Demo Classics: Breaking the ST’s Borders

The Union Demo arrived in January 1989 with contributions from TEX, The Carebears, TNT-Crew, Level 16, Delta Force and The Replicants. Its most striking achievements included graphics in an area familiar to every ST owner: the supposedly empty screen borders.

Photo from 1989. From left to right: Michael Bittner (Hexogen, TNT-Crew), Thorsten Mutschall (Gogo, The Carebears), Michael Raasch (Daryl, TEX), Richard Karsmakers (Cronos, ST NEWS), Erik Simon (ES, TEX, behind the Cambridge Z88), Jochen Hippel (Mad Max, TEX), Gunter Bitz (6719, TEX), Andreas Franz (Ilja, Level 16), Udo Fischer (-ME-, TEX) and Tarik Ahmia (ST Magazin).

Level 16’s fullscreen (see opener) demonstrated how far carefully timed code could push the machine’s video hardware. The trick involved changing video settings at precise points during the display process, allowing graphics to extend beyond the normal picture area.

Opening the lower border, for example, involved briefly switching the video frequency setting from 50 to 60 Hz near the end of the normal display. Removing the side borders required tighter synchronisation and repeated intervention on individual scanlines.

This consumed processing time. A PAL ST scanline occupies 512 CPU cycles; border-opening instructions and the actual demo effect had to fit into that schedule. Additional graphics also meant more screen memory to handle.

The Union Demo’s interactive menu made its collection easy to explore, but the technical achievement was less forgiving: every moving object, scrolling message and border switch competed for a precisely measured share of the processor’s time.

The difficult part was combining everything reliably. A border routine might work on one machine yet produce shifting graphics on another. Stabilisation switches helped keep the Shifter’s output aligned, while the remaining instructions had to be arranged around the display timing.

That makes fullscreen worth distinguishing from simply drawing a larger picture. The programmer was controlling when the hardware continued fetching and displaying graphics.

Recommended reading: Explore the Atari ST demoscene, including The Union Demo and explanations of fullscreen and sync-scrolling techniques, in BREAKIN’ THE BORDERS – Atari ST vol. 1.