An OS provides an environment within which programs are executed. The design of a new O/S as a major task.
It is important that the goals of the system be well defined before the design begins.
System components:
We can create a system as large and complex as an only by portioning it into smaller pieces. Each of these pieces should be well- defined portion of the system, with defined inputs, outputs and function.
Obviously, not all system has the structure but the goal of supporting the types of system components as
1) Process management
2) Main Memory management
3) File management
4) Secondary storage management
5) I/O system
6) Networking
7) Protection system
8) Command- Interpreter system
Process Management:
A program does nothing unless its instruction executed by the CPU. A process can be thought as a program in execution.
But when we explore it
I. A batch job is a process
II. A time-shared user is a process
III. A system task, such as pooling output to a printer also is a process.
A process needs certain resources as:
I. CPU time
II. Memory
III. Files and I/O Devices to accomplish the task.
These resources are either given to the process when it is created or allocated to it while it is running. CPU executes one instruction of the process after another, until the process computes. Two processes may be associated with the same program. It is common to have a single program that spawns many processes at it runs. A process is a unit of work in a system. A system consists of a collection of processes.
Some of which:
O/S processes: those executes system codes
User processes: executes user codes.
All these processes can potentially executes concurrently.
The operating system is responsible for the following activities in connection with process management.
I. Creation and deletion of both user and system process
II. Suspension and resumption of processes
III. Process synchronization
IV. Deadlock Handling
Main Memory Management:
Main memory is a large array of words or bytes ranging in size from hundreds of thousands to number of millions.
Each words and bytes has its own address. Main memory is a quickly accessible data shared by CPU and I/O devices.
The central processor reads instruction from main memory during data –fetch cycle and both read and write data from main memory during the data fetch cycle.
The main memory is a large storage device that the CPU is able to address and access directly.
The operating system is responsible for the following activities in connection with main memory management.
I. Keep track of which parts of memory are currently being used and whom.
II. Decide which processes ti be loaded into memory when memory becomes available.
III. Allocate and de allocates memory spaces as needed.
File Management:
File management is one of the visible components of an operating system.
Computers can store information on several types of disk physical media as magnetic tape, magnetic disk and optical disk. Each of these media has its own characteristics and physical organization. Each medium is controlled by a device such as disk drive, tape drive with its unique characteristics. These properties includes:
I. Capacity
II. Data transfer rate
III. Access methods( Sequential and Random Access)
The operating system provides the uniform logical view of information storage.
File:- file is collection of related of information defined by its creator. Commonly file represents programs.
A file consists of sequence of bits, bytes, lines or records who’s meaning are defined by their creators.
The operating system is responsible for the following activities in connection with file management.
I. The creation and deletion of files
II. The creation and deletion of directories
III. Manipulating files and directories
IV. Mapping of files onto secondary storage
V. Backup of files on stable storage media
Secondary storage management:
The main purpose of the computer system is to execute programs. These programs with the data they access must be in the main memory( primary memory) during execution.
But when power off, the computer system must provide sec-storage to back up main memory.
The O/S is responsible for the following activities in connection with the disk management.
I. Free- Space management
II. Storage allocation
III. Disk scheduling
I/O system management:
One of the purpose of an O/S is to hide the peculiarities of specific H/W device from the user.
The O/S consists of
I. A memory management component including buffering, caching and spooling.
II. A general device driver interface
III. Drivers for specific H/W device
Networking:
A distributed system is a collection of processors that do not share memory, peripherals devices or a clock. Instead each processor has its own local memory and processor can communicate one another through various communication lines.
The processors in a distributed system are connected through a communication network, which can be configured in a number of different ways.
Protection system:
If a computer system has multiple user (allow the concurrent execution of multiple processes).
Then the various processes must be protected from one another activities. For that purpose, mechanisms are provided to ensure that the files, memory segments, CPU and other resources can be operated on by only those processes that gained proper authorization from the operating system.
Protection refers to the mechanism for controlling the access of programs, processes or users to the resources defined by the computer system.
Command- interpreter system:
One of the most important system programs for an operating system is the command interpreter. Which is the interface between the user and the operating system.
Some O/S includes the command interpreter in the Kernel.
Other O/S such as Ms-Dos and UNIX, treat the interpreter as a special program that is running when a job initiated or when a user first log on.
Many command are given to the O/S by control statements. When a job is :
I. Started in a batch system
II. When a user log on to a time- sharing system
A program that reads and interprets control statements is executed automatically.
This program is some time called the control card interpreter or the command line interpreter and is known as shell.
Shell:- its function is quite simple . Get the new command and execute it.
O/S is frequently differentiated in the area of the shell with a user-friendly command interpreter.
One style user-friendly interface is the mouse-based windows as Microsoft windows.
In some of these shells, commands are typed on a keyboard and display on a screen or printing terminal with a enter key signaling that a command is complete and ready to execute.
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