Motorola 56000

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The Motorola DSP56000 (aka 56K) is a family of digital signal processor (DSP) chips produced by Motorola Semiconductor (now known as Freescale Semiconductor) starting in the 1980s and is still being produced in more advanced models in the 2000s. The 56k series was quite popular for a time in a number of computers, including the NeXT, Atari Falcon, and SGI Indigo workstations. Upgraded 56k versions are still used today in audio gear, radars, communications devices (like mobile phones) and various other embedded DSP applications. The 56000 was also used as the basis for the updated 96000, which was not commercially successful.

Technical description

The DSP56000 uses fixed-point arithmetic, with 24-bit program words and 24-bit data words. It includes two 24-bit registers, which can also be referred to as a single 48-bit register. It also includes two 56-bit accumulators, each with an 8-bit "extension" (aka headroom); otherwise, the accumulators are similar to the other 24/48-bit registers. Being a Harvard architecture processor, the 56k has two separate memory spaces+buses (and on-chip memory banks in some of the models): a program memory space/bus and a data memory space/bus.

24 bits were selected as the basic word length because it gave the system a reasonable number range and precision for processing audio (sound), the 56000's main concern. 24 bits correspond to a large 144dB dynamic range, sufficient in the 1980s when analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) rarely exceeded 20 bits. One example is ADSL applications, where filters typically require 20 bits of accuracy.[citation needed] The leftmost four bits are considered ample headroom for calculations.

Applications and Variants

In most designs the 56000 is dedicated to one single task, because digital signal processing using special hardware is mostly real-time and does not allow any interruption. For somewhat more mildly demanding tasks which are not time-critical, or more of a simple "if-then" type, designers normally use a separate CPU or MCU.

The addition of SIMD instructions to most desktop computer CPUs have meant that dedicated DSP chips like the 56000 have partly retreated from some application fields, but they continue to be used widely in communications and other professional uses. To this end the 56800 series added a complete MCU which created a single-chip "DSPcontroller" solution, while the opposite occurred in the 68456—a 68000 with a 56000 on it.

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