AGSP
Any Given Screen Position
Putting the C64's whole screen anywhere by combining VSP for the horizontal and a line crunch for the vertical - the trick behind full-screen scrolling games and demos, and identifiable by the blanked lines it leaves at the top.
AGSP — Any Given Screen Position — is what you get by putting two earlier C64 tricks at right angles to each other:
A VSP (Wanker/DMA delay) is used for the horizontal position and a line crunch is used for the vertical position.
VSP shifts the screen sideways. Linecrunch shifts it vertically. Together they place the whole display anywhere, at scroll speeds no software copy could reach — which is why AGSP underlies full-screen scrolling on a machine with no hardware scroll worth the name.
How to spot it
The technique leaves a signature at the top of the screen:
The top five and a bit lines are “wasted” by the line crunch, if you see any demos with the top few lines blanked out and the screen below is moving at a fast rate then you can probably guess it is using a similar method.
What it costs
Both halves are timing-critical. VSP needs a stable raster, and the reference code is PAL-only — “to make it work for NTSC you have to do some minor timing modifications for each scan-line.”
Getting the vertical half exactly right matters more here than in a demo effect, which is why Codebase64’s smooth-linecrunch article warns AGSP builders specifically that its example scrolls by 7 pixels rather than 8 — fine for a demonstration, wrong for “anything else that requires precise positioning.”
Who did it first
The firsts list records it under its parts rather than its name: LineCrunch + VSP, by Exilon/MDT/Horizon in Bonanza, November 1988 — thirteen months after VSP’s own first appearance.
See also
- VSP — the horizontal half
- Linecrunch — the vertical half
- Stable raster — the prerequisite
- Hardware scroll