Blitter
Encyclopedia
In a computer
system, a blitter is a circuit, sometimes as a coprocessor
or a logic block on a microprocessor
, that is dedicated to the rapid movement and modification of data within that computer's memory
. A blitter is capable of copying large quantities of data from one memory area to another relatively quickly, and in parallel with the CPU
.
The name comes from the acronym BLIT
, which stands for BLock Image Transfer. A typical use for a blitter is the movement of a large bitmap
in a 2D computer game
or demo
.
s with raster-graphics
output, the screen buffer
was normally held in main memory and drawn under the control of the CPU. For many simple graphics routines, like sprite
support or flood filling polygons, large amounts of memory had to be manipulated. A typical reason to move large data areas arose when drawing bitmap
s, such as when drawing the next frame during a computer game
or demo. An image that moves smoothly across the screen might give rise to the need for a bitmap (representing the image) to be moved every frame. Game designers had to design their game's graphics so that the total amount of bitmap data transfer required to draw each frame was within the capacity of the CPU.
As graphics hardware became more sophisticated, framebuffer
s grew larger with higher resolutions and colour depth. Games designers wanted to use larger images to represent game elements, and to maintain acceptable framerates. Graphics operations then required more and more data transfer speed to accomplish these bitmap moves. This work tied down the CPU, preventing it from operating on other tasks or making it infeasible to transfer large images within the finite time allowed for processing between frames.
s manufactured in the 1980s included a graphics coprocessor
that contained a blitter. The CPU would send a description of the necessary bit blit
operations to the blitter, which would then carry out the operation much faster than the CPU could, and in parallel.
The Commodore Amiga
was the first personal computer
to use a full-featured blitter, and the first US patent filing to use the term blitter was "Personal computer apparatus for block transfer of bit-mapped image data," assigned to Commodore-Amiga, Inc. On top of the ability to copy and manipulate large areas of graphics, the hardware that contained the Amiga's blitter also included line drawing and area-filling hardware.
Later models of the Atari ST
also included a blitter co-processor, which was named in all capitals as the BLITTER chip. One story states that manufacturing delays deferred its introduction into the ST line until after the first STs had shipped. Another is that the Atari ST's main competitor, the Amiga, was famous for its blitter, and so Atari introduced one as well. Although Atari planned an upgrade to allow dealers to install the blitter chip, this plan was later dropped. Instead, the BLITTER was introduced on the Mega series, and then also supported on most later machines (except the Atari TT
).
Graphics-oriented software (especially games) running on systems that did not have a blitter needed to find other methods of transferring large bitmaps. Some games were written across platforms, some of which contained a blitter and some of which didn't. A typical example would be a game written for the Atari ST and Amiga. Both machines contained similar hardware, including the MC68000
processor, but the Amiga contained a blitter and the early Ataris did not. Such a game could not rely on the presence of a blitter. One approach to handling this was to load all available 68000 data register
s with data from the bitmap in memory, and then push the data into the frame buffer in as few operations as possible.
Blitting was not the only solution to providing high-performance graphics in performance-limited machines. A more common solution in early machines was the use of sprites
, which used two different graphics pathways to draw images that were then combined in the video display circuitry into a single image. Sprites were small bitmaps that were positioned on the screen independent of the normal bitmap background, allowing them to be moved on-screen by adjusting the values of several timers. The video circuitry started drawing the sprites after the timer had expired, allowing them to be drawn for little cost, and avoiding the need to move memory around to provide the illusion of motion. The downside of this approach is that the sprite systems generally had hard-coded limits to the number of sprites they could display, often between two (the Atari VCS
) and eight (Commodore 64
). Blitters offered the ability to have any number of objects, limited only by the performance of the blitter and the memory it talked to. As the performance of these circuits increased, the flexibility of the blitter overwhelmed any performance advantage in the sprite approach, and many sprite systems became blitters in disguise.
Typically, a computer program would put information into certain registers describing what memory transfer needed to be completed and the logical operations to perform on the data, then trigger the blitter to begin operating. The CPU is then free to begin some other operation while the blitter operates.
The destination for the transfer is usually the frame buffer. However, a blitter can also be used for non-graphics work. For example, an area of memory might be zeroed (filled with zeroes) using a blitter more quickly than can be accomplished with the CPU. Additionally, simple mathematical operations can be built from basic logical operations.
The image at right helps illustrate how a blitter may use a 'mask' to decide which pixels to transfer and which to leave untouched. The mask operates like a stencil
, showing which pixels in the source image will be written to destination memory. The logical operation would be Dest = ((Background) AND (Mask)) OR (Sprite).
