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The Intel 8087, announced in 1980, was the first x87 floating-point coprocessor for the 8086 line of microprocessors. The purpose of the 8087 was to speed up computations for floating-point arithmetic, such as addition, subtraction, multiplication, division, and square root. It also computed transcendental functions such as exponential, logarithmic or trigonometric calculations, and besides floating-point it could also operate on large binary and decimal integers. The performance enhancements were from approximately 20% to over 500%, depending on the specific application. The 8087 could perform about 50,000 FLOPS〔 using around 2.4 watts.〔 Only arithmetic operations benefited from installation of an 8087; computers used only with such applications as word processing, for example, would not benefit from the extra expense (around $150〔) and power consumption of an 8087. The sales of the 8087 received a significant boost when IBM included a coprocessor socket on the IBM PC motherboard. Development of the 8087 led to the IEEE 754-1985 standard for floating-point arithmetic. There were later x87 coprocessors for the 80186 (not used in PC-compatibles), 80286, 80386, and 80386SX processors. Starting with the 80486, the later Intel x86 processors did not use a separate floating point coprocessor; floating point functions were provided integrated with the processor. ==Design and development== Intel had previously manufactured the 8231 ''Arithmetic processing unit'', and the 8232 ''Floating Point Processor''. These were designed for use with 8080 or similar processors and used an 8-bit data bus. They were interfaced to a host system either through programmed I/O or a DMA controller.〔Intel ''Component Data Catalog 1980'', Intel catalog no. C-864/280/150K/CP, pages 8-21, 8-28〕 The 8087 was initially conceived by Bill Pohlman, the engineering manager at Intel who oversaw the development of the 8086 chip. Bill took steps to be sure that the 8086 chip could support a yet-to-be-developed math chip. In 1977 Pohlman got the go ahead to design the 8087 math chip. Bruce Ravenel was assigned as architect, and John Palmer was hired to be co-architect and mathematician for the project. The two came up with a revolutionary design with 64 bits of mantissa and 16 bits of exponent for the longest format real number, with a stack architecture CPU and 8 84-bit stack registers, with a computationally rich instruction set. The design solved a few outstanding known problems in numerical computing and numerical software: rounding error problems were eliminated for 64-bit operands, and numerical mode conversions were solved for all 64-bit numbers. Palmer credited William Kahan's writings on floating point as a significant influence on their design.〔Julio Sanchez and Maria P. Cannon, Software Solutions for Engineers and Scientists, p. 96〕 The 8087 design languished for almost a year due to its aggressive design. Eventually, the design was given to Intel Israel, and chip designer Rafi Nave was assigned to implement the chip. Ravenel, Palmer and Nave were awarded patents for aspects of the design.〔(Patent 4,484,259 )〕 Robert Koehler and John Bayliss were also awarded a patent for the technique where some instructions with a particular bit pattern were offloaded to the coporocessor.〔(Patent 4,270,167 )〕 The 8087 had 45,000 transistors and was manufactured as a 3 μm depletion load HMOS circuit. It worked in tandem with the 8086 or 8088 and introduced about 60 new instructions. Most 8087 assembly mnemonics begin with F, such as FADD, FMUL, FCOM and so on, making them easily distinguishable from 8086 instructions. The binary encodings for all 8087 instructions begin with the bit pattern 11011, decimal 27, the same as the ASCII character ESC although in the higher order bits of a byte; similar instruction prefixes are also sometimes referred to as "escape codes". When the 8088 saw the escape code, it would defer to the 8087 until it was ready. The codes are encoded in 6 bits across 2 bytes, beginning with the escape sequence: ┌───────────┬───────────┐ │ 1101 1xxx │ mmxx xrrr │ └───────────┴───────────┘ The first three x's are the first three bits of the floating point opcode. Then two m's, then the latter half three bits of the floating point opcode, followed by three r's. The m's and r's specify the addressing mode information.〔Assembly Language and Systems Programming for the IBM PC and Compatibles, Karen A. Lemone, page 302〕 Application programs had to be written to make use of the special floating point instructions. At run time, software could detect the coprocessor and use it for floating point operations. When detected absent, similar floating point functions had to be calculated in software or the whole coprocessor could be emulated in software for more precise numerical compatibility.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Intel 8087」の詳細全文を読む スポンサード リンク
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