(Or "i486", "iAPX 80486", and "Intel DX4" but usually just "486"). A range of Intel CISC microprocessors which is part of the Intel 80x86 family of processors. The 486s are very similar to their immediate predecessor, the Intel 80386DX. The main differences are that the 486 has an optimised instruction set, has an on-chip unified instruction and data cache, an optional on-chip floating-point unit (FPU), and an enhanced bus interface unit. These improvements yield a rough doubling in performance over an Intel 80386 at the same clock rate. There are several suffixes and variants including: Intel 486SX - a 486DX with a faulty FPU that has been disabled in the factory. Intel 486DX - 486SX with a working FPU. 486DX-2 - runs at twice the external clock rate. 486SX-2 - runs at twice the external clock rate. 486SL - 486DX with power conservation circuitry. 486SL-NM - 486SX with power conservation circuitry; SL enhanced suffix, denotes a 486 with special power conservation circuitry similar to that in the 486SL processors. 487 - 486DX with a slightly different pinout for use in 486SX systems. OverDrive - 486DX-2 with a slightly different pinout for use in 486SX systems. RapidCAD - 486DX in a special package with a companion FPU dummy package for use in Intel 80386 systems. Intel DX4, Cyrix Cy486SLC. External clock rates include 16MHz, 20MHz, 25MHz, 33MHz, 40MHz, although 16Mhz is rare now, and the 20MHz processors are often clock doubled. The 486 processor has been licensed or reverse engineered by other companies such as IBM, AMD, Cyrix, and Chips & Technologies. Some are almost exact duplicates in specications and performance, some aren't. The successor to the 486 is the Pentium.