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A blitter is a circuit, sometimes as a coprocessor or a logic block on a microprocessor, dedicated to the rapid movement and modification of data within a computer's memory. A blitter can copy large quantities of data from one memory area to another relatively quickly, and in parallel with the CPU, while freeing up the CPU's more complex capabilities for other operations. A typical use for a blitter is the movement of a bitmap, such as windows and fonts in a graphical user interface or images and backgrounds in a 2D computer game. The name comes from the bit blit operation of the 1973 Xerox Alto, which stands for bit-block transfer. A blit operation is more than a memory copy, because it can involve data that's not byte aligned (hence the bit in bit blit), handling transparent pixels (pixels which should not overwrite the destination), and various ways of combining the source and destination data.
Blitters have largely been superseded by programmable graphics processing units.
In early computers with raster-graphics output, the frame buffer was normally held in main memory and updated via software running on the CPU. For many simple graphics routines, like compositing a smaller image into a larger one (such as for a video game) or drawing a filled rectangle, large amounts of memory had to be manipulated, and many CPU cycles were spent fetching and decoding instructions for short repetitive loops of load/store instructions. For CPUs without caches, the bus requirement for instructions was as significant as data. Further, as a single byte usually held between 2 (for 16-colors) and 8 (for monochrome) pixels, the data was not naturally aligned for the CPU, so extra shifting and masking operations were required.
Blitters in personal computing
The 1973 Xerox Alto, where the term bit blit originated, had a bit block transfer instruction implemented in microcode, making it much faster than the same operation written on the CPU. The microcode was implemented by Dan Ingalls.
The MS-DOS compatible Mindset contained a custom VLSI chip to move rectangular sections of a bitmap. The hardware handled transparency and eight modes for combining the source and destination data. Released in 1984, the Mindset was claimed to have graphics up to 50x faster than PCs of the time, but the system was not successful.
The Commodore Amiga, released the following year, also has a full-featured blitter. 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. The blitter performs a "4 operand" boolean operation (typically destination:=op(destination, source, mask))
Later models of the Atari ST include a blitter co-processor, which was stylized as the BLiTTER chip. It was introduced on the Mega series, and then also supported on most later machines (except the Atari TT).
Blitters in arcade games
1982's Robotron: 2084 from Williams Electronics includes two blitter chips which allow the game to have up to 80 simultaneously moving objects. Performance was measured at roughly 910 KB/second. The blitter operates on 4-bit (16 color) pixels where color 0 is transparent, allowing for non-rectangular shapes. Williams used the same blitters in other games from the time period, including Sinistar and Joust.
The TMS34010, released in 1986, is a general purpose 32-bit processor with additional blitter-like instructions for manipulating bitmap data. It is optimized for cases that normally take extra processing on the CPU, such as handling transparent pixels, working with non-byte aligned data, and converting between bit depths. The TMS34010 served as both CPU and GPU for a number of games in the late 1980s and early 1990s, including Hard Drivin', Narc, Smash TV, Mortal Kombat, and NBA Jam.
Typically, a computer program puts information into certain registers describing what memory transfer needs to be completed and the logical operations to perform on the data. The CPU then triggers the blitter to begin operating. The CPU is free for other processing while the blitter is working, though the blit running in parallel uses memory bandwidth.
To copy data with transparent portions—such as sprites—a color can be designated to be ignored during the blit (such as color 0). On other systems, a second 1 bit per pixel image may be used as a "mask" to indicate which pixels to transfer and which to leave untouched (see image at right). The mask operates like a stencil. The logical operation would be destination := (background AND mask) OR sprite.
Hardware sprites are small bitmaps that can be positioned independently, composited together with the background on-the-fly by the video chip, so no actual modification of the frame buffer occurs. Sprite systems are more efficient for moving graphics, typically requiring 1/3 the memory cycles because only image data—not CPU instructions—need to be fetched, with the subsequent compositing happening on-chip. The downside of sprites is a limit of moving graphics per scanline, which can range from three (Atari 2600) to eight (Commodore 64 and Atari 8-bit family) to significantly higher for 16-bit arcade hardware and consoles, and the inability to update a permanent bitmap (making them unsuitable for general desktop GUI acceleration).
- Direct Memory Access (DMA)
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