Jackintosh

In 1985, the computer world was in a 16-bit arms race. Commodore had the Amiga, Apple had the Macintosh, and Atari—under the new leadership of Jack Tramiel—had the ST (Sixteen/Thirty-two). While the Amiga was the undisputed king of games and video, the Atari ST carved out a unique niche that kept it relevant for a decade. The secret wasn’t a blitter or a copper; it was a simple video mode known as ST High.

The Shifter Chip

The video generator in the ST was custom silicon named the “Shifter”. It was simpler than the Amiga’s Denise. It didn’t handle sprites (the ST had no hardware sprites; everything was drawn by the CPU). It fetched words from shared system RAM and shifted them out to the monitor.

The Shifter offered three modes:

Low Res: 320x200, 16 colors (from a palette of 512). Similar to the Amiga/C64.
Med Res: 640x200, 4 colors. An odd “interlaced-style” resolution but progressive.
High Res: 640x400, Monochrome (1-bit). 72Hz refresh rate.

The Power of High Res Mono

It’s hard to overstate how crisp 640x400 at 72Hz looked in 1985. The standard PC CGA card was outputting a fuzzy, flickering 60Hz. The Amiga’s high-res mode flickered horribly because it was interlaced (drawing odd/even lines in alternate frames).

The Atari ST High mode, paired with the dedicated SM124 monitor, was rock solid. It looked like paper. This singular feature made the ST the “poor man’s Macintosh” (often called the “Jackintosh” after Jack Tramiel).

Desktop Publishing (DTP): Software like Calamus allowed users to design print-ready layouts.
MIDI Sequencing: Cubase and Notator flourished on the ST because the high resolution allowed for long, detailed timelines of music notes on a single screen.

Memory Layout: Planar Nightmare?

In color modes, the ST used a Planar memory layout, similar to the Amiga and EGA. 16 colors require 4 bits per pixel. Instead of storing them as a “nibble” (packed), the ST stored them in 4 separate 16-bit words interlaced together. To read a single pixel’s color, the CPU had to read 4 words and mask out the specific bit from each. This made software sprite rendering slower than on packed-pixel systems (like the VGA Mode 13h).

However, in High Res Mono, the memory was linear. 1 bit = 1 pixel. 0 was white, 1 was black (or vice versa depending on the OS setting). This was incredibly fast for the 68000 CPU to manipulate. Blitting a window or scrolling text was just a matter of the MOVE.L (Move Long) instruction.

The Sync-Scroll Hack

The Shifter had a major limitation: it had no hardware scrolling. To scroll a game screen, the CPU had to redraw the entire frame—a massive task. However, demo coders (specifically from groups like The Union and Automation) found a loophole.

The Shifter had internal counters that tracked where the electron beam was. By switching between 50Hz and 60Hz modes at precise CPU cycles within a single scanline, coders could trick the Shifter into “missing” a beat or starting the line early. This effectively shifted the display by bytes or even pixels.

These “Sync Scrolling” techniques allowed later ST games and demos (Enchanted Land, Ooh Cricky Wot A Scorcher) to feature smooth, multi-directional scrolling that the hardware designers insisted was impossible. It required cycle-exact assembly code; if you upgraded your CPU accelerator, the screen would often dissolve into garbage because the timing was thrown off.

Legacy

While the Amiga is remembered for its soul (the Copper and Paula), the Atari ST is remembered for its utility. It was the workhorse of the European techno scene and the German publishing industry. The Shifter proved that you didn’t need sprites or 4096 colors to be revolutionary—sometimes, you just needed a really, really sharp image.