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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 421
390
INPUT/OUTPUT
CHAP. 5
5.5.2 Clock Software
All the clock hardware does is generate interrupts at known intervals. Every-
thing else involving time must be done by the software, the clock driver. The exact
duties of the clock driver vary among operating systems, but usually include most
of the following:
1.
Maintaining the time of day.
2.
Preventing processes from running longer than they are allowed to.
3. Accounting for CPU usage.
4. Handling the
alarm
system call made by user processes.
5.
Providing watchdog timers for parts of the system itself.
6.
Doing profiling, monitoring, and statistics gathering.
The first clock function, maintaining the time of day (also called the
real time
)
is not difficult. It just requires incrementing a counter at each clock tick, as men-
tioned before. The only thing to watch out for is the number of bits in the time-of-
day counter. With a clock rate of 60 Hz, a 32-bit counter will overflow in just over
2 years. Clearly the system cannot store the real time as the number of ticks since
Jan. 1, 1970 in 32 bits.
Three approaches can be taken to solve this problem. The first way is to use a
64-bit counter, although doing so makes maintaining the counter more expensive
since it has to be done many times a second. The second way is to maintain the
time of day in seconds, rather than in ticks, using a subsidiary counter to count
ticks until a whole second has been accumulated. Because 2
32
seconds is more than
136 years, this method will work until the twenty-second century.
The third approach is to count in ticks, but to do that relative to the time the
system was booted, rather than relative to a fixed external moment. When the back-
up clock is read or the user types in the real time, the system boot time is calcu-
lated from the current time-of-day value and stored in memory in any convenient
form. Later, when the time of day is requested, the stored time of day is added to
the counter to get the current time of day. All three approaches are shown in
Fig. 5-29.
The second clock function is preventing processes from running too long.
Whenever a process is started, the scheduler initializes a counter to the value of
that process’ quantum in clock ticks. At every clock interrupt, the clock driver
decrements the quantum counter by 1. When it gets to zero, the clock driver calls
the scheduler to set up another process.
The third clock function is doing CPU accounting. The most accurate way to
do it is to start a second timer, distinct from the main system timer, whenever a
process is started up. When that process is stopped, the timer can be read out to tell
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