Time Stamp Counter

All 80 x 86 microprocessors include a CLK input pin, which receives the clock signal of an external oscillator.

Starting with the Pentium, many recent 80 x 86 microprocessors include a 64-bit Time Stamp Counter (TSC ) register that can be read by means of the rdtsc assembly language instruction. This register is a counter that is incremented at each clock signal — if, for instance, the clock ticks at 400 MHz, the Time Stamp Counter is incremented once every 2.5 nanoseconds.

Linux takes advantage of this register to get much more accurate time measurements than those delivered by the Programmable Interval Timer. To do this, Linux must determine the clock signal frequency while initializing the system. In fact, since this frequency is not declared when compiling the kernel, the same kernel image may run on CPUs whose clocks may tick at any frequency.

The task of figuring out the actual frequency of a CPU is accomplished during the system's boot. The calibrate_tsc( ) function computes the frequency by counting the number of clock signals that occur in a relatively long time interval, namely 50.00077 milliseconds. This time constant is produced by properly setting up one of the channels of the Programmable Interval Timer (see the next section). The long execution time of calibrate_tsc( ) does

Continue reading here: Clock tick multiplied by 232 613 Programmable Interval Timer

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