Where did 33/66/100(99) FSB originate from?

BCinSC

Platinum Member
Oct 11, 1999
2,084
0
0
Back in the days of the IBM XT, it was 4.77MHZ, then 8 & 12MHz for 286; the 386 was 16, 20, 25, and 33 (plus AMD's 40MHz, though I know there were some other bastard schemes out there with 6 & 10MHz XT, and 10 & 16MHz 286). The Intel camp seemed to settle on 33.333MHz for the 386/486 (and original Pentium?). At some point, the jump to 66.667MHz came along, and then 100 and now 133. How did that come to be? Why 33.333 and the subsequent double, triple and quadruple? How does the timing crystal work?
 

AndyHui

Administrator Emeritus<br>Elite Member<br>AT FAQ M
Oct 9, 1999
13,140
5
81
We got to 33, synchronous motherboard and CPU speeds, then motherboard makers found that it was hard to keep up sending 50MHz to the board while the CPU was racing along at 50MHz.....

That's how we got double clock with the Intel 486 DX2/50 and DX2/66. It was cheaper to produce a system where the motherboard was operating at 25 or 33MHz rather than doing something like a 486 DX/50. If you ever compared the prices between a DX/50 and a DX2/50, you will know what I mean. The speed of the processor was allowed to go up while the motherboard didn't need to. We got stuck with those speeds as Intel simply used a multiplier on the chip in order to scale the frequency.

The Intel Pentium brought in the 66MHz FSB as it was a big jump in performance over the 33MHz 486 bus....and we've been stuck with that sort of speed since the 1st Pentium/60 and 66s in 1992.
 

BCinSC

Platinum Member
Oct 11, 1999
2,084
0
0
But why 33? And I forgot about the 486/50 on 50MHz board. Even had a couple at work way back when and yes, they were REALLY expensive and frequently incompatible with add-in cards.
 

ASK THE COMMUNITY