A program that controls the execution of application programs

An interface between applications and hardware

The main objectives of an OS:

• Convenience – Targets the user – ease of use
• Efficiency – Don’t want to waste resources
• Ability to evolve – Addressing issues, compatibility and security

Program development

• Editors and debuggers.

Program execution

• OS handles scheduling of numerous tasks required to execute a program

Access I/O devices

• Each device will have unique interface
• OS presents standard interface to users

Controlled access to files

• Accessing different media but presenting a common interface to users
• Provides protection in multi-access systems

System access

• Controls access to the system and its resources

Error detection and response

• Internal and external hardware errors
• Software errors
• Operating system cannot grant request of application

Accounting

• Collect usage statistics
• Monitor performance

A computer is a set of resources for the movement, storage, and processing of data.

The OS is responsible for managing these resources.

The OS functions in the same way as an ordinary computer software

• It is a program that is executed by the CPU

Operating system relinquishes control of the processor

Operating systems will evolve over time

• Hardware upgrades plus new types of hardware – such as Multicore processors
• New services – such as Video conferencing
• Fixes

OS has to keep up with hardware advancements to make sure all resources are used effectively

It may be easier to understand the key requirements of an OS by considering the evolution of Operating Systems

Stages include:

• Serial Processing
• Simple Batch Systems
• Multiprogrammed batch systems
• Time Sharing Systems

User Mode – Where games or browsers run

• User program executes in user mode
• Certain areas of memory protected from user access – To protect the system
• Certain instructions may not be executed

Kernel Mode – ‘The heart of the system’

• Monitor executes in kernel mode
• Privileged instructions may be executed, all memory accessible.

Processor must wait for I/O instruction to complete before preceding

• When one job needs to wait for I/O, the processor can switch to the other job

Operating Systems are among the most complex pieces of software ever developed

Major advances include:

• Processes
• Memory management
• Information protection and security
• Scheduling and resource management
• System

Fundamental to the structure of OS’s

A process is:

• A program in execution
• An instance of a running program
• The entity that can be assigned to and executed on a processor
• A single sequential thread of execution, a current state, and an associated set of system resources.

Error in designing an OS are often subtle and difficult to diagnose.

Errors typically include:

• Improper synchronisation
• Failed mutual exclusion – When processes deal with memory, we don’t want other programs to get in the way
• Non-determinate program operation
• Deadlocks – Create an infinite loop

Deadlocks explained

It is possible for two or more programs to be hung up waiting for each other.

• e.g. two programs may each require two I/O devices to perform some operation (e.g., disk to tape copy).
• One of the programs has seized control of one of the devices and the other program has control of the other device.
• Each is waiting for the other program to release the desired resource.

Such a deadlock may depend on the chance timing of resource allocation and release.

A process consists of 

• An executable program
• Associated data needed by the program
• Execution context of the program (or “process state”) – Running, waiting for resources etc

The execution context contains all information the operating system needs to manage the process

The OS has 5 principal storage management responsibilities

• Process isolation
• Automatic allocation and management
• Support of modular programming – Access to and use of code libraries
• Protection and access control
• Long-term storage

File system implements long-term store – The way memory is organised

Virtual memory allows programs to address memory from a logical point of view

• Without regard to the limits of physical memory

Allows process to be comprised of a number of fixed-size blocks, called pages.

Virtual address is a page number and an offset within the page.

Each page may be located anywhere in main memory.

The problem involves controlling access to computer systems and the information stored in them.

Main issues are:

• Availability – Available all the time
• Confidentiality – Only available to us
• Data integrity – Data should be there as we last left it. Nobody should be able to tamper with it.
• Authenticity – Before we get access to the data, we should authenticate ourselves.

Key responsibility of an OS is managing resources

Resource allocation policies must consider:

• Fairness
• Differential responsiveness – Some things are more important than others e.g. Monitor drivers before running games
• Efficiency

• View the system as a series of levels

• Each level performs a related subset of functions

• Each level relies on the next lower level to perform more primitive functions

• This decomposes a problem into a number of more manageable sub problems

Most early OS are a monolithic kernel

• Most OS functionality resides in the kernel.

A microkernel assigns only a few essential functions to the kernel

• Address spaces
• Interprocess communication (IPC)
• Basic scheduling

A Thread is a dispatchable unit of work which executes sequentially and is interruptible

A Process is a collection of one or more threads which can run concurrently.

Symmetric multiprocessing (SMP)

An SMP system has

• multiple processors
• These processors share same main memory and I/O facilities
• All processors can perform the same functions

The OS of an SMP schedules processes or threads across all of the processors.

SMP Advantages

Performance

• Allowing parallel processing

Availability

• Failure of a single process does not halt the system

Incremental Growth

• Additional processors can be added

Scaling – There is always a threshold, at which things are no longer more efficient.

Provides the illusion of

• a single main memory space and
• single secondary memory space

Early stage of development

Used for adding modular extensions to a small kernel

Enables programmers to customise an operating system without disrupting system integrity

 From Windows 2000 on Windows development developed to exploit modern 32-bit and 64-bit microprocessors

Designed for single users who run multiple programs

Main drivers are:

• Increased memory and speed of microprocessors
• Support for virtual memory
• Windows draws heavily on the concepts of object-oriented design.

Windows OS, protected subsystem, and applications all use a client/server model

• Common in distributed systems, but can be used internal to a single system

Processes communicate via RPC

• Multi-user, multitasking operating system
• It can be used as the master control program in workstations and servers.
• Hundreds of commercial applications are available
• In its heydays, UNIX was rapidly adopted and became the standard OS in universities.

• The unfriendly, terse, inconsistent, and non-mnemonic user interface
• UNIX OS is designed for a slow computer system, so you can't expect fast performance.
• Shell interface can be treacherous because typing mistake can destroy files.
• Versions on various machines are slightly different, so it lacks consistency.
• UNIX does not provide any assured hardware interrupt response time, so it does not support real time response time systems.

Although monolithic, the kernel is structures as a collection of modules

• Loadable modules
• An object file which can be linked and unlinked at run time

Characteristics:

• Dynamic Linking
• Stackable modules

• Support multitasking

• Programs consist of one or more processes, and each process have one or more threads

• It can easily co-exists along with other Operating systems.

• It can run multiple user programs

• Individual accounts are protected because of appropriate authorisation

• Linux is a replica of UNIX but does not use its code.

• There's no standard edition of Linux

• Linux has patchier support for drivers which may result in misfunctioning of the entire system.

• Linux is, for new users at least, not as easy to use as Windows.

• Many of the programs we are using for Windows will only run on Linux only with the help of a complicated emulator. For example. Microsoft Office.

• Linux is best suitable for a corporate user. It's much harder to introduce in a home setting.