A vertical blank interrupt (or VBI) is a hardware feature found in some legacy computer systems that generate a video signal.

Cathode-ray tube A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms on an oscilloscope, a ...

based video display circuits generate vertical blanking and vertical sync pulses when the display picture has completed and the raster is being returned to the start of the display. With VBI, the vertical blank pulse is also used to generate an

interrupt request In a computer, an interrupt request (or IRQ) is a hardware signal sent to the processor that temporarily stops a running program and allows a special program, an interrupt handler, to run instead. Hardware interrupts are used to handle events s ...

for the computer's microprocessor. The interrupt service routine can then modify data in the video display memory while it is not being read to avoid screen tearing effects. This was particularly useful in simple

home computers Home computers were a class of microcomputers that entered the market in 1977 and became 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 s ...

and

video game A video game or computer game is an electronic game that involves interaction with a user interface or input device (such as a joystick, game controller, controller, computer keyboard, keyboard, or motion sensing device) to generate visual fe ...

consoles that relied upon a central microprocessor to generate text or graphic displays. More advanced home computers featuring hardware sprites often supported the more flexible horizontal blank interrupt instead in order to allow them to be multiplexed. As the VBI will be generated at the start of every displayed frame (50 Hz for

PAL Phase Alternating Line (PAL) is a color encoding system for analog television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...

, 60 Hz for

NTSC NTSC (from National Television System Committee) is the first American standard for analog television, published and adopted in 1941. In 1961, it was assigned the designation System M. It is also known as EIA standard 170. In 1953, a second ...

), it is a useful timebase in systems lacking other timing sources. VBIs are used in some home computers to perform regular functions like scanning the keyboard and joystick ports. It can also be used to implement a basic form of multitasking as well as a buffered graphics screen via page flipping, if hardware permits. Modern

protected mode In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units (CPUs). It allows system software to use features such as Memory_segmentation, segmentation, virtual mem ...

operating systems generally do not support VBIs as access to hardware interrupts for unprivileged user programs could compromise the system stability. Instead, various

API An application programming interface (API) is a connection between computers or between computer programs. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how to build ...

s like

DirectX Microsoft DirectX is a collection of application programming interfaces (APIs) for handling tasks related to multimedia, especially game programming and video, on Microsoft platforms. Originally, the names of these APIs all began with "Direct" ...

provide efficient and safe ways to present graphics free of tear and flicker. For computers that support VBIs see the page about

raster interrupt A raster interrupt (also called a horizontal blank interrupt) is an interrupt signal in a legacy computer system which is used for display timing. It is usually, though not always, generated by a system's graphics chip as the scan lines of a fra ...

References

{{reflist Video game development

See also

References