13 Facts About 8087


Intel 8087, announced in 1980, was the first x87 floating-point coprocessor for the 8086 line of microprocessors.

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Purpose of the 8087 was to speed up computations for floating-point arithmetic, such as addition, subtraction, multiplication, division, and square root.

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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 and power consumption of an 8087.

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Sales of the 8087 received a significant boost when IBM included a coprocessor socket on the IBM PC motherboard.

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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 sometimes referred to as "escape codes".

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Intel 8087 IBM PC CPU

Main CPU program continued to execute while the 8087 executed an instruction; from the perspective of the main 8086 or 8088 CPU, a coprocessor instruction took only as long as the processing of the opcode and any memory operand cycle, after which the CPU would begin executing the next instruction of the program.

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Instruction prefetch queues of the 8086 and 8088 make the time when an instruction is executed not always the same as the time it is fetched, a coprocessor such as the 8087 cannot determine when an instruction for itself is the next instruction to be executed purely by watching the CPU bus.

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The 8087 maintains its own identical prefetch queue, from which it reads the coprocessor opcodes that it actually executes.

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When Intel designed the 8087, it aimed to make a standard floating-point format for future designs.

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The 8087 looked for instructions that commenced with the "11011" sequence and acted on them, immediately requesting DMA from the main CPU as necessary to access memory operands longer than one word, then immediately releasing bus control back to the main CPU.

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The 8087 was able to detect whether it was connected to an 8088 or an 8086 by monitoring the data bus during the reset cycle.

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Intel 8087 coprocessors were fabricated in two variants: one with ceramic side-brazed DIP and one in hermetic DIP, and were designed to operate in the following temperature ranges:.

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Unlike later Intel coprocessors, the 8087 had to run at the same clock speed as the main processor.

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