Intel SL3QA - Pentium III 550 MHz Processor Specification page 91

Specification Clarifications
Counting Clocks
The count of cycles, also known as clockticks, forms a the basis for measuring how
long a program takes to execute. Clockticks are also used as part of efficiency ratios
like cycles per instruction (CPI). Processor clocks may stop ticking under
circumstances like the following:
The processor is halted when there is nothing for the CPU to do. For example, the
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processor may halt to save power while the computer is servicing an I/O request.
When Hyper-Threading Technology is enabled, both logical processors must be
halted for performance-monitoring counters to be powered down.
The processor is asleep as a result of being halted or because of a power-
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management scheme. There are different levels of sleep. In the some deep sleep
levels, the time-stamp counter stops counting.
There are three ways to count processor clock cycles to monitor performance. These
are:
Non-halted clockticks — Measures clock cycles in which the specified logical
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processor is not halted and is not in any power-saving state. When Hyper-
Threading Technology is enabled, this these ticks can be measured on a per-
logical-processor basis.
• Non-sleep clockticks — Measures clock cycles in which the specified physical
processor is not in a sleep mode or in a power-saving state. These ticks cannot
be measured on a logical-processor basis.
• Time-stamp counter — Some processor models permit clock cycles to be
measured when the physical processor is not in deep sleep (by using the time-
stamp counter and the RDTSC instruction). Note that such ticks cannot be
measured on a per-logical-processor basis. See Section 10.8 for detail on
processor capabilities.
The first two methods use performance counters and can be set up to cause an
interrupt upon overflow (for sampling). They may also be useful where it is easier for
a tool to read a performance counter than to use a time stamp counter (the
timestamp counter is accessed using the RDTSC instruction).
For applications with a significant amount of I/O, there are two ratios of interest:
Non-halted CPI — Non-halted clockticks/instructions retired measures the CPI
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for phases where the CPU was being used. This ratio can be measured on a
logical-processor basis when Hyper-Threading Technology is enabled.
Nominal CPI — Time-stamp counter ticks/instructions retired measures the CPI
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over the duration of a program, including those periods when the machine halts
while waiting for I/O.
15.10.9.3
The time-stamp counter increments when the clock signal on the system bus is active
and when the sleep pin is not asserted. The counter value can be read with the
RDTSC instruction.
Specification Update
Incrementing the Time-Stamp Counter
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