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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 53
22
INTRODUCTION
CHAP. 1
Pipelines cause compiler writers and operating system writers great headaches be-
cause they expose the complexities of the underlying machine to them and they
have to deal with them.
Fetch
unit
Fetch
unit
Fetch
unit
Decode
unit
Decode
unit
Execute
unit
Execute
unit
Execute
unit
Execute
unit
Decode
unit
Holding
buffer
(a)
(b)
Figure 1-7.
(a) A three-stage pipeline. (b) A superscalar CPU.
Even more advanced than a pipeline design is a
superscalar
CPU, shown in
Fig. 1-7(b).
In this design, multiple execution units are present, for example, one
for integer arithmetic, one for floating-point arithmetic, and one for Boolean opera-
tions. Two or more instructions are fetched at once, decoded, and dumped into a
holding buffer until they can be executed. As soon as an execution unit becomes
available, it looks in the holding buffer to see if there is an instruction it can hand-
le, and if so, it removes the instruction from the buffer and executes it.
An implica-
tion of this design is that program instructions are often executed out of order. For
the most part, it is up to the hardware to make sure the result produced is the same
one a sequential implementation would have produced, but an annoying amount of
the complexity is foisted onto the operating system, as we shall see.
Most CPUs, except very simple ones used in embedded systems, have two
modes, kernel mode and user mode, as mentioned earlier. Usually, a bit in the PSW
controls the mode.
When running in kernel mode, the CPU can execute every in-
struction in its instruction set and use every feature of the hardware. On desktop
and server machines, the operating system normally runs in kernel mode, giving it
access to the complete hardware. On most embedded systems, a small piece runs
in kernel mode, with the rest of the operating system running in user mode.
User programs always run in user mode, which permits only a subset of the in-
structions to be executed and a subset of the features to be accessed. Generally, all
instructions involving I/O and memory protection are disallowed in user mode.
Setting the PSW mode bit to enter kernel mode is also forbidden, of course.
To obtain services from the operating system, a user program must make a
sys-
tem call
, which traps into the kernel and invokes the operating system. The
TRAP
instruction switches from user mode to kernel mode and starts the operating sys-
tem. When the work has been completed, control is returned to the user program at
the instruction following the system call.
We will explain the details of the system
call mechanism later in this chapter. For the time being, think of it as a special kind
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