Horizontal blank interrupt
Encyclopedia
A horizontal blank interrupt 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 horizontal blank
, which takes about 10 μS.
The technique was only really useful before the entire display could be addressed directly with high speed CPUs
attached to large frame buffers, that is, in the days of 8-bit
systems. In this case the CPU was not terribly fast, and that 10 μS might be enough to run perhaps 40 to 60 instructions. That was enough to change a few registers in the display hardware however, which is why this technique was useful.
For instance, both the Bally Astrocade
and Atari 8-bit family
(originally intended to be a console) included HBI support. Both could display only four colors per pixel, but which four could be selected from a palette of 256, selected by storing the color number in one of four registers. By changing the values of the palette registers during the HBI, the system could select a new set of four colors on every line, leading to a number of "rainbow" displays with all 256 colors on screen (four per line).
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 horizontal blank
Horizontal blank
Horizontal blanking interval refers to a part of the process of displaying images on a computer monitor or television screen via raster scanning. CRT screens display images by moving beams of electrons very quickly back and forth from the left to right side of the screen...
, which takes about 10 μS.
The technique was only really useful before the entire display could be addressed directly with high speed CPUs
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
attached to large frame buffers, that is, in the days of 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...
systems. In this case the CPU was not terribly fast, and that 10 μS might be enough to run perhaps 40 to 60 instructions. That was enough to change a few registers in the display hardware however, which is why this technique was useful.
For instance, both the Bally Astrocade
Bally Astrocade
The Astrocade is an early video game console and simple computer system designed by a team at Midway, the videogame division of Bally. It was marketed only for a limited time before Bally decided to exit the market. The rights were later picked up by a third-party company, who re-released it and...
and 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...
(originally intended to be a console) included HBI support. Both could display only four colors per pixel, but which four could be selected from a palette of 256, selected by storing the color number in one of four registers. By changing the values of the palette registers during the HBI, the system could select a new set of four colors on every line, leading to a number of "rainbow" displays with all 256 colors on screen (four per line).