. The modern GPU is essentially a very advanced blitter and shares with earlier blitters the goal of rapidly copying, transforming, and writing transformed bitmaps (more generally textures), to a framebuffer. But, while early blitters were limited to performing simple logical operations on the target data, modern GPUs add the ability to modify these bitmaps in mathematically advanced ways that help produce more interesting effects, such as shading to produce illumination effects and blending for transparency. Additionally, modern GPUs can write bitmaps to destination memory in such a way so as to provide the illusion of depth. While earlier blitters were limited to a linear one-to-one mapping of source pixels to destination pixels, modern GPUs can instead map source pixels to destination pixels in a non-linear way to produce the appearance of perspective. Support for more advanced mathematical transformations also allows the modern GPU to transform blocks of coordinates in a 3d space. This ability, combined with shading and non-linear mapping between source and destination pixels helps to create the fluid 3d experience found in many of today's games.
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...
system, a blitter is a circuit, sometimes as a coprocessor
Coprocessor
A coprocessor is a computer processor used to supplement the functions of the primary processor . Operations performed by the coprocessor may be floating point arithmetic, graphics, signal processing, string processing, or encryption. By offloading processor-intensive tasks from the main processor,...
or a logic block on a microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
, that is dedicated to the rapid movement and modification of data within that computer's memory
Random-access memory
Random access memory is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order with a worst case performance of constant time. Strictly speaking, modern types of DRAM are therefore not random access, as data is read in...
. A blitter is capable of copying large quantities of data from one memory area to another relatively quickly, and in parallel with the CPU
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...
.
The name comes from the acronym BLIT
Bit blit
Bit BLIT is a computer graphics operation in which several bitmaps are combined into one using a raster operator....
, which stands for BLock Image Transfer. A typical use for a blitter is the movement of a large bitmap
Bitmap
In computer graphics, a bitmap or pixmap is a type of memory organization or image file format used to store digital images. The term bitmap comes from the computer programming terminology, meaning just a map of bits, a spatially mapped array of bits. Now, along with pixmap, it commonly refers to...
in a 2D computer game
Personal computer game
A PC game, also known as a computer game, is a video game played on a personal computer, rather than on a video game console or arcade machine...
or demo
Demo (computer programming)
A demo is a non-interactive multimedia presentation made within the computer subculture known as the demoscene. Demogroups create demos to demonstrate their abilities in programming, music, drawing, and 3D modeling...
.
The historical need for a blitter
In early computerComputer
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...
s with raster-graphics
Raster graphics
In computer graphics, a raster graphics image, or bitmap, is a data structure representing a generally rectangular grid of pixels, or points of color, viewable via a monitor, paper, or other display medium...
output, the screen buffer
Screen buffer
In computing, screen buffer is a part of computer memory used by a computer application for the representation of the content to be shown on the computer display....
was normally held in main memory and drawn under the control of the CPU. For many simple graphics routines, like sprite
Sprite (computer graphics)
In computer graphics, a sprite is a two-dimensional image or animation that is integrated into a larger scene...
support or flood filling polygons, large amounts of memory had to be manipulated. A typical reason to move large data areas arose when drawing bitmap
Bitmap
In computer graphics, a bitmap or pixmap is a type of memory organization or image file format used to store digital images. The term bitmap comes from the computer programming terminology, meaning just a map of bits, a spatially mapped array of bits. Now, along with pixmap, it commonly refers to...
s, such as when drawing the next frame during a computer game
Personal computer game
A PC game, also known as a computer game, is a video game played on a personal computer, rather than on a video game console or arcade machine...
or demo. An image that moves smoothly across the screen might give rise to the need for a bitmap (representing the image) to be moved every frame. Game designers had to design their game's graphics so that the total amount of bitmap data transfer required to draw each frame was within the capacity of the CPU.
As graphics hardware became more sophisticated, framebuffer
Framebuffer
A framebuffer is a video output device that drives a video display from a memory buffer containing a complete frame of data.The information in the memory buffer typically consists of color values for every pixel on the screen...
s grew larger with higher resolutions and colour depth. Games designers wanted to use larger images to represent game elements, and to maintain acceptable framerates. Graphics operations then required more and more data transfer speed to accomplish these bitmap moves. This work tied down the CPU, preventing it from operating on other tasks or making it infeasible to transfer large images within the finite time allowed for processing between frames.
Blitters in home computing
Computer manufacturers introduced blitters to help lessen this graphics burden on the CPU, or to allow more complex graphics. Several home computerHome 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 manufactured in the 1980s included a graphics coprocessor
Coprocessor
A coprocessor is a computer processor used to supplement the functions of the primary processor . Operations performed by the coprocessor may be floating point arithmetic, graphics, signal processing, string processing, or encryption. By offloading processor-intensive tasks from the main processor,...
that contained a blitter. The CPU would send a description of the necessary bit blit
Bit blit
Bit BLIT is a computer graphics operation in which several bitmaps are combined into one using a raster operator....
operations to the blitter, which would then carry out the operation much faster than the CPU could, and in parallel.
The Commodore Amiga
Amiga
The Amiga is a family of personal computers that was sold by Commodore in the 1980s and 1990s. The first model was launched in 1985 as a high-end home computer and became popular for its graphical, audio and multi-tasking abilities...
was the first personal computer
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
to use a full-featured blitter, and the first US patent filing to use the term blitter was "Personal computer apparatus for block transfer of bit-mapped image data," assigned to Commodore-Amiga, Inc. On top of the ability to copy and manipulate large areas of graphics, the hardware that contained the Amiga's blitter also included line drawing and area-filling hardware.
