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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 423
Current time
Next signal
Figure 5-30.
Simulating multiple timers with a single clock.
Note that during a clock interrupt, the clock driver has several things to do—
increment the real time, decrement the quantum and check for 0, do CPU ac-
counting, and decrement the alarm counter. However, each of these operations has
been carefully arranged to be very fast because they have to be repeated many
times a second.
Parts of the operating system also need to set timers. These are called
dog timers
and are frequently used (especially in embedded devices) to detect
problems such as hangs. For instance, a watchdog timer may reset a system that
stops running. While the system is running, it regularly resets the timer, so that it
never expires. In that case, expiration of the timer proves that the system has not
run for a long time, and leads to corrective action—such as a full-system reset.
The mechanism used by the clock driver to handle watchdog timers is the same
as for user signals. The only difference is that when a timer goes off, instead of
causing a signal, the clock driver calls a procedure supplied by the caller. The pro-
cedure is part of the caller’s code. The called procedure can do whatever is neces-
sary, even causing an interrupt, although within the kernel interrupts are often
inconvenient and signals do not exist. That is why the watchdog mechanism is pro-
vided. It is worth nothing that the watchdog mechanism works only when the
clock driver and the procedure to be called are in the same address space.
The last thing in our list is profiling. Some operating systems provide a mech-
anism by which a user program can have the system build up a histogram of its
program counter, so it can see where it is spending its time. When profiling is a
possibility, at every tick the driver checks to see if the current process is being pro-
filed, and if so, computes the bin number (a range of addresses) corresponding to
the current program counter. It then increments that bin by one. This mechanism
can also be used to profile the system itself.
5.5.3 Soft Timers
Most computers have a second programmable clock that can be set to cause
timer interrupts at whatever rate a program needs. This timer is in addition to the
main system timer whose functions were described above.
As long as the interrupt
frequency is low, there is no problem using this second timer for application-spe-
cific purposes. The trouble arrives when the frequency of the application-specific

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