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Intel 80486
The Intel 486 ("''four-eighty-six''"), also known as the i486 or 80486 was a higher performance follow-up to the Intel 80386 microprocessor. The 486 was introduced in 1989 and was the first tightly〔The 386, 286, and even the 8086 all had overlapping fetch, decode, execution (calculation), and write back; however, tightly pipelined usually means that all stages perform their respective duties within the same length time slot. In contrast loosely pipelined implies that some kind of buffering is used to decouple the units and allow them to work more independently. Both the original 8086 and the x86-chips of today are "loosely pipelined" in this sense, while the 486 and the original Pentium worked in a "tightly pipelined" manner for typical instructions. This included most "CISC" type instructions as well as the simple load/store-free "RISC-like" ones, although the most complex also used some dedicated microcode control.〕 pipelined x86 design as well as the first x86 chip to use more than a million transistors, due to a large on-chip cache and an integrated floating-point unit. It represents a fourth generation of binary compatible CPUs since the original 8086 of 1978.
A 50 MHz 486 executes around 40 million instructions per second on average and is able to reach 50 MIPS peak performance.
The i486 does not have the usual 80-prefix because of a court ruling that prohibits trademarking numbers (such as 80486). Later, with the introduction of the Pentium brand, Intel began branding its chips with words rather than numbers.
==Background==
The 486 was announced at Spring Comdex in April 1989. At the announcement, Intel stated that samples would be available in the third quarter of 1989 and production quantities would ship in the fourth quarter of 1989.〔486 32-bit CPU breaks new ground in chip density and operating performance. (Intel Corp.) (product announcement) EDN | May 11, 1989 | Pryce, Dave〕 The first 486-based PCs were announced in late 1989, but some advised that people wait until 1990 to purchase an 486 PC because there were early reports of bugs and software incompatibilities.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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