Later models of the Atari ST
Atari ST
The Atari ST is a home/personal computer that was released by Atari Corporation in 1985 and commercially available from that summer into the early 1990s. The "ST" officially stands for "Sixteen/Thirty-two", which referred to the Motorola 68000's 16-bit external bus and 32-bit internals...
also included a blitter co-processor, which was named in all capitals as the BLITTER chip. One story states that manufacturing delays deferred its introduction into the ST line until after the first STs had shipped. Another is that the Atari ST's main competitor, the Amiga, was famous for its blitter, and so Atari introduced one as well. Although Atari planned an upgrade to allow dealers to install the blitter chip, this plan was later dropped. Instead, the BLITTER was introduced on the Mega series, and then also supported on most later machines (except the Atari TT
Atari TT030
-History:Atari Corporation realized that to remain competitive, they needed to begin taking steps to exploit the power offered by other processors in the Motorola 68000 series. At that time, the best option was the 68020. It was the first true "thirty-two bit bus/thirty-two bit instruction" chip...
).
Graphics-oriented software (especially games) running on systems that did not have a blitter needed to find other methods of transferring large bitmaps. Some games were written across platforms, some of which contained a blitter and some of which didn't. A typical example would be a game written for the Atari ST and Amiga. Both machines contained similar hardware, including the MC68000
Motorola 68000
The Motorola 68000 is a 16/32-bit CISC microprocessor core designed and marketed by Freescale Semiconductor...
processor, but the Amiga contained a blitter and the early Ataris did not. Such a game could not rely on the presence of a blitter. One approach to handling this was to load all available 68000 data register
Processor register
In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...
s with data from the bitmap in memory, and then push the data into the frame buffer in as few operations as possible.
Blitting was not the only solution to providing high-performance graphics in performance-limited machines. A more common solution in early machines was the use of sprites
Sprite (computer graphics)
In computer graphics, a sprite is a two-dimensional image or animation that is integrated into a larger scene...
, which used two different graphics pathways to draw images that were then combined in the video display circuitry into a single image. Sprites were small bitmaps that were positioned on the screen independent of the normal bitmap background, allowing them to be moved on-screen by adjusting the values of several timers. The video circuitry started drawing the sprites after the timer had expired, allowing them to be drawn for little cost, and avoiding the need to move memory around to provide the illusion of motion. The downside of this approach is that the sprite systems generally had hard-coded limits to the number of sprites they could display, often between two (the Atari VCS
Atari 2600
The Atari 2600 is a video game console released in October 1977 by Atari, Inc. It is credited with popularizing the use of microprocessor-based hardware and cartridges containing game code, instead of having non-microprocessor dedicated hardware with all games built in...
) and eight (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...
). Blitters offered the ability to have any number of objects, limited only by the performance of the blitter and the memory it talked to. As the performance of these circuits increased, the flexibility of the blitter overwhelmed any performance advantage in the sprite approach, and many sprite systems became blitters in disguise.
Operation
The destination for the transfer is usually the frame buffer. However, a blitter can also be used for non-graphics work. For example, an area of memory might be zeroed (filled with zeroes) using a blitter more quickly than can be accomplished with the CPU. Additionally, simple mathematical operations can be built from basic logical operations.
The image at right helps illustrate how a blitter may use a 'mask' to decide which pixels to transfer and which to leave untouched. The mask operates like a stencil
Stencil
A stencil is a thin sheet of material, such as paper, plastic, or metal, with letters or a design cut from it, used to produce the letters or design on an underlying surface by applying pigment through the cut-out holes in the material. The key advantage of a stencil is that it can be reused to...
, showing which pixels in the source image will be written to destination memory. The logical operation would be Dest = ((Background) AND (Mask)) OR (Sprite).
Current technology
Blitters have evolved into the modern graphics processing unitGraphics 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...
. The modern GPU is essentially a very advanced blitter and shares with earlier blitters the goal of rapidly copying, transforming, and writing transformed bitmaps (more generally textures), to a framebuffer. But, while early blitters were limited to performing simple logical operations on the target data, modern GPUs add the ability to modify these bitmaps in mathematically advanced ways that help produce more interesting effects, such as shading to produce illumination effects and blending for transparency. Additionally, modern GPUs can write bitmaps to destination memory in such a way so as to provide the illusion of depth. While earlier blitters were limited to a linear one-to-one mapping of source pixels to destination pixels, modern GPUs can instead map source pixels to destination pixels in a non-linear way to produce the appearance of perspective. Support for more advanced mathematical transformations also allows the modern GPU to transform blocks of coordinates in a 3d space. This ability, combined with shading and non-linear mapping between source and destination pixels helps to create the fluid 3d experience found in many of today's games.