INTRODUCTION

So far we have seen an operating system provides the environment within which programs are executed.  Internally, operating systems vary greatly in their makeup, since they are organized along many different lines.  The design of a new operating system is a major task.  It is important that the goals of the system be well defined before the design begins.  These goals form the basis for choices among various algorithms and strategies.

9.1     DEFINITION AND OVERVIEW

We can view an operating system from several vantage points.  Let's consider three of such views:

• Services that the system provides

• Interface employed by the system

• Components and their interconnections

 

9.2     OPERATING SYSTEMS SERVICES
An operating system provides an environment for the execution of programs.  It makes certain services available to programs and to the users of those programs.  The specific services provided, of course, differ from one operating system to another, but we can identify common classes.

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USER SPECIFIC SERVICES

• User interface.  Almost all operating systems have a user interface (UI).  This interface can take several forms.  Most commonly, a graphical user interface (GUI) is used.  Here, the interface is a window system with a mouse that serves as a pointing device to direct I/O, choose from menus, and make selections and a keyboard to enter text.  Mobile systems such as phones and tablets provide a touch-screen interface, enabling users to slide their fingers across the screen or press buttons on the screen to select choices.  Another option is a command-line interface (CLI), which uses text commands and a method for entering them (say, a keyboard for typing in commands in a specific format with specific options).  Some systems provide two or all three of these variations.

• Program execution.  The system must be able to load a program into memory and to run that program.  The program must be able to end its execution, either normally or abnormally (indicating error).

• I/O operations.  A running program may require I/O, which may involve a file or an I/O device.  For specific devices, special functions may be desired (such as reading from a network interface or writing to a file system).  For efficiency and protection, users usually cannot control I/O devices directly.  Therefore, the operating system must provide a means to do I/O.

• File-system manipulation.  The file system is of particular interest.  Obviously, programs need to read and write files and directories.  They also need to create and delete them by name, search for a given file, and list file information.  Finally, some operating systems include permissions management to allow or deny access to files or directories based on file ownership.  Many operating systems provide a variety of file systems, sometimes to allow personal choice and sometimes to provide specific features or performance characteristics.

• Communications.  There are many circumstances in which one process needs to exchange information with another process.  Such communication may occur between processes that are executing on the same computer or between processes that are executing on different computer systems tied together by a network.  Communications may be implemented via shared memory, in which two or more processes read and write to a shared section of memory, or message passing, in which packets of information in predefined formats are moved between processes by the operating system.

• Error detection.  The operating system needs to be detecting and correcting errors constantly.  Errors may occur in the CPU and memory hardware (such as a memory error or a power failure), in I/O devices (such as a parity error on disk, a connection failure on a network, or lack of paper in the printer), and in the user program (such as an arithmetic overflow or an attempt to access an illegal memory location).  For each type of error, the operating system should take the appropriate action to ensure correct and consistent computing.  Sometimes, it has no choice but to halt the system.  At other times, it might terminate an error-causing process or return an error code to a process for the process to detect and possibly correct.

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SYSTEM SPECIFIC SERVICES​

​Another set of operating-system functions exists not for helping the user but rather for ensuring the efficient operation of the system itself.  Systems with multiple processes can gain efficiency by sharing the computer resources among the different processes.

• Resource allocation.  When there are multiple processes running at the same time, resources must be allocated to each of them.  The operating system manages many different types of resources.  Some (such as CPU cycles, main memory, and file storage) may have special allocation code, whereas others (such as I/O devices) may have much more general request and release code.  For instance, in determining how best to use the CPU, operating systems have CPU-scheduling routines that take into account the speed of the CPU, the process that must be executed, the number of processing cores on the CPU, and other factors.  There may also be routines to allocate printers, USB storage drives, and other peripheral devices.

• Logging.  We want to keep track of which programs use how much and what kinds of computer resources.  This record keeping may be used for accounting (so that users can be billed) or simply for accumulating usage statistics.  Usage statistics may be a valuable tool for system administrators who wish to reconfigure the system to improve computing services.

• Protection and security.  The owners of information stored in a multiuser or networked computer system may want to control use of that information.  When several separate processes execute concurrently, it should not be possible for one process to interfere with the others or with the operating system itself.  Protection involves ensuring that all access to system resources is controlled.  Security of the system from outsiders is also important.  Such security starts with requiring each user to authenticate himself or herself to the system, usually by means of a password, to gain access to system resources.  It extends to defending external I/O devices, including network adapters, from invalid access attempts and recording all such connections for detection of break-ins.  If a system is to be protected and secure, precautions must be instituted throughout it.  A chain is only as strong as its weakest link.

 

9.3     USER AND OPERATING SYSTEMS INTERFACE

There are several ways for users to interface with the operating system.  Two fundamental ones are:

• Command Line or command interpreters

• Graphical User Interface

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9.4     SYSTEM CALLS

System calls provide an interface to the services made available by an operating system.  These calls are generally available as functions written in C and C++, although certain low-level tasks (for example, tasks where hardware must be accessed directly) may have to be written using assembly-language instructions.  System calls can be grouped roughly into six major categories:

• Process control

create process, terminate process
load, execute
get process attributes, set process attributes
wait event, signal event
allocate and free memory

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• File management

create file, delete file
open, close
read, write, reposition
get file attributes, set file attributes

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• Device management

request device, release device
read, write, reposition
get device attributes, set device attributes
logically attach or detach devices

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• Information maintenance

get time or date, set time or date
get system data, set system data
get process, file, or device attributes
set process, file, or device attributes

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• Communications

create, delete communication connection
send, receive messages
transfer status information
attach or detach remote devices

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• Protection

get file permissions
set file permissions