Intel 80486 Processor

关于 Intel 80486

80486（简称 486 或是 i486）是英特尔第一个内部包含浮点运算器（FPU）的中央处理器（DX 版CPU），并在 x86 系列中首次使用了 RISC (精简指令集)技术，从而提升了每周期执行指令的速度。486 还采用了更进步的数据总线，大大提高了处理器与内存的数据交换速度。由于这些改进，同样速度的 i486DX 的性能比拥有 80387 数学辅助运算器的 80386 快了两倍有余。不过 486SX 早期的产品反而比 80386 高阶的产品还慢。

从 80486 DX 开始，浮点运算器开始内建于微处理器中，并开始使用倍频，所以微处理器可以以比主板更高的频率运作。好笑的是，SX 往往就是浮点运算器有问题的 DX，英特尔把其浮点运算器关闭后拿来当 SX 卖。当时 486 也是一堆公司争相仿冒的对象。当时对于数学运算比较不注重（或是想省钱）的用户，都会选择比较便宜的 SX，可是当发现其运算能力不足的时候则需要添购 80487 浮点运算器，这样花费反而不划算。可是其实较早推出的 80486 DX 早就内建了浮点运算器，所以这种情况很少会发生，当然 80487 也变成非常少见的产品。

A80486SX-16 只有 16MHz，是 80486 里面最慢的产品，数量很少。A80486DX ES 则是 486 的工程版，为 486 正式推出之前的样本，也是鲜少见到的产品。

Introduction

The 486 is very similar to its predecessor, the 386. Main differences are an optimised instruction set, 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 386 at the same clock rate.

The 486 processor has been licensed or reverse engineered by other companies such as IBM, AMD and Cyrix. Some manufacturers made hybrid 386/486 CPUs (Cxrix Cx486DLC, Texas Instruments TX486DLC), having a 486 instruction set and a 386-compatible pinout. The RapidCAD is a 486 DX with 386 pinout and dummy FPU chip for upgrading 386 systems to 486 technology.

The 486SX processors were the same as their DX counterparts in design, but the built in math co processor was disabled. There was a 487 which was actually a 486 with the math co processor enabled. If the motherboard had this slot for the 487, the SX cpu would be disabled and the 487 cpu would take over all processing.

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