The processor is the part of the computer that actually does the computations. This is sometimes called an MPU (for main processor unit) or CPU (for central processing unit or central processor unit).
A
processor typically contains an arithmetic/logic unit (ALU), control
unit (including processor flags, flag register, or status register), internal
buses, and sometimes special function units (the most common special function
unit being a floating point unit for floating point arithmetic).
Some
computers have more than one processor. This is called multi-processing.
The
major kinds of digital processors are: CISC, RISC, DSP, and hybrid.
CISC
stands for Complex Instruction Set Computer. Mainframe computers and
minicomputers were CISC processors, with manufacturers competing to offer the
most useful instruction sets. Many of the first two generations of
microprocessors were also CISC.
RISC
stands for Reduced Instruction Set Computer. RISC came about as a result of
academic research that showed that a small well designed instruction set
running compiled programs at high speed could perform more computing work than
a CISC running the same programs (although very expensive hand optimized
assembly language favored CISC).
DSP
stands for Digital Signal Processing. DSP is used primarily in dedicated
devices, such as MODEMs, digital cameras, graphics cards, and other specialty
devices.
Hybrid
processors combine elements of two or three of the major classes of processors.