Blitter

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, while freeing up the CPU's more complex ISA for more general operations.

The name comes from the bit blit operation, which stands for bit-block transfer. A typical use for a blitter is the movement of a bitmap, such as windows and fonts in a graphical user interface or sprites and backgrounds in a 2D computer game. 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), and because it may need to handle transparent pixels—pixels which should not overwrite the destination data.

In early computers with raster-graphics output, the screen buffer was normally held in main memory and updated using software running on the CPU. For many simple graphics routines, like sprite support or flood filling polygons, large amounts of memory had to be manipulated, and many CPU cycles were spent fetching and decoding instructions for repetitive loops of simple shift/masking operations. For CPUs without caches, this bus requirement for instructions was as significant as data.

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.

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