About 4 years ago I finally bit the bullet and purchased my first laptop. I was always bothered that I could build a top-of-the-line desktop computer for so much cheaper (about a third of the cost of this particular laptop), which is mainly why I've mostly been a desktop guy my whole life. I really needed portability though so I finally caved.

I was aware that laptop CPUs employed frequency scaling to conserve power (i.e. SpeedStep). When my AC adapter wasn't plugged in I noticed that my CPU speed was much lower than its "native" clock speed under Windows System Properties. I thought it might be fun to write a program to give me my current clock speed, because I found myself repeating the same steps often to pull up System Properties because I was curious what the current clock speed was. I originally intended to make a little applet that ran in the system tray that would give me a tooltip with the current CPU speed or something, but I never ended up completing it. I did, however, find out how to get this information programatically through the Win32 API and made a small command-line program to test it.

Initially I had a real problem finding any really useful information. After lots of googling I must have hit the right combination of search keywords, because I finally stumbled upon CallNtPowerInformation. To use it, one must include powrprof.h and link with powrprof.lib. However, there are a few catches...

First, powrprof.h hasn't been C++-proofed like most of the other headers in the Platform SDK. You'll get linker errors if you don't take care before including it in a C++ program:

extern "C" {
#include <powrprof.h>
}

The second issue is a structure that is for some reason missing from powrprof.h, PROCESSOR_POWER_INFORMATION. I couldn't find it anywhere 4 years ago, and apparently it's still missing from my current copy of the Platform SDK that comes with Visual C++ 2008 Express Edition. In order to retrieve current processor speed, we need to call CallNtPowerInformation with the ProcessorInformation enumerated value as the first parameter and an array of PROCESSOR_POWER_INFORMATION structures as the output buffer (one for each installed processor/core in the system). Luckily for us, the structure is defined in MSDN so we can just define it ourselves:

typedef struct _PROCESSOR_POWER_INFORMATION {
  ULONG  Number;
  ULONG  MaxMhz;
  ULONG  CurrentMhz;
  ULONG  MhzLimit;
  ULONG  MaxIdleState;
  ULONG  CurrentIdleState;
} PROCESSOR_POWER_INFORMATION , *PPROCESSOR_POWER_INFORMATION;

As you can see from the definition of PROCESSOR_POWER_INFORMATION, this structure provides with CPU speed information such as the maximum speed, current speed, and idle states (although I admit I don't know much about the idle states bit--I should probably research it).

A related API function that is also useful is GetPwrCapabilities, which tells us whether or not the processor supports frequency throttling/scaling/SpeedStep and what the throttling percentages are.