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The Intel 80486
Dan Knight - September 2001
From the 8080 through the 80386, CPUs gained most of their improved performance from greater clock speed and a wider data bus. With the next generation, both Intel and Motorola (in their 680x0 family) worked on making the processor more efficient.
For both companies, this included a level 1 cache on the chip itself (8 KB on the 486) and the integration of the FPU (floating point unit, a math coprocessor) into the CPU itself. FPU integration eliminated the bottleneck when moving 64 or 80 bits of data between the CPU and FPU over a 32-bit data bus.
As a general rule of thumb, the 486 offered two to two-and-a-half times the performance of the 386 at the same clock speed. (Likewise for the Motorola 68040 vs. the earlier 68030.) This actually let companies release lower MHz computers with improved performance.
Byte magazine (May 1993) noted that the 486 had a MIPS (million instructions per second) rating of 20 (at 25 MHz) to 54 (66 MHz). This comes to about 0.8 MIPS/MHz, which is 2.4x the rating of the 386DX (0.33 MIPS/MHz), although we must keep in mind that MIPS is not an accurate pedictor of real world CPU performance.
The original 80486, also known as the 486 DX (and sometimes DX2), had 1.2 million transistors using 1 micron technology. This included the CPU, FPU, and data cache. Speeds ranged from 25 MHz to 50 MHz. The 486 could address 4 GB of RAM and 64 TB (terabytes) of virtual memory. The core of the processor ran at twice bus speed.
A more powerful version, the 486 DX4, runs the processor at 3x bus speed. This shipped in 75 and 100 MHz speeds.
As with the earlier 386 family, Intel released a lower cost version of the 486 and dubbed it 486 SX. The 486 SX was deliberately crippled by disabling the FPU, but the integrated cache still made it a decent performer. It was also clocked at lower speeds than the 486 DX: 16, 20, 25, and 33 MHz.
As noted on the 486 Processors Chart, AMD, Cyrix, and IBM also made 486-class processors. AMD offered SX versions at 33- 66 MHz, DX versions from 25-80 MHz, and DX4 versions ranging from 75 MHz to 120 MHz. Cyrix 486DX processors came in 33-80 MHz versions, while their DX4 variant ran at 75 anhd 100 MHz speeds. The IBM 486 processors have the same specs as the Cyrix processors; I believe this is because IBM build Cyrix's processors.
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