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    order to read system time. Instead, a mechanism that shares kernel time of

    order to read the system time. Instead, a mechanism that shares kernel

    the day counters with userspace address spaces is deployed. On the kernel

    time of the the day counters with userspace address spaces is deployed. On

    side, during system initialization, HelenOS allocates a frame of physical

    the kernel side, during system initialization, HelenOS allocates a frame

    memory and stores the time of the day counters there. The counters have

    of physical memory and stores the time of the day counters there. The

    the following structure:</para>

    counters have the following structure:</para>

    ns) asks the kernel to map this frame into its address space. Other,

    ns) asks the kernel to map this frame into its address space. Other

    non-privileged, tasks then use IPC to communicate read-only sharing of

    non-privileged tasks then use IPC to receive read-only share of this

    this memory. Reading time in a userspace task is therefore just a matter

    memory. Reading time in a userspace task is therefore just a matter of

    of reading memory.</para>

    reading memory.</para>

    platforms. Now the second point is that 32-bit platforms cannot atomically

    platforms. Unfortunately, on 32-bit platforms it is usually impossible to

    read two 32-bit counters either. However, a generic protocol is used to

    read atomically two 32-bit counters either. However, a generic protocol is

    guarantee that sequentially read times will create a non-decreasing

    used to guarantee that sequentially read times will create a

    sequence.</para>

    non-decreasing sequence.</para>

    <para>The problematic part is updating both seconds and microseconds once

    <para>The problematic part is incrementing seconds counter and clearing

    in a second. Seconds must be incremented and microseconds must be reset.

    microseconds counter together once every second. Seconds must be

    However, without any synchronization, the two kernel stores and the two

    synchronization, the two kernel stores and the two userspace reads can

    userspace reads can arbitrarily interleave. Furthemore, the reader has no

    arbitrarily interleave. Furthemore, the reader has no chance to detect

    chance to detect that the counters were updated only from half. Therefore

    that the counters were updated only paritally. Therefore three counters

    three counters are used in HelenOS.</para>

    are used in HelenOS.</para>

    increments the second second counter. When only microseconds need to be

    increments the second second counter. When only microseconds needs to be

    userspace task must always read all three counters and in reversed order.

    userspace task must always read all three counters in reversed order. A

    A read memory barrier operation must be issued between each two reads. A

    read memory barrier operation must be issued between each two reads. A

    userspace library solves this situation by returning zero instead of the

    userspace library solves this situation by returning higher of them with

    value read from the microsecond counter.</para>

    microseconds set to zero.</para>