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understand the linux kernel 3rd
Besides the Real Time Clock and the Time Stamp Counter, IBM-compatible PCs include another type of time-measuring device called Programmable Interval Timer(PIT).
The role of a PIT is similar to the alarm clock of a microwave oven: it makes the user aware that the cooking time interval has elapsed. Instead of ringing a bell,
this device issues a special interrupt called timer interrupt, which notifies the kernel that one more time interval has elapsed.[]
Another difference from the alarm clock is that the PIT goes on issuing interrupts forever at some fixed frequency established by the kernel. Each IBM-compatible PC includes at least one PIT, which is usually implemented by an 8254 CMOS chip using the 0x40-0x43 I/O ports.
As we'll see in detail in the next paragraphs, Linux programs the PIT of IBM-compatible PCs to issue timer interrupts on the IRQ 0 at a (roughly) 1000-Hz frequency that is, once every 1 millisecond. This time interval is called a tick, and its length in nanoseconds is stored in the tick_nsec variable.
On a PC, tick_nsec is initialized to 999,848 nanoseconds (yielding a clock signal frequency of about 1000.15 Hz), but its value may be automatically adjusted by the kernel if the computer is synchronized with an external clock (see the later section "The adjtimex( ) System Call"). The ticks beat time for all activities in the system; in some sense, they are like the ticks sounded by a metronome while a musician is rehearsing.
A few macros in the Linux code yield some constants that determine the frequency of timer interrupts. These are discussed in the following list.
HZ yields the approximate number of timer interrupts per second that is, their frequency. This value is set to 1000 for IBM PCs.
CLOCK_TICK_RATE yields the value 1,193,182, which is the 8254 chip's internal oscillator frequency.
LATCH yields the ratio between CLOCK_TICK_RATE and HZ, rounded to the nearest integer. It is used to program the PIT. |
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發(fā)表于 2009-10-09 16:52
