{{unreferenced, date=February 2008 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 (oscilloscope), pictur ...

based video display circuits generate

vertical blanking In a raster scan display, the vertical blanking interval (VBI), also known as the vertical interval or VBLANK, is the time between the end of the final visible line of a frame or field and the beginning of the first visible line of the next fra ...

and vertical sync pulses when the display picture has completed and the

raster Raster may refer to: * Raster graphics, graphical techniques using arrays of pixel values * Raster graphics editor, a computer program * Raster scan, the pattern of image readout, transmission, storage, and reconstruction in television and compu ...

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 In computer systems programming, an interrupt handler, also known as an interrupt service routine or ISR, is a special block of code associated with a specific interrupt condition. Interrupt handlers are initiated by hardware interrupts, softwar ...

can then run specific software to 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 and

video game Video games, also known as computer games, are electronic games that involves interaction with a user interface or input device such as a joystick, controller, keyboard, or motion sensing device to generate visual feedback. This fee ...

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 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 ...

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 colour encoding system for analogue 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 The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplement ...

), 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 virtual memory, paging and safe multi-tasking d ...

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 way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...

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 fr ...

See also