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A semiconductor chip, or chip set, that implements the central processor of a computer. Microprocessors consist of, at a minimum, an ALU and a control unit. They are characterized by speed, word length (internal and external), architecture, and instruction set, which may be either fixed or microprogrammed. It is the combination of these characteristics and not just the cycle time that determines the performance of a microprocessor.

Most microprocessors have a fixed instruction set. Microprogrammed processors have a control store containing the microcode or firmware that defines the processor' instruction set; such processors may either be implemented on a single chip or constructed using bit-slice architecture elements. RISC microprocessors are designed to execute a small number of simple instructions extremely fast.

The processor' architecture determines what register, stack, addressing, and I/O facilities are available, as well as defining the processor' primitive data types. The data types, which are the fundamental entities that can be manipulated by the instruction set, have included bit, nibble (4 bits), byte (8 bits), word (16 bits), double words (32 bits), and quadruple words (64 bits). Note that a word is usually defined as the number of bits in the processor' internal data bus rather than always being 16 bits. Instructions generally include arithmetic, logical, flow-of-control, and data movement (between stacks, registers, memory, and I/O ports).

The first microprocessor, the four-chip set Intel 4004, appeared in 1971 accompanied by considerable debate about its utility and marketability. It was the outcome of an idea proposed by Ted Hoff of Intel Corp. for a calculator that could implement a simple set of instructions in hardware but permitted complex sequences of them to be stored in a read-only memory (ROM). The result of his proposal was a design for a four-chip set consisting of a CPU, ROM, RAM, and a shift-register chip, the chip design proceeding in 1970 under the direction of Federico Faggin, later the founder of Zilog, Inc. The Intel 4004 had a 4-bit data bus, could address 4.5 Kbytes of memory, and had 45 instructions. Its 8-bit counterpart, the Intel 8008, was introduced in 1974 and its improved derivative, the Zilog Z80, in 1976. By this time there were over 50 microprocessors on the market.

The next generation of microprocessors included the Zilog Z8000, Motorola 68000, Intel 8086, National 16000, as well as the older Texas Instruments 9900 and Digital Equipment Corporation LSI-11. All of these chips use a 16-bit-wide external data bus. Microprocessors that use 32-bit external data buses include the Intel386, Intel486, Motorola 68030, and Digital' VAX 78032 and 78132 (processor and FPA). Modern processors, for example recent products in Intel' Pentium family and its derivatives and in AMD' Athlon family, use a 64-bit external data bus and 32- or 64-bit data types. RISC microprocessor chips with a 64-bit architecture include the PowerPC and SPARC families. See also multi-core processor.

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