[2The PIT is also used to drive an audio amplifier connected to the computers internal speaker

As we shall see in detail in the next paragraphs, Linux programs the PIT to issue timer interrupts on the IRQ0 at a (roughly) 100-Hz frequency — that is, once every 10 milliseconds. This time interval is called a tick, and its length in microseconds is stored in the tick variable. 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.

Generally speaking, shorter ticks result in higher resolution timers, which help with smoother multimedia playback and faster response time when performing synchronous I/O multiplexing (poll( ) and select( ) system calls). This is a trade-off however: shorter ticks require the CPU to spend a larger fraction of its time in Kernel Mode — that is, a smaller fraction of time in User Mode. As a consequence, user programs run slower. Therefore, only very powerful machines can adopt very short ticks and afford the consequent overhead. Currently, most Hewlett-Packard's Alpha and Intel's IA-64 ports of the Linux kernel issue 1,024 timer interrupts per second, corresponding to a tick of roughly 1 millisecond. The Rawhide Alpha station adopts the highest tick frequency and issues 1,200 timer interrupts per second.

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 number of timer interrupts per second — that is, their frequency. This value is set to 100 for IBM PCs and most other hardware platforms.

• clock_tick_rate yields the value 1,193,180, which is the 8254 chip's internal oscillator frequency.

• latch yields the ratio between clock_tick_rate and hz. It is used to program the PIT.

The first PIT is initialized by init_iRQ( ) as follows:


outb_p(LATCH & 0xff , 0x40); outb(LATCH >> 8 , 0x40);

The outb( ) C function is equivalent to the outb assembly language instruction: it copies the first operand into the I/O port specified as the second operand. The outb_p( ) function is similar to outb( ), except that it introduces a pause by executing a no-op instruction. The first outb_ p( ) invocation is a command to the PIT to issue interrupts at a new rate. The next two outb_ p( ) and outb( ) invocations supply the new interrupt rate to the device. The 16-bit latch constant is sent to the 8-bit 0x4 0 I/O port of the device as two consecutive bytes. As a result, the PIT issues timer interrupts at a (roughly) 100-Hz frequency (that is, once every 10 ms).

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