Raster line interrupt

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The raster line interrupt is a hardware interrupt that is triggered when a computer's video chip begins to display a specific line on the screen .

Raster line interrupt programming was extensively applied to home computers in the 1980s. It soon became an integral part of many computer games written for 8-bit home computers.

The image build-up on screens takes place line by line. The video chip that outputs the image data contains, among other things, a counter for the image line currently to be output, the so-called raster line. If the video chip supports a raster line interrupt, the program can specify a line at which this interrupt is to be triggered. If the raster line counter reaches this value, the video chip signals an interrupt request to the processor. This interrupts the current program and executes an interrupt routine (interrupt handler). At the end of the interrupt routine, the processor continues with the interrupted program.

The raster line interrupt makes it easier to execute program parts when a certain screen line is reached, i.e. to synchronize them with the image structure. The program does not have to actively wait for this position to be reached, but is informed by the interrupt request signaled by the video chip. For example, it is very easy to change graphic modes ( split screen ) while the image is being created, or to switch colors that normally apply to the overall image. The raster interrupt also simplifies the simultaneous display of a larger number of hardware- generated sprites than originally provided by the system by changing the screen positions of sprites that are no longer required from the screen area already swept over to the screen area still to be drawn by the electron beam (e.g. when playing games for the Commodore 64 ).

hardware

A well-known classic graphics chip in which the manufacturer has built in the mechanisms for a raster line interrupt is the VIC-II ( MOS Technology 6569 and similar variants). This was installed in the Commodore 64, among other things. Contemporary 8-bit hardware such as the Atari 800 , home computers based on the MSX standard or the Schneider CPC also dominated it. But even more recent hardware supports raster line interrupts, such as B. the Megadrive , the Super Nintendo , the Game Boy Advance or the GameCube , where the technology was often used in games for various optical effects.

literature

Web links

Individual evidence

  1. Display List Interrupt programming on the Atari 800: Article at Atariarchives.org (English)
  2. Programming the Sega Megadrive: Documentation at Gamefaqs.com (English)
  3. Display interrupt programming on the Gameboy Advance: Wiki page at NYC Resistor (English)