The 8088 debuted at 5 (& 8) Mhz; IBM derated it a bit for the PC because
4.77Mhz*3 = 14.318 = 4 * 3.57Mhz (TV color burst frequency).
An IBM (pre-XT) motherboard could be pushed to about 7.5 Mhz by splitting
the clock signal paths. FPU was separate. Ten Mhz chips were offered.
This chip had competition from the 8086, 80186, and NEC V20.
Intel offered the 286 with 6, 8, 10, and 12.5 Mhz on one data sheet.
AMD got to 16 on this one, but an early data sheet lists 4, 6, and 8
(and says reprinted by permission of Intel). FPU was separate.
I don't recall a 286-20. The original IBM AT was designed with CPU
clocked at 6Mhz, FPU at 4Mhz.
The 386 debuted at 12.5 and 16 Mhz. These had no cache
on-chip. On-motherboard cache was common for 25Mhz and above.
Intel's offerings topped out at 33Mhz, AMD offered 40
eventually. The 386 core was also throttled down to 16-bit bus width
in the 386SX, which had speeds as low as 10Mhz and up to 20
or higher. FPU was separate. Cyrix offered a faster FPU.
TI eventually offered a clock-doubled near 486 with cache in a 386 pinout.
The 486 was the first to offer an on-chip FPU, came out at 20 and
25 Mhz and went to 100 Mhz (core=3x memory bus)
in the Intel line, 133 (4x) elsewhere (AMD?). The 486 socket's performance
could be stretched a little further by using a Pentium Overdrive chip
from Intel; at 83 Mhz (2.5x) giving 1.7x the performance of a 486-66
in real world finite element analysis (ANSYS). The 486SX was a
no-fpu 486, with full memory bus width.
The Pentium I's in various subflavors went from 60 & 66 at announcement,
to 200 (nonMMX) and 233 in the MMX type.
I think the Pentium Pro's went 166, 180, and 200.
Pentium II's included the 266 Mhz speed.
Ken
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