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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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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 422
SEC. 5.5
CLOCKS
391
(a)
(b)
(c)
Time of day in ticks
Time of day
in seconds
Counter in ticks
System boot time
in seconds
Number of ticks
in current second
64 bits
32 bits
32 bits
Figure 5-29.
Three ways to maintain the time of day.
how long the process has run. To do things right, the second timer should be saved
when an interrupt occurs and restored afterward.
A less accurate, but simpler, way to do accounting is to maintain a pointer to
the process table entry for the currently running process in a global variable. At
every clock tick, a field in the current process’ entry is incremented. In this way,
every clock tick is ‘‘charged’’ to the process running at the time of the tick. A
minor problem with this strategy is that if many interrupts occur during a process’
run, it is still charged for a full tick, even though it did not get much work done.
Properly accounting for the CPU during interrupts is too expensive and is rarely
done.
In many systems, a process can request that the operating system give it a
warning after a certain interval. The warning is usually a signal, interrupt, message,
or something similar. One application requiring such warnings is networking, in
which a packet not acknowledged within a certain time interval must be retrans-
mitted. Another application is computer-aided instruction, where a student not pro-
viding a response within a certain time is told the answer.
If the clock driver had enough clocks, it could set a separate clock for each re-
quest. This not being the case, it must simulate multiple virtual clocks with a single
physical clock. One way is to maintain a table in which the signal time for all
pending timers is kept, as well as a variable giving the time of the next one. When-
ever the time of day is updated, the driver checks to see if the closest signal has oc-
curred. If it has, the table is searched for the next one to occur.
If many signals are expected, it is more efficient to simulate multiple clocks by
chaining all the pending clock requests together, sorted on time, in a linked list, as
shown in Fig. 5-30. Each entry on the list tells how many clock ticks following the
previous one to wait before causing a signal. In this example, signals are pending
for 4203, 4207, 4213, 4215, and 4216.
In Fig. 5-30, the next interrupt occurs in 3 ticks. On each tick,
Next signal
is
decremented. When it gets to 0, the signal corresponding to the first item on the list
is caused, and that item is removed from the list. Then
Next signal
is set to the
value in the entry now at the head of the list, in this example, 4.
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