Vertical blank interrupt
Encyclopedia
A vertical blank interrupt (or VBI) is a programming technique used in some systems, notably video games and consoles, to allow program
code to be run in the periods when the display hardware is turned off, waiting for the TV
to complete its vertical blank.
Since the vertical blank period occurs at the start of every displayed frame (PAL: 50 times a second, NTSC: 60 times a second), timing the code to run during this period guarantees a regular "heartbeat" which is useful for timing-dependent functions. Simple routines like reading the joystick or updating the display can be placed in the VBI, and then basically ignored while the main program runs. Many systems used this to develop a crude sort of multitasking
.
The VBI is particularly useful if the display hardware includes some sort of page flipping support, in which case the hardware can be set up to draw a new frame during the period where there is no drawing taking place. This guarantees a flicker-free display with almost no effort.
This was widely used on the Atari 8-bit family
, whose ANTIC display chip included a register pointing to the start of the display memory. The main program would draw the new frame into a buffer in memory, and then a small piece of code in the VBI would change the ANTIC register. The result was excellent animation from a simple piece of hardware.
Computer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
code to be run in the periods when the display hardware is turned off, waiting for the TV
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
to complete its vertical blank.
Since the vertical blank period occurs at the start of every displayed frame (PAL: 50 times a second, NTSC: 60 times a second), timing the code to run during this period guarantees a regular "heartbeat" which is useful for timing-dependent functions. Simple routines like reading the joystick or updating the display can be placed in the VBI, and then basically ignored while the main program runs. Many systems used this to develop a crude sort of multitasking
Computer multitasking
In computing, multitasking is a method where multiple tasks, also known as processes, share common processing resources such as a CPU. In the case of a computer with a single CPU, only one task is said to be running at any point in time, meaning that the CPU is actively executing instructions for...
.
The VBI is particularly useful if the display hardware includes some sort of page flipping support, in which case the hardware can be set up to draw a new frame during the period where there is no drawing taking place. This guarantees a flicker-free display with almost no effort.
This was widely used on 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...
, whose ANTIC display chip included a register pointing to the start of the display memory. The main program would draw the new frame into a buffer in memory, and then a small piece of code in the VBI would change the ANTIC register. The result was excellent animation from a simple piece of hardware.