Measure the exact time (number of cycles) of a set of instructions

Question

I have some sectors on my drive with poor reading. I could measure the reading time required by each sector and then compare the time of the good sectors and the bad sectors.

I could use a timer of the processor to make the measurements. How do I write a program in C/Assembly that measures the exact time it takes for each sector to be read?

So the procedure would be something like this:

Start the timer
Read the disk sector
Stop the timer
Read the time measured by the timer

Answer 1

The most useful functionality is the "rdtsc" instruction (ReaD Time Stamp Counter) which is incremented every time the processor's internal clock increments. For a 3 Ghz processor it increments 3 billion times per second. It returns a 64 bit unsigned integer containing the number of clock cycles since the processor was powered on.

Obviously the difference between two read-outs is the number of elapsed clock cycles consumed for executing the code sequence in-between. For a 3 Ghz machine you could use any of the following algorithms to convert to parts of seconds:

(time_difference+150)/300 gives a rounded off elapsed time in 0.1 us (tenths of microseconds) (time_difference+1500)/3000 gives a rounded off elapsed time in us (microseconds) (time_difference+1500000/3000000 gives a rounded off elapsed time in ms (milliseconds)

The 0.1 us algorithm is the most precise value you can use without having to adjust for read-out overhead.