Atari ST, architecture and hardware of this classic PC

The era of 16-bit computers was not only a matter of the PC and the Macintosh, but it had two worthy rivals in the form of the Commodore Amiga that we already discussed in its day and the Atari ST, which we will remember in this article. And the thing is that the Atari 16-bit computer is remembered above all for being the computer for musicians, but it was more than that and that is why it deserves a small article in the form of tribute.

The Atari ST was a computer launched by Atari Corporation in 1985, which was the product of the spin-off of the legendary Atari after the sale by Time Warner of the company in 1984. Which was divided into two halves. The first Atari Games call ended up in the hands of Japan’s Namco and contained the games and software division. While the second called Atari Corporation was created by the Tramiel family after the patriarch, Jack Tramiel, was fired by the shareholders of the company he had founded: Commodore.

Atari ST

The first project for a 16-bit computer by Atari had to be the Amiga, since the design team for the 8-bit Atari was the same as the one that had made the Commodore Amiga, but in the end Atari Corporation had to create your own design. Which was influenced by the greater success of the Tramiel in their previous company, the Commodore 64.


Despite the curiosity of competing against its old company, the Atari ST was not designed to compete against the Commodore Amiga, but to face the Apple Macintosh and the IBM PC AT. Nor can we forget that the Atari ST had been presented months before the presentation of the Amiga. The business strategy was to take the same aggressive approach that they had with the Commodore 64, but in the business market rather than the home market.

The Motorola 68000

Motorola 68000

The Atari ST like its rival, the Commodore Amiga, made use of the Motorola 68000 , the same CPU as its rival, the Commodore Amiga. With the difference that in the ST it worked a little faster than the Amiga when going to 8 MHz instead of the 7.16 MHz of the Commodore PC. Remember that the 68000 is a CPU with 32-bit registers and instructions, 16-bit data bus and 24-bit addressing. The latter limited the amount of RAM in systems with such a CPU to 16 MB at most, however this was better than the 1 MB limit of Intel‘s 8086 and 80286 and their 20 addressing bits.

As for the performance of this CPU, it was better than the 8086 and this gave the Atari ST better performance than the original IBM PC and XT PC. Although inferior to the 80286, but the strength of the ST was to offer a computer at a competitive price compared to the IBM PC and the Macintosh. Battle that lost against the PC due to the market share of this and the enormous amount of software available for the PC.

The acronym ST stands for Sixteen Thirty Two, which translated into Spanish means sixteen thirty-two. In reference to the size in bits of the Motorola CPU registers and data bus. But, the CPU is not the only thing that a computer has inside it, so we are going to do a review of the rest of the hardware.

As for RAM, the first ST was released in two variants. The first called 520ST with 512 KB as standard and the second with 1 MB as standard under the name 1040ST. So from the outset it had much more RAM than the first Macintosh and the Commodore Amiga. The ST also had 192 KB of ROM memory, which included a series of routines to speed up the operation of the operating system.

Atari ST support processors

Atari ST entrañas

A computer not only lives on its CPU, but also on supporting processors or accelerators. Which free the processor from certain tasks, perform them more efficiently or add capabilities that they did not have before. The four supporting chips were totally custom and created by the Atari Corporation itself. Their names? They didn’t try very hard as these were: GLUE, MMU, DMA, and SHIFTER.

Although they were not the only support chips, since in the design they added other chips already available on the market . As is the driver for 3.5 inch floppy drives WD 1772 , the 6850 driver for the I / O interfaces. The Motorola 68901 for interrupt management and the YM2149 sound generator which was based on three square wave channels and one noise channel. Rudimentary compared to the Paula chip in the Amiga, but much better than the screeching speaker found in PCs.

MMU and DMA unit are key in ST

One of the limitations that the 68000 had compared to Intel’s 80286 was the lack of an MMU unit , which was key to being able to run a complex operating system. For the ST they added an MMU unit in the form of a separate chip that is responsible for generating the memory addresses for the rest of the chips, but it cannot access the RAM directly, a function performed by the DMA unit with which it works together. so the rest of the logic can access not only the RAM, but the data on the disk as well.

