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    <section>

    <title>System clock</title>

    <para>

    Every hardware architecture supported by HelenOS must support some kind of a device that can be

    programmed to yield periodic time signals (i.e. clock interrupts). Some architectures have

    external clock that is merely programmed by the kernel to interrupt the processor multiple

    times in a second. This is the case of ia32 and amd64 architectures<footnote><para>When

    running in uniprocessor mode.</para></footnote>, which use i8254 or a compatible chip to

    achieve the goal.

    </para>

    <para>

    Other architectures' processors typically contain two registers. The first

    register is usually called a compare or a match register and can be set to an arbitrary value

    by the operating system. The contents of the compare register then stays unaltered until it is

    written by the kernel again. The second register, often called a counter register, can be also

    written by the kernel, but the processor automatically increments it after every executed

    instruction or in some fixed relation to processor speed. The point is that a clock interrupt is

    generated whenever the values of the counter and the compare registers match. Sometimes, the scheme of

    two registers is modified so that only one register is needed. Such a register, called a decrementer,

    then counts towards zero and an interrupt is generated when zero is reached.

    </para>

    <para>

    In any case, the initial value of the decrementer or the initial difference between the counter and

    the compare registers, respectively, must be set accordingly to a known relation between the real time

    and the speed of the decrementer or the counter register, respectively.

    </para>

    </section>