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The Intel i486 (also called 486 or 80486) is a range of Intel CISC microprocessors which is part of the Intel x86 family of processors. The i486's predecessor was the Intel 80386 processor. The i486 was named without the usual 80-prefix, and with its successor, the Pentium processor, Intel dropped number-based naming altogether.
From a software point of view, the instruction set of the i486 family is very similar to its predecessor, the Intel 80386, with the addition of only a few extra instructions.
From a hardware point of view, however, the architecture of the i486 is a vast improvement. It has an on-chip unified instruction and data cache, an optional on-chip floating-point unit (FPU), and an enhanced bus interface unit. In addition, under optimal conditions, the processor core can sustain an execution rate of one instruction per clock cycle. These improvements yield a rough doubling in performance over an Intel 80386 at the same clock rate. However, some low-end i486 models were actually slower than the highest-speed 386s, especially so with the 'SX' i486s.
A 25 MHz version was introduced in April 1989, a 33 MHz version in May 1990, and a 50 MHz version in June 1991.
There are several suffixes and variants including:
- Intel 80486SX - a i486DX with its FPU disabled, although the earlier variants were simply normal i486s with defective FPUs. In later versions, the FPU was removed from the die to reduce its area and thus reduce cost.
- Intel 80486DX - same as above, with a working FPU.
- Intel 80486DX2 - the internal processor clock runs at twice the clock rate of the external bus clock.
- Intel 80486SX2 - same as the i486DX2, but with the FPU disabled.
- Intel 80486SL - i486DX with power conservation circuitry. Mainly for use in portables computers.
- Intel 80486SL-NM - i486SX with power conservation circuitry; SL enhanced suffix, denotes a i486 with special power conservation circuitry similar to that in the i486SL processors.
- Intel 80487 - i486DX with a slightly different pinout for use in i486SX systems as a FPU.
- Intel 80486 OverDrive - i486SX, i486SX2, i486DX2 or i486DX4. Marked as upgrade processors, some models had different pinouts or voltage handling abilities from 'standard' chips of the same speed stepping.
- Intel 80486DX4 - designed to run at triple clock rate (not quadruple as often believed).
External clock rates include 16MHz, 20MHz, 25MHz, 33MHz, 40MHz and 50MHz. Some later i486 motherboards provided unofficial and undocumented support for 60 and 66MHz, however.
The i486 processor has been licensed or reverse engineered by other companies such as IBM, Texas Instruments, AMD, Cyrix, and Chips and Technologies. Some are almost exact duplicates in specifications and performance, some are not.
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