As we have said before, the MMU was key to moving a complex operating system , in the case of the ST it was the TOS, Tramiel Operating System. However, unlike the Amiga operating system, it was not multitasking. Nor was it a problem since the IBM PC and Macintosh equivalents weren’t either. In reality, TOS had a graphical mouse interface called GEM that was combined with GEMDOS, an operating system with characteristics similar to MS-DOS, all to show a user interface very similar to that of the Apple Macintosh.

This was because the MMU included in the ST, unlike the x86 MMUs, did not support virtual addressing by pages or segments. Being the pair of the MMU and the DMA unit rather a way to give addressing and unified access to all the components of the system that needed to access the RAM of the system.

GLUE, the most important Custom Chip


Among the four custom chips in the Atari ST, the most important is called GLUE. Which was in charge of keeping the CPU and the rest of the support chips together, hence its name, which in Spanish translates as glue. What was GLUE’s job? Generating the different operating frequencies of the system, for example it was responsible for generating the frequency of 8 MHz for the CPU, 2 MHz for the YM2149 or 500 KHz for the I / O interfaces such as MIDI and keyboard ports.

It was also responsible for generating the video signals for the SHIFTER, such as the horizontal and vertical synchronization periods (HSYNC and VSYNC) l, as well as the blank signal times (HBLANK and VBLANK) and whether the screen is active or not. . In general, all support chips are designed to depend on each other when performing their function, which was done with the aim of saving costs in the final hardware design. Therefore, the GLUE was also part of the video circuitry of the system by generating the necessary signals to correctly send the information to the screen.

The Atari ST video system


The ST was never thought of as a video game machine, unlike the Amiga, which was born as a design for a console that was expanded to be a computer. The ST has a very rudimentary video system that lacks elements such as scrolling or scrolling support or even a generator of sprites or moving objects. These elements were also not found on the IBM PC and the Apple Macintosh.

At that time, the screen generation was carried out by the CPU and what was known as a graphic system, the only thing it did was transfer the image buffer created by the CPU into a video signal. Because RAM at the time was single-channel, this meant that in all systems when the image was being drawn on the screen, the CPU and other elements could not access the RAM.

Modos de video de Atari ST

The image buffer was generated by the CPU, which generated 32 KB of data in a specific part of the system’s RAM and supported the following resolutions in its image buffer:

  • 640 x 400 monochrome, 1 bit color.
  • 640 x 200 and 4 colors on screen, 2 bits of color.
  • 320 x 200 and 16 colors on screen, 4 bits of color.

Like the IBM EGA and the Commodore Denise, it was based on bit planes, but could only display up to 16 colors on screen like the EGA, although a larger color palette was available thanks to its 9-bit DAC that gave it access up to 512 colors. So in general it was inferior to the Amiga in the graphic section.

The Atari STe

Atari STE

Despite Atari’s attempt to compete face-to-face with IBM and Apple, the Atari brand so associated with video games and the lack of software did not allow them to have a significant market share. And even though the Atari ST ended up being successful in the home market for the first few years, most users used it for video games. Computers in homes were little more than glorified video game consoles, the ST’s hardware was short for it.

The demand for video games in the ST was answered by the Atari STE, an improved version of the base hardware of the ST added a series of improvements over the original hardware:

  • Increased color palette from 512 colors to 4096 bits by adding one bit per RGB component.
  • Screen scrolling or hardware scrolling, freeing the 68K from performing this task.
  • Added a new coprocessor called Blitter, which had the same homonymous functionality as the Agnus component of the Commodore Amiga with the same name.
  • A DMA audio unit that allowed to reproduce PCM tracks in stereo with 8-bit quality and up to 50 KHz sampling frequency.
  • The ability to expand the system RAM up to 4MB using standard 30-pin SIMMs.

Of all the added hardware, the most important was that of the Blitter, since it allowed the transfer of data blocks from one part of the memory to another with data transformation operations on the fly. This made it possible to improve the graphics and speed up certain routines in the programs. However, the STE continued with the limitation of 16 simultaneous colors on the screen and for Atari it was not enough to face a cheap Commodore Amiga 500 and a PC that would eventually prove intractable and would become the sole survivor of said battle. .