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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 420
SEC. 5.5
CLOCKS
389
this base signal can be multiplied by a small integer to get frequencies up to several
gigahertz or even more. At least one such circuit is usually found in any computer,
providing a synchronizing signal to the computer’s various circuits. This signal is
fed into the counter to make it count down to zero. When the counter gets to zero,
it causes a CPU interrupt.
Crystal oscillator
Counter is decremented at each pulse
Holding register is used to load the counter
Figure 5-28.
A programmable clock.
Programmable clocks typically have several modes of operation.
In
one-shot
mode
, when the clock is started, it copies the value of the holding register into the
counter and then decrements the counter at each pulse from the crystal. When the
counter gets to zero, it causes an interrupt and stops until it is explicitly started
again by the software. In
square-wave mode
, after getting to zero and causing the
interrupt, the holding register is automatically copied into the counter, and the
whole process is repeated again indefinitely. These periodic interrupts are called
clock ticks
.
The advantage of the programmable clock is that its interrupt frequency can be
controlled by software. If a 500-MHz crystal is used, then the counter is pulsed
every 2 nsec. With (unsigned) 32-bit registers, interrupts can be programmed to oc-
cur at intervals from 2 nsec to 8.6 sec. Programmable clock chips usually contain
two or three independently programmable clocks and have many other options as
well (e.g., counting up instead of down, interrupts disabled, and more).
To prevent the current time from being lost when the computer’s power is
turned off, most computers have a battery-powered backup clock, implemented
with the kind of low-power circuitry used in digital watches. The battery clock can
be read at startup.
If the backup clock is not present, the software may ask the user
for the current date and time. There is also a standard way for a networked system
to get the current time from a remote host.
In any case the time is then translated
into the number of clock ticks since 12
A.M.
UTC
(
Universal Coordinated Time
)
(formerly known as Greenwich Mean Time) on Jan. 1, 1970, as UNIX does, or
since some other benchmark moment. The origin of time for Windows is Jan. 1,
1980. At every clock tick, the real time is incremented by one count. Usually util-
ity programs are provided to manually set the system clock and the backup clock
and to synchronize the two clocks.
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