Hi Cameron, The rdtsc function has its origin in the rdtsc instruction on an x86 CPU, it returns a 64-bit value, the number of cycles since power on. I can see from the rdtsc function for PowerPC that it is implemented using a routine, so it's exact performance isn't critical I would presume. The nearest instruction on a 601 is mfspr rd,6 which loads the decrementer. This is a user-level instruction on the 601 (not on the 603 or later). So, -DEC roughly the tb. According to the Ppc601 user manual, dec uses the 7.8125MHz RTC clock as its time base (pg H-3), but on page B-8 it says the power architecure decrements dec every nanosecond. So, we could do something like: static __inline__ unsigned long long rdtsc(void) { asm { mfspr regUpper,6 }; return (long long)-regUpper<<32; } In this case, the effective frequency of the clock is 7.8125MHz*2^32. For the PowerPc601. On initialisation I would probably read the processor version register (PVR) to determine the processor; and if it was a 601 I'd substitute the PowerPC 601 code and then flush the code and data caches. -cheers from julz @P