Technical Q&A HW31

UpTime's values are consistently slow?

Q: We're using UpTime and finding the values are consistently slow when running on certain accelerator cards. How can we get the correct timing values?

A: 8500/9500/8600/9600 machines measure the computer's bus speed against the VIA timer, and use this value to look up the timing information from a table in the ROM. This table has data for all integral values between 25 and 66 mhz, as well as 33.333 mhz. Some later machines also support integral speeds between 67 and 125 mhz. The timing information is used to set the time base, which converts from absolute time to nanoseconds.

If the bus timing is incorrect, or does not exactly match one of the values specified in the ROM table, then the time base will be set incorrectly, and UpTime will be consistently off. For example, if the actual bus speed is 44.5 mhz, then UpTime will actually run at 44.5/45, or about 99% of the correct value. This causes UpTime to drift from other clocks on the Mac.

Accelerator card vendors should choose timings that are in the list described above. Apple is investigating options that may fix this in future OS releases, but no plans or decisions have been announced.