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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 54
SEC. 1.3
of procedure call that has the additional property of switching from user mode to
kernel mode.
As a note on typography, we will use the lower-case Helvetica font
to indicate system calls in running text, like this:
It is worth noting that computers have traps other than the instruction for ex-
ecuting a system call. Most of the other traps are caused by the hardware to warn
of an exceptional situation such as an attempt to divide by 0 or a floating-point
underflow. In all cases the operating system gets control and must decide what to
do. Sometimes the program must be terminated with an error. Other times the
error can be ignored (an underflowed number can be set to 0).
Finally, when the
program has announced in advance that it wants to handle certain kinds of condi-
tions, control can be passed back to the program to let it deal with the problem.
Multithreaded and Multicore Chips
Moore’s law states that the number of transistors on a chip doubles every 18
months. This ‘‘law’’ is not some kind of law of physics, like conservation of mo-
mentum, but is an observation by Intel cofounder Gordon Moore of how fast proc-
ess engineers at the semiconductor companies are able to shrink their transistors.
Moore’s law has held for over three decades now and is expected to hold for at
least one more.
After that, the number of atoms per transistor will become too
small and quantum mechanics will start to play a big role, preventing further
shrinkage of transistor sizes.
The abundance of transistors is leading to a problem: what to do with all of
them? We saw one approach above: superscalar architectures, with multiple func-
tional units. But as the number of transistors increases, even more is possible.
obvious thing to do is put bigger caches on the CPU chip. That is definitely hap-
pening, but eventually the point of diminishing returns will be reached.
The obvious next step is to replicate not only the functional units, but also
some of the control logic.
The Intel Pentium 4 introduced this property, called
(Intel’s name for it), to the x86 processor, and
several other CPU chips also have it—including the SPARC, the Power5, the Intel
Xeon, and the Intel Core family.
To a first approximation, what it does is allow the
CPU to hold the state of two different threads and then switch back and forth on a
nanosecond time scale.
(A thread is a kind of lightweight process, which, in turn,
is a running program; we will get into the details in Chap. 2.)
For example, if one
of the processes needs to read a word from memory (which takes many clock
cycles), a multithreaded CPU can just switch to another thread. Multithreading
does not offer true parallelism.
Only one process at a time is running, but
thread-switching time is reduced to the order of a nanosecond.
Multithreading has implications for the operating system because each thread
appears to the operating system as a separate CPU.
Consider a system with two
actual CPUs, each with two threads. The operating system will see this as four
CPUs. If there is only enough work to keep two CPUs busy at a certain point in

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