Why is my Stopwatch.Frequency so low?

Question

  Debug.WriteLine(\"Timer is high-resolution: {0}\", Stopwatch.IsHighResolution);
  Debug.WriteLine(\"Timer frequency: {0}\", Stopwatch.Frequency);

Res

Answer 1

3,579,545 is the magic number. That's the frequency in Hertz before dividing it by 3 and feeding it into the 8053 timer chip in the original IBM PC. The odd looking number wasn't chosen by accident, it is the frequency of the color burst signal in the NTSC TV system used in the US and Japan. The IBM engineers were looking for a cheap crystal to implement the oscillator, nothing was cheaper than the one used in every TV set.

Once IBM clones became widely available, it was still important for their designers to choose the same frequency. A lot of MS-DOS software relied on the timer ticking at that rate. Directly addressing the chip was a common crime.

That changed once Windows came around. A version of Windows 2 was the first one to virtualize the timer chip. In other words, software wasn't allowed to directly address the timer chip anymore. The processor was configured to run in protected mode and intercepted the attempt to use the I/O instruction. Running kernel code instead, allowing the return value of the instruction to be faked. It was now possible to have multiple programs using the timer without them stepping on each other's toes. An important first step to break the dependency on how the hardware is actually implemented.

The Win32 API (Windows NT 3.1 and Windows 95) formalized access to the timer with an API, QueryPerformanceCounter() and QueryPerformanceFrequency(). A kernel level component, the Hardware Adaption Layer, allows the BIOS to pass that frequency. Now it was possible for the hardware designers to really drop the dependency on the exact frequency. That took a long time btw, around 2000 the vast majority of machines still had the legacy rate.

But the never-ending quest to cut costs in PC design put an end to that. Nowadays, the hardware designer just picks any frequency that happens to be readily available in the chipset. 3,325,040,000 would be such a number, it is most probably the CPU clock rate. High frequencies like that are common in cheap designs, especially the ones that have an AMD core. Your number is pretty unusual, some odds that your machine wasn't cheap. And that the timer is a lot more accurate, CPU clocks have typical electronic component tolerances.