Performance Management Guide

POWER-based-Architecture-Unique Timer Access

Note: The following discussion applies only to the POWER and POWER2 architectures (and the 601 processor chip). The code examples will function correctly in a POWER-based system, but some of the instructions will be simulated. Because the purpose of accessing the processor timer is to obtain high-precision times with low overhead, simulation makes the results much less useful.

The POWER and POWER2 processor architectures include two special-purpose registers (an upper register and a lower register) that contain a high-resolution timer. The upper register contains time in seconds, and the lower register contains a count of fractional seconds in nanoseconds. The actual precision of the time in the lower register depends on its update frequency, which is model-specific.

Assembler Routines to Access the POWER Timer Registers

The following assembler-language module (timer.s) provides routines (rtc_upper and rtc_lower) to access the upper and lower registers of the timer:

           .globl  .rtc_upper
.rtc_upper: mfspr   3,4         # copy RTCU to return register
            br
 
           .globl  .rtc_lower
.rtc_lower: mfspr   3,5         # copy RTCL to return register
            br

C Subroutine to Supply the Time in Seconds

The following module (second.c) contains a C routine that calls the timer.s routines to access the upper and lower register contents. It returns a double-precision real value of time in seconds.

double second()
{
  int ts, tl, tu;
 
  ts = rtc_upper();     /* seconds                          */
  tl = rtc_lower();     /* nanoseconds                      */
  tu = rtc_upper();     /* Check for a carry from           */
  if (ts != tu)         /* the lower reg to the upper.      */
    tl = rtc_lower();   /* Recover from the race condition. */
  return ( tu + (double)tl/1000000000 );
}

The subroutine second() can be called from either a C routine or a FORTRAN routine.

Note: Depending on the length of time since the last system reset, the second.c module might yield a varying amount of precision. The longer the time since reset, the larger the number of bits of precision consumed by the whole-seconds part of the number. The technique shown in the first part of this appendix avoids this problem by performing the subtraction required to obtain an elapsed time before converting to floating point.