Blitter

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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.

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The historical need for a blitter

In early computers 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 bitmaps, 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, framebuffers 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 computers 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.

One of the first blitter-like units was used in the Bally Astrocade. The Astrocade was set up so all addresses below 0x4000 were dedicated to ROM. The graphics chip was wired to notice all attempts to write to this address range, and then "flip" the address to its own memory area in RAM, applying a function to the data being written. Using XOR as the function, basic blitting operations were very easy to implement.

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