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Operating System

A layer of (systems) software that manages the hardware resources. Also provides a friendly interface to the users by hiding the complexity of the hardware. Acts as a virtual machine to application software. Microsoft Windows, Linux, and Mac OS are a few examples.

OS provides common interfaces of the hardware to software regardless of the specific features of its underlying hardware. Thus, the program needed only to be compatible with this virtual machine interface.

Handles:

  • CPU scheduling
  • Memory management
  • Storage management

Kernel

Core of an OS. Loads before any of the user programs and remains in the memory. Responsible for managing CPU, memory, disk, processes.

Major services of OS

An operating system provides two major services.

OS as an Extended Machine

OS hides specific details about the underlying hardware. Instead, provide a simpler and uniform interfaces for application software and users.

For example: saving a file to a secondary storage device. Writing mechanism is physically different for floppy disk, hard disk, and magnetic tape. However, application software authors or the end users do not need to be aware of those details. For them, saying “save this file in that place” to the OS is enough. All other details will be handled by the OS.

OS as a Resource Manager

OS manages all hardware resources. OS decides which programs run at which time, how much memory is allocated to each program, where to write data, how to deal with concurrent users, and many other things.

History

1st generation

Cannot be considered as real operating systems. Those days programs were written by changing the wire in a plugboard. When a different program is to be executed the plugboard needs to be rewired.

A plugboard is a large circuit board where different vacuum tubes are connected to form a circuit. The tip of the wire includes a plug, therefore instead of soldering those wires they were directly plugged into the plugboard.

2nd generation

Were mainly batch systems. Very low performant.

Steps

  • A “job” is submitted in the form of punch cards
  • It is transferred into a magnetic tape. Multiple jobs with similar needs are batched together.
  • Tape is inserted into main computer which does the processing.
  • Output is written to a tape (which might then be converted to a hardcopy)

3rd generation

Uses multiprogramming for better performance.

Multiple jobs are kept in the memory at the same time. A job is run by the CPU until it is blocked due to some I/O activity. Then OS switches to one of the remaining jobs. No user interaction.

Time-sharing systems

Uses multitasking which is a variation of multiprogramming. Improves resource utilization. CPU is never idle. Allows users to interact with their programs while it is being executed.

4th generation

Aka. desktop systems or personal computer OSs. Currently used.

Functions of an Operating System

Functionality of an OS slightly varies based on the type.

Common tasks carried out by a typical OS:

  • Memory management - allocates memory for programs. handles swapping and reversing. The objective is to utilize limited memory in an efficient manner.
  • Spooling print jobs: All the print jobs will be collected onto the hard disk and later they are scheduled by the OS for printing.
  • Configuring devices: allows easy access to devices, their installation and configuration.
  • Monitoring system performance: A modern OS can display how system resources are utilized.
  • Administering security: supports multiple concurrent users while making sure each user uses the system without interference by others. It also enforces authentication and authorization.
  • Managing storage media and files: makes sure the secondary storage devices are optimally used. all the hard work of saving and retrieving files to and from disks, timestamping, setting various file attributes such as read-only, hidden and so on are handled by the OS.

Main classes of operating systems:

  • Microsoft OS - Started with MS-DOS. Later evolved into Windows.
  • UNIX - Led to different variants such as Linux, Sun Solaris, FreeBSD, AIX, HP-UX and so on.
  • Mainframe OS - From IBM. Some of the well-known ones are OS/360, OS/390 and OS/400.
  • MacOS - For Apple Macintosh computers

DOS

DOS was the first disk-based OS developed for IBM-PC by Microsoft. The objective was to keep the OS, application programs and all user files on a disk and managing them through set of commands called DOS commands. Simple to use and learn. Closed source and paid.

There are 2 well-known variants:

  • Personal Computer DOS - developed and sold with IBM PCs
  • Microsoft-DOS - sold in open market.

Windows

DOS with a GUI. Inspired by Apple Lisa. Initial versions of Windows were just an application over DOS; GUI was issuing DOS commands. Windows 98 also supported multitasking. Closed source and paid.

Supports most of the hardware and software components. Mostly used OS in the consumer market. However, compared to some other OSs Windows is not so stable, does not use resources in an effective way and have many security issues.

UNIX

Has many variants and versions which are still being used. It was developed as a time-sharing system for minicomputers and was initially mostly used by universities. Very stable. Highly reputed for its security, reliability, robustness, and performance.

It was developed by engineers for engineers. Therefore, it was harder for an average person to effectively use UNIX or any of its variants. As a result, UNIX was not so popular among the average users. However, modern variants with GUIs are much better in terms of user friendliness.

Linux

A UNIX-like OS developed originally for home PCs. Main objective was to make it simple so that anyone can understand and improve. Free and open source. Stable, reliable, and efficient. Most used OS in servers. Requires less disk space, memory and processing power. Better at handling multiple users.

Now, runs on many platforms including Intel, PowerPC, Macintosh, and Sun Sparc.