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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 119
and the cake ingredients are the input data. The process is the activity consisting of
our baker reading the recipe, fetching the ingredients, and baking the cake.
Now imagine that the computer scientist’s son comes running in screaming his
head off, saying that he has been stung by a bee. The computer scientist records
where he was in the recipe (the state of the current process is saved), gets out a first
aid book, and begins following the directions in it. Here we see the processor being
switched from one process (baking) to a higher-priority process (administering
medical care), each having a different program (recipe versus first aid book).
When the bee sting has been taken care of, the computer scientist goes back to his
cake, continuing at the point where he left off.
The key idea here is that a process is an activity of some kind.
It has a pro-
gram, input, output, and a state.
A single processor may be shared among several
processes, with some scheduling algorithm being accustomed to determine when to
stop work on one process and service a different one. In contrast, a program is
something that may be stored on disk, not doing anything.
It is worth noting that if a program is running twice, it counts as two processes.
For example, it is often possible to start a word processor twice or print two files at
the same time if two printers are available. The fact that two processes happen to
be running the same program does not matter; they are distinct processes. The op-
erating system may be able to share the code between them so only one copy is in
memory, but that is a technical detail that does not change the conceptual situation
of two processes running.
2.1.2 Process Creation
Operating systems need some way to create processes.
In very simple sys-
tems, or in systems designed for running only a single application (e.g., the con-
troller in a microwave oven), it may be possible to have all the processes that will
ever be needed be present when the system comes up.
In general-purpose systems,
however, some way is needed to create and terminate processes as needed during
operation. We will now look at some of the issues.
Four principal events cause processes to be created:
1. System initialization.
Execution of a process-creation system call by a running process.
A user request to create a new process.
Initiation of a batch job.
When an operating system is booted, typically numerous processes are created.
Some of these are foreground processes, that is, processes that interact with
(human) users and perform work for them. Others run in the background and are
not associated with particular users, but instead have some specific function. For

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