Raster interrupt
Encyclopedia
A raster 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 computer
s and video game console
s during the 8-bit
era were somewhat limited in their capabilities. For instance, the MOS Technology VIC-II
chip used in the Commodore 64
could only handle eight sprites
simultaneously. Raster interrupts were sometimes incorporated into these video chips in order to allow skilled programmers to transcend these limitations. An 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, multiple graphics modes could be mixed on one screen, the number of sprites could be greatly multiplied, and a portion of the screen could be scrolled
while another part remained stationary.
Most, though not all, popular graphics chips of the 8-bit era supported raster interrupts in some form. The C64's VIC-II had a powerful and flexible raster interrupt system, which was used extensively by the demoscene
. The ANTIC
chip used by the Atari 8-bit family
included display list interrupts, which performed much the same function. Some systems did not include native support for raster interrupts, and had to compensate in other ways. The Nintendo Entertainment System
's PPU
graphics chip did not support true raster interrupts - an interrupt could be set to trigger during the vertical blank interval, but not at any arbitrary scan line - and 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 was quite CPU-intensive, and 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.
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
interrupt
Interrupt
In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
signal that is used for display timing purposes. It is usually, though not always, generated by the system's graphics chip
Graphics processing unit
A graphics processing unit or GPU is a specialized circuit designed to rapidly manipulate and alter memory in such a way so as to accelerate the building of images in a frame buffer intended for output to a display...
.
Often, the graphics chips used in home computer
Home computer
Home computers were a class of microcomputers entering the market in 1977, and becoming increasingly common during the 1980s. They were marketed to consumers as affordable and accessible computers that, for the first time, were intended for the use of a single nontechnical user...
s and video game console
Video game console
A video game console is an interactive entertainment computer or customized computer system that produces a video display signal which can be used with a display device to display a video game...
s during the 8-bit
8-bit
The first widely adopted 8-bit microprocessor was the Intel 8080, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M operating system. The Zilog Z80 and the Motorola 6800 were also used in similar computers...
era were somewhat limited in their capabilities. For instance, the MOS Technology VIC-II
MOS Technology VIC-II
The VIC-II , specifically known as the MOS Technology 6567/8562/8564 , 6569/8565/8566 , is the microchip tasked with generating Y/C/composite video graphics and DRAM refresh signals in the Commodore 64 and C128 home computers.Succeeding MOS's original VIC , the VIC-II was one of the two chips...
chip used in the Commodore 64
Commodore 64
The Commodore 64 is an 8-bit home computer introduced by Commodore International in January 1982.Volume production started in the spring of 1982, with machines being released on to the market in August at a price of US$595...
could only handle eight sprites
Sprite (computer graphics)
In computer graphics, a sprite is a two-dimensional image or animation that is integrated into a larger scene...
simultaneously. Raster interrupts were sometimes incorporated into these video chips in order to allow skilled programmers to transcend these limitations. An 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, multiple graphics modes could be mixed on one screen, the number of sprites could be greatly multiplied, and a portion of the screen could be scrolled
Scrolling
In computer graphics, filmmaking, television production, and other kinetic displays, scrolling is sliding text, images or video across a monitor or display. "Scrolling", as such, does not change the layout of the text or pictures, or but incrementally moves the user's view across what is...
while another part remained stationary.
Most, though not all, popular graphics chips of the 8-bit era supported raster interrupts in some form. The C64's VIC-II had a powerful and flexible raster interrupt system, which was used extensively by the demoscene
Demoscene
The demoscene is a computer art subculture that specializes in producing demos, which are non-interactive audio-visual presentations that run in real-time on a computer...
. The ANTIC
ANTIC
Alphanumeric Television Interface Controller is an early video system chip used in the Atari 8-bit family of microcomputers as well as the Atari 5200 in the 1980s. The chip was patented by Atari, Inc. in 1981...
chip used by the Atari 8-bit family
Atari 8-bit family
The Atari 8-bit family is a series of 8-bit home computers manufactured from 1979 to 1992. All are based on the MOS Technology 6502 CPU and were the first home computers designed with custom coprocessor chips...
included display list interrupts, which performed much the same function. Some systems did not include native support for raster interrupts, and had to compensate in other ways. The Nintendo Entertainment System
Nintendo Entertainment System
The Nintendo Entertainment System is an 8-bit video game console that was released by Nintendo in North America during 1985, in Europe during 1986 and Australia in 1987...
's PPU
Picture Processing Unit
The PPU , more specifically known as Ricoh RP2C02 / RP2C07 , is the microprocessor in the Nintendo Entertainment System responsible for generating video signals from graphic data stored in memory....
graphics chip did not support true raster interrupts - an interrupt could be set to trigger during the vertical blank interval, but not at any arbitrary scan line - and instead required polling of a "hit flag" that indicated when the first sprite was being drawn. Although early games like Super Mario Bros.
Super Mario Bros.
is a 1985 platform video game developed by Nintendo, published for the Nintendo Entertainment System as a sequel to the 1983 game Mario Bros. In Super Mario Bros., the player controls Mario as he travels through the Mushroom Kingdom in order to rescue Princess Toadstool from the antagonist...
, Castlevania
Castlevania
Castlevania, known as in Japan, is a video game series created and developed by Konami. The series debuted in Japan on September 26, 1986, with the release of for the Family Computer Disk System , followed by an alternate version for the MSX 2 platform on October 30...
, and The Legend of Zelda managed to produce effective split-screen scrolling with this method, it was quite CPU-intensive, and some later cartridges incorporated MMC
Multi-Memory Controller
Multi-Memory Controllers or Memory Management Controllers are different kinds of special chips designed by various video game developers for use in Nintendo Entertainment System cartridges...
circuitry (most prominently Nintendo's MMC3 chip) that kept track of the PPU's address and data lines and generated raster interrupts.