Intel 80486

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The Intel 80486 microprocessor (alias i486 or Intel486) was a higher performance follow up on the Intel 80386. Introduced in 1989, it was the first tightly[1] 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 80486 executed around 40 million instructions per second on average and was able to reach 50 MIPS peak.

The i486 was without the usual 80-prefix because of a court ruling that prohibited trademarking numbers (such as 80486). Later, with the introduction of the Pentium brand, Intel began branding its chips with words rather than numbers.

Contents

Background

The 80486 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.[2] The first 80486-based PCs were announced in late 1989, but some advised that people wait until 1990 to purchase an 80486 PC because there were early reports of bugs and software incompatibilities.[3]

Improvements

The instruction set of the i486 is very similar to its predecessor, the Intel 80386, with the addition of only a few extra instructions, such as CMPXCHG which executes the compare-and-swap atomic operation and the XADD which executes the fetch-and-add atomic operation returning the original value, unlike the ADD instruction that only returned some flags.

From a performance point of view, the architecture of the i486 is a vast improvement over the 80386. It has an on-chip unified instruction and data cache, an on-chip floating-point unit (FPU), except in the SX and SL models, and an enhanced bus interface unit. Simple instructions (such as ALU reg, reg) execute in one clock cycle. These improvements yield a rough doubling in ALU performance over the 386 at the same clock rate. A 16-MHz 486 therefore has a performance similar to a 33-MHz 386 (or 286), and the older design has to reach 50 MHz to be comparable with a 25-MHz 486 part.[4]

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