blitter

Blitter (from BLIT or Block Image Transfer) is a co-processor chip dedicated to memory data transfers, usually independently of the CPU using bit blit methods.

History

Previously the computer's CPU was given the job of moving bitmaps around in memory. But as graphics operations became more complex, this tied down the computer because the CPU had to perform other tasks.

The blitter was introduced to help lessen the graphics burden on the CPU. For some time in the 1980s several home computers included a graphics co-processor for this task. The CPU would send the bit blit operations to the blitter, which would then carry out the operation much faster than the CPU could.

One of the first personal computers that used this solution was the Commodore Amiga. The Amiga's chipset included a blitter. On top of the ability to copy and manipulate large areas of graphics, the Amiga's blitter also included line drawing and area-filling hardware.

Later models of the Atari ST also included a blitter co-processor. One story goes that manufacturing delays caused it to be introduced into the ST line after the first ST's were delivered. Another is that the Atari ST's main competitor, the Amiga, was famous for its blitter, and so Atari introduced one. Although Atari planned an upgrade to allow dealers to install the blitter chip, this plan was later dropped. Atari STE and up (except the Atari TT) had a blitter chip.

Alternative methods

In software (mostly games) for systems that did not have a blitter, other methods of transferring large graphics were necessary. This also applied to cross-platform software that could not rely on the presence of a blitter, but, for whatever reason, was not optimised for the features of each platform.

One approach to handling this, on MC68000 processors at least, was to load all available processor registers with data from memory, and then putting them back into memory in as few operations as possible. One Bitmap Brothers game, at least, used instructions for stack pushing and popping to accomplish this.

Current Technology

Today, Blitter functionality is one of the many specialized tasks that graphics processing units are optimized to perform quickly.

See also

• Bit blit
• Direct Memory Access

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