TSC unstable on Intel Pentium M processor 750

["Mao Yilu" <ylmao@mail.ustc.edu.cn>]

Hi,

I masked all the interrupts except keyboard interrupt before the instruction “hlt”, So the CPU ran nothing until I click the keyboard button. And I used “rdtscll” and “do_gettimeofday” to get the halt time. But the result of “do_gettimeofday” was longer than the one of “rdtscll” about 3s per minute. I don’t know why.
I timed some math job using the same way in contrary to the “hlt” instruction. The TSC was not correct either. In 3 minutes, the result of “do_gettimeofday” was longer than the one of “rdtscll” about 5s.
The processor is Intel Pentium M processor 750. From the software developer’s manual of Intel, the processor clock of Pentium M processors is impacted by Intel SpeedStep technology, while some other processors is not. Maybe this is another feature Pentium M processors own.
Pentium M processors support 5 C-states. I am sure the TSC doesn’t stop in C0 and C1 states. But I am not sure about other states. Maybe other states will stop the clock, and “hlt” instruction will make the CPU into deeper state.
These are my guess. I wanna the truth. Thank you.

Mao Yilu