The Operating System of the Atari ST

From Atari Wiki
Jump to navigation Jump to search

The Operating Sytem of the Atari ST

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).

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.

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 dependant routines. The code for the machine independent routines was the same across machines, and whenever it did anything it called the machine dependant 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 theses libraries.


   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. Controling 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.


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.


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.

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).

Wrapup of TOS

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


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.


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.

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.

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.

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,

	"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.

1987 by John Ogawa

Back to TOS