It is
built so that these features can  be  defined,  accessed, and used with a
relatively small number of routines.  It also allows limited multitasking
through desk accessories.  In  addition  to  all  that, GEM allows inter-
process communication and control.  I said it was complex!

GEM  consists  of  two  libraries   of  routines,  the  AES  (Application
Environment Services) and the VDI (Virtual Device Interface).  The AES is
responsible for drawing and getting  input  from  windows and dialogs, as
well as  coordination  of  multiple  input  sources  and  the interaction
between concurrently running programs.   It  is  primarily concerned with
input.

The VDI is responsible for complex output.  It handles fancy text output,
high level graphics output, and output  to devices other than the screen.
In the next two sub-sections I'll describe the AES and VDI libraries in a
little more detail.

3.1 The AES
-----------
The AES, as I mentioned before, is  mostly concerned with input. While it
is responsible for drawing windows  and  dialogs, it's most important job
is to receive input through those  windows and dialogs.  The evnt_button,
evnt_timer, and evnt_multi calls are input related calls in AES.

Another function of  the  AES  is  to  coordinate  more  than one running
program.    Inter-program    messages    and    coordination    are   AES
responsibilities.

Central to the AES is the  notion  of  an object.  An object is (usually)
something to be drawn in a window (like a slider bar) or a dialog (like a
button).  Objects can also be text  or  boxes. A collection of objects is
organized into a tree,  which  gives  information about the relationships
between the objects. Once drawn, some  object  can be acted on (selected,
moved, sized, etc.), and the results  of these actions comprise the input
that the AES is charged  with  obtaining.  The  AES attempts to provide a
smooth and easily programmed interface between the user and the program.


3.2 The VDI
------------
The VDI routines are  primarily  concerned  with  output.  The VDI offers
complex graphics capabilities using  simple  and easily handled routines.
It also handles fonts and text output.

One of the most important VDI functions is to provide access to a variety
of output devices.  VDI routines are  built  so that the device that they
are outputting onto is not specific.  One can draw a circle on the screen
or on a plotter with the same routine.  The device independent portion of
the VDI is the GDOS.

Its routines control the  peripheral  devices  and  allow output from VDI
routines to them.

The  important  concept  in  the  VDI  library  is  the  workstation.   A
workstation is a device that  can  handle  output  from the VDI routines.
Printers, plotters, and the  screen  are  examples of workstations.  Each
VDI routine is given the workstation it is expected to output to. The VDI
provides a flexible means for presenting complex output to the user.

3.3 Wrapup of GEM
------------------
Well, there is GEM.  As you  might  guess,  the GEM routines are built on
the BIOS, XBIOS, and Line  A  routines.   GEN lets the programmer present
sophisticated information elegantly and receive  complex feedback using a
library of high level routines.  GEM  is  the  highest  level of the ST's
operating system.

4.0 A final wrap up
-------------------
I've done my best to  describe  the  ST's  operating  system and all it's
interconnections, hierarchies, and overlaps.  Negotiating your way around
all those routines as a programmer can be very confusing.

5.0 Where to read
-----------------
Most of the information in this article was gleamed from all of the books
listed below. In addition,  Tim  Oren's  ProGem  series was an invaluable
tutorial on the ins and outs of GEM.

        "Programmer's Guide to GEM" by Phillip Balma and William
                Fitler. Sybex, 1986.

        "Atari ST Internals" by K. Gerits, L. Englisch and R.
                Bruckmann. Abacus Software, 1985.

        "Atari ST GEM Programmer's Reference" by Norbert
                Szczepanowski and Bernd Gunther. Abacus Software,
                1986.

        "Mark Williams C for the Atari ST, version 2.0" (manual).
                Mark Williams Co., 1986.

And of course, many thanks to all the people in the ST community who have
answered my questions. I'll probably have a lot more. :-

6.0 Where to reach me
---------------------
Feel free to  write  me  and  correct  anything  I  have  said  or ask me
questions. Though for the latter, remember  that I probably know about as
much about the ST as you do!

   I can be reached at:

        GEnie:  TUSHIE

        USMail: John Ogawa
                1028 W, Lewis Dr.
                San Diego, CA   92013

Happy Hacking!

tush
5/19/87
