static inline unsigned long do_fast_gettimeoffset(void)
{
register unsigned long eax, edx;
/* Read the Time Stamp Counter */
rdtsc(eax,edx);
/* .. relative to previous jiffy (32 bits is enough) */
eax -= last_tsc_low; /* tsc_low delta */
/*
* Time offset = (tsc_low delta) * fast_gettimeoffset_quotient
* = (tsc_low delta) * (usecs_per_clock)
* = (tsc_low delta) * (usecs_per_jiffy/clocks_per_jiffy)
*
* Using a mull instead of a divl saves up to 31 clock cycles
* in the critical path.
*/
__asm__("mull %2"
:"=a" (eax), "=d" (edx)
:"rm" (fast_gettimeoffset_quotient),
"" (eax));
/* our adjusted time offset in microseconds */
return delay_at_last_interrupt + edx;
}
Suppose I wanted to do the same thing as above but with both the upper and
lower 32 bits what is read by rdtsc, what would I do?
The intel documents have some code that looks like this:
unsigned time, time_low, time_high;
unsigned mhz = 150000000; // 150 MHz processor
__asm rdtsc // Read time stamp to EAX
__asm mov time_low, eax
__asm mov time_high, edx
Sleep (35000); // Sleep for 35 seconds
__asm rdtsc
__asm sub eax, time_low // Find the difference
__asm sub edx, time_high
__asm div mhz // Unsigned divide EDX:EAX by mhz
__asm mov time, eax
printf("Seconds: %u\n", time);
What exactly does the div line do?
Anton
--
Kernelnewbies: Help each other learn about the Linux kernel.
Archive: http://mail.nl.linux.org/kernelnewbies/
FAQ: http://kernelnewbies.org/faq/
