Motorola 56000

The Motorola 56000 was a series of DSP chips produced by Motorola largely in the 1980s and early 1990s. They were popular for a time in a number of computers, including the NeXT boxes, two models of the Apple Macintosh, and the SGI Indigo. Modified versions were also used in a number of communications uses.

The DSP56000 used fixed-point math with 24-bit program words and 24-bit data words. It included two functional units, meaning it could process two streams of data at the same time. Each path included two 24-bit registers, which could also be referred to as a single 48-bit register. It also included two 56-bit accumulators, which added an 8-bit "extension" to what was otherwise similar to the other 24/48-bit registers.

24 bits was selected as the basic word size deliberately: high-end audio systems quantize sound at 20-24 bits per sample, and use two channels for stereo. This was also useful in ADSL applications, where filters typically required 20 bits of accuracy.

The instruction set included some basic logic and looping, but was otherwise dedicated entirely to math. This made the design suitable only as an "add-on" chip in a design that included some other CPU to handle general purpose tasks and input/output.

The addition of SIMD instructions to most desktop CPUs have meant dedicated DSPs like the 56000 have largely disappeared from computer uses. They continue to be used widely in communications; better manufacturing has allowed them to fall in price and they can now be used in devices like modems. To this end the 56800 series also added a complete microcontroller which created a single-chip solution, while the opposite occurred in the 68456 which is a 68000 with a 56000 on it.

The latest model of the basic 56000 is the the 3rd generation 563xx family. The 56000 was also used as the basis for the updated 96000.

Links:

Motorola Digital Signal Processors (http://e-www.motorola.com/webapp/sps/site/taxonomy.jsp?nodeId=01M938562922795)