Every Amiga thread on the internet eventually gets an Amiga owner explaining screens to people who never used the machine. Carl Svensson finally wrote the primer: what a screen is, why it existed, and what it bought the user on 7 MHz hardware with very little RAM.

A screen is a display area the operating system treats as an object in its own right. A program can open as many as it likes, at whatever resolution and colour depth it likes, and the hardware composites them:

  • Colour comes from indexed palettes — a small set of per-screen registers, each value picked from a 12-bit colour space (24-bit on AGA)
  • Pixels are planar: each bitplane holds one bit of a pixel’s colour index. One bitplane gives 2 colours, five give 32 on the original chipset, eight give 256 on AGA
  • OCS low-res is 320×256 (320×512 interlaced) in up to 32 colours; high-res is 640×256 in up to 16, plus overscan
  • A sixth bitplane in low-res unlocks HAM (all 4096 colours on screen, with caveats) or EHB, a 32-colour palette plus half-brightness copies of it

Planar storage is what makes the rest affordable. A text editor can run on a 2-colour screen while a paint program uses 32 colours beside it, and no memory is spent on bits a pixel does not need. Screens can be arbitrarily sized and positioned — a 320×50 strip at the bottom of the display is a legal screen.

Because the copper co-processor can change resolution, colour depth and palette at any point in the redraw cycle, screens of different shapes and colour counts can overlap on the same display. That is what makes the party trick possible:

  • Switching between screens is instant
  • A screen can be dragged down with the mouse to reveal the one behind it, long before anyone called that a compositor
  • Dual playfields give two overlapping screens an alpha channel, because colour index 0 on the front layer becomes transparent

Svensson is honest about the payoff: screen dragging is more impressive than useful, since full-screen switching is faster. The real value is in the workflows it enabled — a file manager where slamming the pointer to either edge of the screen and clicking jumps to the parent directory, or an 80×24 terminal that “just feels right” on a widescreen monitor for reasons of proportion rather than pixels.

The lesson is a design one, not nostalgia. Screen boundaries gave the pointer an edge to hit and gave whole applications a fixed frame to design against; a window manager built out of overlapping rectangles has no equivalent, which is why those affordances mostly do not exist today. The Amiga got productive multitasking out of a constrained display model rather than raw speed — and the constraints are where the interesting ideas came from.