Raster interrupt

The raster line is a hardware interrupt that is triggered when the video chip of a computer begins the representation of a particular line on the screen.

The Rasterzeileninterruptprogrammierung has been applied extensively to the home computers of the 1980s. She soon became an integral part of many written for 8- bit home computer computer games.

The screen layout on screens takes place line by line. The video chip, which outputs the image data, to contain, inter alia, a count of the currently being output image line, the so-called raster line. Supports the video chip a raster line, a line can be set at which this interrupt is to be triggered by the program. The raster line counter reaches this value, the video chip an interrupt signal to the processor. This interrupts the current program and executes an interrupt routine (interrupt handler ) from. At the end of the interrupt routine, the processor proceeds to the interrupted program.

The raster line makes it easier to run parts of the program when a certain line of the screen, so to synchronize with the screen layout. The program does not wait for actively reaching this position, but is informed by the signaled from the video chip interrupt request ( IRQ). Thus, for example, very easy to move during the image construction Modes ( split screen) or normally applicable for the overall image color switch. The simultaneous representation of a larger number of hardware -generated sprites as originally provided by the system by changing the screen positions no longer required sprites from the already swept in the still from the electron beam to draw the screen is simplified by the raster interrupt (eg games for the Commodore 64 ).

Hardware

A well-known classic graphics chip in which the manufacturer has incorporated the mechanisms for a raster line, the VIC -II ( MOS Technology 6569 and similar variants). This was installed among other things in the Commodore 64. Contemporary 8-bit hardware as the Atari 800 home computer after the MSX standard or the Amstrad CPC also dominated him. But even younger Rasterzeileninterrupts hardware supports, such as the Mega Drive, Super Nintendo, the Gameboy Advance or Gamecube, where the technique was often used in games for various optical effects.