Display list

A display list (or display file) is a series of graphics commands that define an output image. The image is created (rendered) by executing the commands to combine various primitives. This activity is most often performed by specialized display or processing hardware partly or completely independent of the system's CPU for the purpose of freeing the CPU from the overhead of maintaining the display, and may provide output features or speed beyond the CPU's capability.

For a display device without a frame buffer, such as the old vector graphics displays, the commands were executed every fraction of a second to maintain and animate the output. In modern systems, the commands need only be executed when they have changed or in order to refresh the output (e.g., when restoring a minimized window).

A display list can represent both two- and three-dimensional scenes. Systems that make use of a display list to store the scene are called retained mode systems as opposed to immediate mode systems.

One of the earliest popular systems with true display list was the Atari 8-bit family. The display list (actually called so in Atari terminology) is a series of instructions for ANTIC, the video co-processor used in these machines. This program, stored in the computer's memory and executed by ANTIC in real time, can specify blank lines, any of six text modes and eight graphics modes, which sections of the screen can be horizontally or vertically fine scrolled, and trigger Display List Interrupts (called Raster interrupts or HBI on other systems).

The Amstrad PCW family contains a Display List function called the 'Roller RAM'. This is a 512-byte RAM area consisting of 256 16-bit vectors into RAM, one for each line of the 720 × 256 pixel display. Each vector identifies the location of 90 bytes of monochrome pixels that hold the line's 720 pixel states. The 90 bytes of 8 pixel states are actually spaced at 8-byte intervals, so there are 7 unused bytes between each byte of pixel data. This suits how the text-orientated PCW constructs a typical screen buffer in RAM, where the first character's 8 rows are stored in the first 8 bytes, the second character's rows in the next 8 bytes and so on. The Roller RAM was implemented to speed up display scrolling as it would have been unacceptably slow for its 3.4 MHz Z80 to move up the 23 KB display buffer 'by hand' i.e. in software. The Roller RAM starting entry used at the beginning of a screen refresh is controlled by a Z80-writable I/O register. Therefore, the screen can be scrolled simply by changing this I/O register.

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