Amiga Screens: A Primer | datagubbe.se

Amiga Screens: A Primer Autumn 2026 One of the unwritten rules of the Internet seems to be that whenever something Amiga-related is mentioned, at least one Amiga fan (myself included) must show up and try to explain the concept of screens. Amiga screens can have different resolutions, we'll tell you, and one can drag them, we'll say, and other Amiga users rally in agreement, while non-Amiga users probably still don't get what's so great about screens. Until now, when this text has been written, in the hope of converting unsuspecting normies into full-blown Amiga screen lovers. For practical purposes, this text will focus on the original Amiga graphics hardware, called OCS (Original ChipSet). Some hardware limitations were removed in the subsequent ECS (Enhanced ChipSet) and AGA (Advanced Graphics Architecture) upgrades, but the same basic principles and user experience still apply. A typical Amiga screen, showing a Workbench desktop with a shell window open. A Screen is a Screen is a Screen The specific meaning of screen on the Amiga comes from the operating system, which uses this term to refer to a particular type of display area because it is, well, a screen. Amiga games and demo programmers aren't as bothered by this concept; the Amiga Hardware Reference Manual, for example, refers to a display area as a "playfield", and a demo coder might talk about raster splits, but for simplicity, let's stick to screen. Hence, a screen on the Amiga is, basically, an area onto which graphics is drawn. Amiga screens can have different resolutions and colour depths, and a program can open any number of different-resolution screens to display graphics. Today, we mostly use a single, fixed-resolution display area, which is a combined effect of how modern operating systems and flatscreen monitors work. In the heydays of CRT monitors, however, opening different-resolution displays was commonplace. An image viewer running on a VGA-capable MS-DOS machine, for example, might use a 16-colour, 720x400 pixel text mode resolution for browsing files, and then open a new 256-colour 320x200 display when viewing an image. These variations in resolution and colour depth existed on basically all home computers, and were hardware-enforced tradeoffs to achieve reasonable speed and memory consumption for different use cases. Memory was very expensive at the time (Oh, how history repeats itself!) and the Amiga, which in its stock hardware configuration relied on a relatively small amount of RAM being shared between the CPU, video and audio hardware, offered a high level of control over these screen resolutions and colour depths. Indices and Planes The Amiga typically uses indexed palettes, meaning that a limited number of per-screen colour registers contain a user-defined colour value. These values are selected from a 12-bit colour space (or 24-bit, on AGA). For example, colour index 0 might be set to $000, which is black, and index 1 to $F00, which is red. To manipulate the colour value of individual pixels, planar graphics is used, which means that the colour depth of a screen is increased by adding more bitplanes (bpl for short). Each bitplane is stored separately in memory, and in order to change the colour index of a pixel, a bit must be toggled in each plane. Thus, a one-bitplane screen gives two colour indices, two bitplanes gives four and so on, up to five bitplanes and 32 colours on the original Amiga hardware (or 8 bpl and 256 colours on AGA). An illustration of how bitplanes are combined together to represent per-pixel colour indices. (From the Amiga Hardware Reference Manual) On OCS and ECS, the maximum number of bitplanes per screen is determined by its display resolution, and these are designed to make sense on a PAL or NTSC television set. An OCS Amiga offers low-res and high-res. On PAL, low-res is 320x256 pixels (320x512 with interlace) in up to 32 colours (5 bpl). High-res is 640x256 (640x512 with interlace) in up to 16 colours (4 bpl). These base resolutions can be increased slightly by using overscan, which in high-res can be up to 724x283, but isn't guaranteed to be fully visible on all monitor types or television sets. In low-res, a sixth bitplane can be used for HAM (Hold-And-Modify), allowing free use of all of the OCS Amiga's 4096 colours simultaneously (with some caveats), or EHB (Extra Half-Brite) which duplicates a 32 colour palette into 32 additional copies of the original colours, but with half the original brightness value. Deluxe Paint editing an EHB image. Note the colour selector in the bottom right of the screen: The two rightmost columns are "half-brite" copies of the colours in the first two columns. The half-brightness isn't always perfect, since it's limited by the 12-bit colour space. Unlike most of its contemporary competitors, the Amiga has true, preemptive multitasking, for which planar