The document discusses the key concepts and components of an operating system. It explains that an operating system acts as an interface between the user and underlying hardware by managing computer resources and providing common services to applications. It describes important OS concepts such as processes, files, directories, system calls, and the shell interface. The operating system presents the computer as an easy to use virtual machine and handles complex tasks like memory allocation, scheduling, and input/output management.

1. OPERTATING
SYSTEM
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WHAT IS OPERATING SYSTEM
▸Computer, without its software, a is basically a useless lump
of metal.
▸With its software, a computer can:
▸ store, process, and retrieve information;
▸ play music and videos;
▸ send e-mail, search the Internet;
▸ and engage in many other valuable activities to earn its
keep.

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WHAT IS OPERATING SYSTEM
▸Computer software can be divided roughly into two kinds:
▸System programs, which manage the operation of the
computer itself
▸Application programs, which perform the actual work
the user wants.
▸The most fundamental system program (OS) is the
operating system, whose job is to control all the computer's
resources and provide a base upon which the application
programs can be written.

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COMPUTER IS A COMPLEX MACHINE

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COMPUTER IS A COMPLEX MACHINE
▸A modern computer system consists of:
▸One or more processors,
▸Some main memory,
▸Disks, printers, a keyboard, a display, network interfaces,
and other input/output devices.
▸Computer is A COMPLEX SYSTEM.

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MACHINE NEEDS OPERATING SYSTEM
▸Writing programs that keep track of all these components and use
them correctly, let alone optimally, is an extremely difficult job.
▸It is unlikely that many programs could be written at all.
▸Is there some way that:
▸Shields programmers from the complexity of the hardware.
▸Put a layer of software on top of the bare hardware, to manage
all parts of the system, and present the user with an interface
or virtual machine that is easier to understand and program.
▸This layer of software is the OPERATING SYSTEM.

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THE OPERATING SYSTEM AS AN EXTENDED MACHINE
▸The Operating System as an Extended Machine:
▸OS is to present the user with the equivalent of an
extended machine or virtual machine that is easier to
program than the underlying hardware.
▸The Operating System as a Resource Manager:

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OPERATING SYSTEM CONCEPTS
▸System call: The interface between the operating system
and the user programs is defined by the set of "extended
instructions" that the operating system provides.

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OS CONCEPT: PROCESS - A PROGRAM IN EXECUTION
▸Process: is basically a program in execution.
▸Associated with each process is:
▸Its address space,
▸A list of memory locations from some minimum (usually 0) to some maximum, which the
process can read and write.
▸The address space contains the executable program, the program's data, and its stack.
▸Also associated with each process is some set of registers, including the
▸program counter,
▸stack pointer,
▸and other hardware registers,
▸And all the other information needed to run the program.

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OS CONCEPT: FILE
▸OS hides the peculiarities of the disks and other I/O devices
and present the programmer with a nice, clean abstract
model of device-independent FILES.
▸“A File is a resource for storing information, which is
available to a computer program and is usually based on
some kind of durable storage. A file is "durable" in the sense
that it remains available for other programs to use after the
program that created it has finished executing.”
▸To provide a place to keep files, a DIRECTORY as a way of
grouping files together.

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DIRECTORY TREE

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DIRECTORY
▸ROOT DIRECTORY: top of the directory hierarchy.
▸PATHNAME : Every file within the directory hierarchy can be
specified by giving its PATHNAME from ROOT DIRECTORY.
▸Absolute path: ex: /Faculty/Prof.Brown/Courses/CS101
▸Current working directory: ex /Faculty/Prof.Brown/
▸Relation path: ex Courses/CS101
▸Unix/Linux path separator: “/”
▸Windows path separator: “”

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MOUNT
(a) Before mounting, the files on drive 0 are not accessible.
(b) After mounting, they are part of the file hierarchy.
▸mount system call allows the file system on the CD-ROM to be attached to the
root file system wherever the program wants it to be.

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THE SHELL
▸ Primary interface between a user sitting at his terminal and the
operating system.
▸ Ex: csh, ksh, zsh, and bash … and original shell (sh).

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