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Clock doubling in the CPU

The problem with the high clock frequencies is to ensure that other electronic components keep up with the pace. It is rather simple to make data move very fast inside a chip where the print tracks are microscopic. But when we move outside the chip, other problems appear. The other components must be able to keep up with the pace. When the frequency gets too high, the circuit board print tracks start acting as antennae and various forms of "radio noise" appears. Briefly, it becomes expensive to make the rest of the hardware to keep up with these high frequencies.

The solution to this problem was to split the clock frequency in two:
  • A high internal clock frequency, which governs the pace of the CPU.
  • A lower external clock frequency, which governs the pace on the system bus. This is where the CPU exchanges data with RAM and the I/O units.
Intel's 80486DX2 25/50 MHZ was the first chip with clock doubling. It was introduced in 1992 with great potential. For a lower price you could acquire a chip, which provided 90% of the 486DX50 performance. The DX50 runs at 50 MHZ both internally and externally. The DX2 runs at just 25 MHZ on the system bus. This enables lower cost system boards. Also RAM speed demands are much lower.

Clock doubling occurs inside the CPU. If the system board crystal works at 25 MHZ, the CPU will receive a signal every 40 nanosecond (ns). Internally in the CPU, this frequency is doubled to 50 MHZ. Now the clock ticks every 20 ns inside the CPU. This frequency governs all internal transactions, including integer unit, floating point unit, and all memory management unit operations as well as others. The only area still working at 25 MHZ are external data transfers. That is transfers to RAM, BIOS and the I/O ports.

Today the speed problem is in RAM. The ordinary FPM RAM and EDO RAM can function at a maximum of 66 MHZ (possibly 75 MHZ). Therefore, Pentium and similar CPU's are "clocked up" 2-4 times internally. They work well at high frequencies like 166, 200, 233 and 266 MHZ.

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