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Modern Operating Systems by Herbert Bos and Andrew...
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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 125
the interrupt handling and details of actually starting and stopping processes are
hidden away in what is here called the scheduler, which is actually not much code.
The rest of the operating system is nicely structured in process form.
Few real sys-
tems are as nicely structured as this, however.
n – 2
n – 1
Figure 2-3.
The lowest layer of a process-structured operating system handles
interrupts and scheduling. Above that layer are sequential processes.
2.1.6 Implementation of Processes
To implement the process model, the operating system maintains a table (an
array of structures), called the
process table
, with one entry per process.
authors call these entries
process control blocks
.) This entry contains important
information about the process’ state, including its program counter, stack pointer,
memory allocation, the status of its open files, its accounting and scheduling infor-
mation, and everything else about the process that must be saved when the process
is switched from
state so that it can be restarted later
as if it had never been stopped.
Figure 2-4 shows some of the key fields in a typical system. The fields in the
first column relate to process management.
The other two relate to memory man-
agement and file management, respectively.
It should be noted that precisely
which fields the process table has is highly system dependent, but this figure gives
a general idea of the kinds of information needed.
Now that we have looked at the process table, it is possible to explain a little
more about how the illusion of multiple sequential processes is maintained on one
(or each) CPU.
Associated with each I/O class is a location (typically at a fixed lo-
cation near the bottom of memory) called the
interrupt vector
It contains the ad-
dress of the interrupt service procedure. Suppose that user process 3 is running
when a disk interrupt happens.
User process 3’s program counter, program status
word, and sometimes one or more registers are pushed onto the (current) stack by
the interrupt hardware. The computer then jumps to the address specified in the in-
terrupt vector.
That is all the hardware does. From here on, it is up to the software,
in particular, the interrupt service procedure.
All interrupts start by saving the registers, often in the process table entry for
the current process. Then the information pushed onto the stack by the interrupt is

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