Understanding Amiga Screens: An Introductory Guide

Autumn 2026

There is an unspoken law of the internet: whenever the Amiga is mentioned, at least one enthusiast (myself included) will inevitably appear to explain the concept of screens.

We will tell you that these screens can have varying resolutions; we will insist that you can actually drag them around. While fellow Amiga users will nod in fervent agreement, those unfamiliar with the platform often remain bewildered as to why this is such a revolutionary feature.

This guide is written to bridge that gap, transforming the "normie" into a true appreciator of the Amiga's display philosophy.

Scope of this Guide

For the sake of clarity, this primer focuses on the OCS (Original ChipSet). While later iterations like the ECS (Enhanced ChipSet) and AGA (Advanced Graphics Architecture) relaxed certain hardware constraints, the fundamental logic and user experience remained consistent.

![Image: A classic Amiga Workbench desktop featuring a shell window]

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What Exactly is a "Screen"?

In the context of the Amiga, the term "screen" is a specific operating system definition.

Definition: A "Screen" is a designated area of the display onto which graphics are rendered.

Depending on who you talk to, the terminology changes:

• OS Developers: Call it a Screen.
• Hardware Manuals: Refer to it as a playfield.
• Demo Coders: Might discuss raster splits.

Regardless of the name, the core idea is that a program can open multiple screens, each with its own unique resolution and color depth.

Then vs. Now

Modern computing relies on a single, static resolution—a byproduct of how current OSs interact with flat-panel monitors. However, during the era of CRT monitors, shifting resolutions was standard practice.

Example: MS-DOS VGA Comparison

These weren't just choices; they were arbitrary hardware-mandated trade-offs to balance speed and memory usage. Because RAM was incredibly expensive, the Amiga used a shared memory architecture between the CPU, audio, and video hardware, granting the user granular control over these settings.

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The Mechanics: Indices and Planes

The Amiga utilizes indexed palettes. Instead of storing a full color value for every pixel, it uses a register that points to a specific color.

• OCS/ECS: 12-bit color space.
• AGA: 24-bit color space.

For instance, a palette might be defined as:

ColorIndex[0] = $000; // Black
ColorIndex[1] = $F00; // Red

Planar Graphics

To determine which index a pixel uses, the Amiga employs planar graphics. Rather than storing the index in one place, the color depth is expanded by adding "bitplanes" (bpl). To change a pixel's color, you must toggle bits across multiple separate memory planes.

The relationship between bitplanes and colors is exponential: $\text{Total Colors} = 2^n$ (where $n$ is the number of bitplanes)

• 1 bpl $\rightarrow$ 2 colors
• 2 bpl $\rightarrow$ 4 colors
• 5 bpl $\rightarrow$ 32 colors (OCS Max)
• 8 bpl $\rightarrow$ 256 colors (AGA Max)

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Resolutions and Special Modes

On OCS and ECS hardware, the maximum number of bitplanes is tied to the resolution, optimized for the television standards of the time (PAL/NTSC).

PAL Resolution Standards

• Low-Res: $320 \times 256$ (or $320 \times 512$ with interlace) $\rightarrow$ Up to 32 colors (5 bpl).
• High-Res: $640 \times 256$ (or $640 \times 512$ with interlace) $\rightarrow$ Up to 16 colors (4 bpl).
• Overscan: Can push high-res up to $724 \times 283$, though visibility varies by monitor.

Advanced Color Modes

In low-resolution mode, a sixth bitplane can be utilized for:

• HAM (Hold-And-Modify): Allows the use of all 4,096 OCS colors simultaneously (with specific limitations).
• EHB (Extra Half-Brite): Creates a second palette of 32 colors that are half the brightness of the originals.

![Image: A color selector showing standard colors and their corresponding half-brite versions]

Note: Because of the 12-bit color space, the "half-brightness" effect isn't always mathematically perfect.

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Efficiency and Multitasking

The Amiga's approach to graphics was a masterstroke for its preemptive multitasking capabilities. Planar graphics allowed for extreme memory frugality.

1. Resource Allocation: A simple text editor can run on a 2-color screen, leaving plenty of RAM for a simultaneous 32-color graphics application.
2. Memory Precision: By using only the necessary number of bits per pixel, the system avoids wasting memory (unlike systems that might allocate a full byte per pixel regardless of color depth).
3. Flexible Geometry: Screens can be any size and placed anywhere. You could, for example, have a small $320 \times 50$ low-res status bar at the bottom of your display.

Hardware Hijinx

Since the Amiga was envisioned as a gaming powerhouse, its graphics hardware is incredibly flexible.

• Instant Repositioning: Moving a screen happens immediately.
• High-Speed Scrolling: Scrolling is so efficient that the OS allows for a "virtual desktop"—a screen larger than the physical monitor—which the user can simply scroll across.