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A raster interrupt (also called a horizontal blank interrupt) is a computer interrupt signal that is used for display timing purposes. It is usually, though not always, generated by the system's graphics chip.
Often, the graphics chips used in home computers and video game consoles had limited capabilities. Raster interrupts were incorporated into these video chips allowing skilled programmers to transcend these limitations. A software-serviced interrupt would be set to trigger when a given screen line was refreshed, and the interrupt routine could then reload the chip's registers. In doing so, the graphics chip state could be changed at a specific screen position allowing sprites to be repositioned, additional colors inserted, or graphics modes to be changed giving the graphics hardware the appearance of greater capability than it was inherently endowed.
Systems supporting raster interruptsEdit
Several popular home computers and video game consoles included graphics chips supporting raster interrupts or had features that could be combined to work like raster interrupts.
Bally Astrocade (1977)Edit
Atari 8-bit family (ANTIC chip, 1979)Edit
The ANTIC chip used by the Atari 8-bit family includes display list interrupts (DLIs), which are triggered as the display is being drawn. The ANTIC chip itself is considerably powerful and inherently capable of many features which other systems require raster interrupts to duplicate. ANTIC can mix multiple graphics modes on the screen, display horizontal and vertical overscan graphics, and fine scroll selected horizontal regions. DLIs on the Atari are typically used to add additional color to the display and reuse Player/Missile graphics elements.
Commodore 64 (MOS Technology VIC-II chip, 1982)Edit
The C64's VIC-II has a flexible raster interrupt system. Raster interrupts and CPU intervention are necessary to reuse sprites on the screen, mix graphics modes, and selectively scroll screen regions.
Nintendo Entertainment System (PPU chip, 1983)Edit
The Nintendo Entertainment System's PPU graphics chip does not support true raster interrupts - an interrupt can be set to trigger during the vertical blank interval, but not at any arbitrary scan line - instead required polling of a "hit flag" that indicated when the first sprite was being drawn. Although early games like Super Mario Bros., Castlevania, and The Legend of Zelda managed to produce effective split-screen scrolling with this method, it is CPU-intensive. Some later cartridges incorporated MMC circuitry (most prominently Nintendo's MMC3 chip) that kept track of the PPU's address and data lines and generated raster interrupts.
MSX2 (Yamaha V9938, 1985)Edit
MSX2 computers feature a dedicated Yamaha V9938 Video Display Processor (VDP) that, like the NES, supports 'vertical blank interrupts' but requires polling for 'horizontal blank interrupts'. It was commonly used to create split screens, mix multiple video modes together on one screen and to increase the number of unique colours on the screen beyond the official spec. Prominent usage examples are the Aleste series and Konami's Space Manbow.
Commodore Amiga (Copper chip, 1985)Edit
The Amiga computers include a custom coprocessor called the Copper which is dedicated to servicing raster interrupts. The Copper runs a program of simple instructions directing it to wait for a specific vertical scan line and horizontal beam position, then update the contents of a custom chip hardware register. This is typically used for modifying display parameters, such as mixing display modes, reusing sprites, or changing color registers, but can also be used to trigger a CPU interrupt.
Nintendo Game Boy (PPU chip, 1989)Edit
The Game Boy's PPU has support for four raster interrupts : it can be set to trigger at the beginning of a specified scanline, at the end of scanlines (during Horizontal blanking), at the beginning of all scanlines, or at the beginning of Vertical blanking. Its biggest drawback is that the same Interrupt handler is used for the first three, requiring the use of "dispatching" code if more than one condition is used.