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Intel 80386

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The Intel 80386 is a microprocessor which was used as the Central Processing Unit (CPU) of many personal computers from 1986 until 1994 and later. During its design phase,, the processor was code-named simply "P3", the third-generation processor in the x86 line, but was, and is stil frequently, referred to as the i386. Designed and manufactured by Intel, the i386 processor was taped-out in October of 1985. Intel decided against producing the chip before then, as the cost of production would have been uneconomic. Full-function chips were first delivered to customers in 1986. Motherboards for 386-based computer systems were highly elaborate and expensive to produce, but were rationalised upon the 386's mainstream adoption.

The processor was a significant evolution in a long line of processors that stretched back to the Intel 8008. The predecessor of the Intel 80386 was the Intel 80286, a 16-bit processor with a segment-based memory management and protection system. The 80386 added a 32-bit architecture and a paging translation unit, which made it much easier to implement operating systems which used virtual memory.

Intel later introduced the Intel 80486, but not until the introduction of EM64T in 2004 did Intel's processors introduce as important a feature as the 32-bit flat addressing made possible by the 80386. (Other microprocessor architectures, such as the Motorola 68000, had long since supported this form of addressing.) Most applications running on current Intel-based personal computers will still run on the older 80386, albeit very slowly; there were relatively few instructions added to the main instruction set in later generations, and in most cases their usage can be avoided. Building a program for the Intel 80286 was often much harder.

Late in the 80386's production run, Intel introduced the Intel 80386-SX, which was meant to be a low cost version of the i386. The SX series of chips was 32-bit internally, but had a 16-bit external bus (in much the same way that the 8088 in the original IBM PC was a lower cost version of the 8086). The original 80386 was subsequently renamed the Intel 80386-DX to avoid confusion. Neither CPU included a math coprocessor (most motherboards included a socket for an 80387), though the naming would cause some head-scratching later when the Intel 80486 came in a DX variant that did include floating-point capability (while the Intel 80486-SX did not).

The 80386SL was introduced as an alternative processor for laptop computers. The processor offered several power management options, as well as different "sleep" modes to conserve battery power. It also contained support for an external cache of 16 to 64KB. The extra functions caused this variant to have over 3 times as many transistors as the 80386DX. The 80386SL was only available in one clock speed, 25MHz.

Because of the high degree of compatibility, the range of processors compatible with the 80386 is often collectively termed the i386 Architecture; the instruction set for the architecture is now known as IA-32 or, informally, i386.

From a business perspective, the i386 was significant because it was the first significant microprocessor to be single-sourced; it was available only from Intel Corp. Prior to this, the difficulty of making chips and the uncertainty of reliable supply required that any mass-market semiconductor be multi-sourced, that is, made by two or more manufacturers, the second and subsequent ones manufacturing under license from the designer. Single-sourcing the i386 allowed Intel greater control over its development and substantially greater profits in later years. However, AMD introduced its compatible Am386 processor in March 1991 after overcoming legal obstacles, thus breaking Intel's monopoly.

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