Wednesday, June 19, 2013

Intel coprocessors

Main article: x87 i8087 and i80287 microarchitecture. i80387 microarchitecture.

The original IBM PC included a socket for the Intel 8087 floating-point coprocessor (aka FPU) which was a popular option for people using the PC for CAD or mathematics-intensive calculations. In that architecture, the coprocessor sped up floating-point arithmetic on the order of fiftyfold. Users that only used the PC for word processing, for example, saved the high cost of the coprocessor, which would not have accelerated performance of text manipulation operations.

The 8087 was tightly integrated with the 8086/8088 and responded to floating-point machine code operation codes inserted in the 8088 instruction stream. An 8088 processor without an 8087 could not interpret these instructions, requiring separate versions of programs for FPU and non-FPU systems, or at least a test at run time to detect the FPU and select appropriate mathematical library functions.

Intel 80386DX CPU with 80387DX Math Coprocessor

Another coprocessor for the 8086/8088 central processor was the 8089 input/output coprocessor. It used the same programming technique as 8087 for input/output operations, such as transfer of data from memory to a peripheral device, and so reducing the load on the CPU. But IBM didn't use it in IBM PC design and Intel stopped development of this type of coprocessor.

The Intel 80386 microprocessor used an optional "math" coprocessor (the 80387) to perform floating point operations directly in hardware. The Intel 80486DX processor included floating-point hardware on the chip. Intel released a cost-reduced processor, the 80486SX, that had no floating point hardware, and also sold an 80487SX co-processor that essentially disabled the main processor when installed, since the 80487SX was a complete 80486DX with a different set of pin connections.

Intel processors later than the 80486 integrated floating-point hardware on the main processor chip; the advances in integration eliminated the cost advantage of selling the floating point processor as an optional element. It would be very difficult to adapt circuit-board techniques adequate at 75 MHz processor speed to meet the time-delay, power consumption, and radio-frequency interference standards required at gigahertz-range clock speeds. These on-chip floating point processors are still referred to as coprocessors because they operate in parallel with the main CPU.

During the era of 8- and 16-bit desktop computers another common source of floating-point coprocessors was Weitek. These coprocessors had a different instruction set than the Intel coprocessors, and used a different socket, which not all motherboards supported. The Weitek processors did not provide transcendental mathematics functions (for example, trigonometric functions) like the Intel x87 family, and required specific software libraries to support their functions.

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