graphics offers convenient resource frugality. A text editor might work just fine on a 2-colour screen, saving memory that can be used for simultaneously running a graphics program on a 32-colour screen. It's also memory-saving in the sense that only the exactly required number of bits are needed to store a single pixel while keeping memory addressing sane, instead of, say, allocating one byte per pixel and wasting the unused bits. In addition, a screen can be arbitrarily dimensioned and positioned, such as displaying a 320x50 pixel low-res screen at the bottom of the physical display area. Not using more pixels than necessary per screen will also help save memory. Hardware Hijinx The Amiga was originally designed as a games machine, which means it's got lots of hardware features for working with graphics. Repositioning a screen is instant, and scrolling an entire screen is extremely fast, to the point that even the operating system allows the user to configure a desktop screen that's larger than the visible area, and scroll around it using the mouse. It's also easy to change the display resolution and colour depth at arbitrary points in the redraw cycle. This means that several screens can be combined at once, even overlapping, while maintaining a uniform display experience for the end user. Consider the following example: Simultaneous display of two screens with different resolutions and colour depths. The above example has been created using the BASIC dialect AMOS, which has its own take on the screen concept and provides simple abstractions for working with the Amiga's graphics hardware. Any type of graphics operation can still be performed individually on any of the screens, such as drawing, scrolling, repositioning the screen and changing the palette. This fast resolution and colour depth switching is controlled by the Amiga's copper (short for co-processor). The copper works in lockstep with the video rendering hardware and is also used for manipulating colour values and hardware sprites. Among other things, this can create the distinct Amiga feature called "copper gradients" or "copper bars", in which the colour value for a given colour register is changed once per horizontal line, producing striking gradients and more on-screen colours than what can be achieved using the 32 available palette indices. This is an ordinary 4-colour Workbench screen. The background gradient is created using the copper, by changing the value of colour index 0 at regular intervals during video rendering. Combining screens with different resolutions and colour depths has a multitude of use cases. Even if the maximum number of colours per screen is 32, these 32 colours can be different on each screen. Thus, a game might display 64 or more colours simultaneously by using 32 colours for the main game area and 32 different colours for the user interface and/or status display. This can then be combined with copper gradients to further boost the colour count. End User Experience Apart from games, this swift graphics handling is also convenient when running multitasking productivity software, which (at last!) brings us to screen dragging. Because of the low display resolutions offered by home computers and early PCs, most applications ran in full-screen mode, taking over the entire display area to show as much information (and user interface) as possible. When multitasking on the Amiga, the user can quickly switch between entire screens using either a button in the top right of the screen, or a system-wide keyboard shortcut. However, screens can also be dragged by clicking and, well, dragging the screen title bar downwards using the mouse. This will reveal another running program's screen behind it, as illustrated below. An illustration of how screen dragging might look on a user's monitor. I must confess that even though many of us Amiga fans go on about it, the actual usefulness of screen dragging is limited, at least in my personal workflows. However, the effect must have been rather stunning in 1985, when multitasking and colour graphics were rarely seen in combination other than on very expensive Unix workstations. One use case suggestion is that you can drag down your chat program screen just a bit to check on a file download progressing on the web browser screen behind it, but full-screen switching on the Amiga is so effortless that dragging usually feels a bit cumbersome. In order to show just how snappy this screen handling is, I've prepared a short movie clip. It's filmed off a flatscreen monitor connected to an Amiga 600, which is a 7 MHz (that's 0.007 GHz) machine based around essentially the same hardware as the original Amiga 1000 in 1985. Here it's playing some music while also running a text editor and the graphics program Deluxe Paint, and of course performing screen switching and dragging: Click above to watch the movie. Dual Playfields