Operating System Overview: Understanding the Role and Components of an OS, Slides of Computer Science

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OPERATING SYSTEMS

OVERVIEW

WHAT IS AN OPERATING SYSTEM?

• An interface between users and hardware - an environment "architecture”
• Allows convenient usage; hides the tedious stuff
• Allows efficient usage; parallel activity, avoids wasted cycles
• Provides information protection
• Gives each user a slice of the resources
• Acts as a control program.

OVERVIEW

A mechanism for scheduling jobs or processes. Scheduling can be as simple

as running the next process, or it can use relatively complex rules to pick

a running process.

A method for simultaneous CPU execution and IO handling. Processing is

going on even as IO is occurring in preparation for future CPU work.

Off Line Processing; not only are IO and CPU happening concurrently, but

some off-board processing is occurring with the IO.

OVERVIEW

Components

The CPU is wasted if a job waits for I/O. This leads to:

• Multiprogramming ( dynamic switching ). While one job waits for a

resource, the CPU can find another job to run. It means that several jobs

are ready to run and only need the CPU in order to continue.

CPU scheduling is the subject of Chapter 6.

All of this leads to:

• memory management
• resource scheduling
• deadlock protection

which are the subject of the rest of this course.

OVERVIEW

Components

OVERVIEW

Characteristics

Interrupts:

• Interrupt transfers control to the interrupt service routine generally, through the interrupt vector , which contains the addresses of all the service routines.
• Interrupt architecture must save the address of the interrupted instruction.
• Incoming interrupts are disabled while another interrupt is being processed to prevent a lost interrupt.
• A trap is a software-generated interrupt caused either by an error or a user request.
• An operating system is interrupt driven.

OVERVIEW

Hardware

Support

These are the devices that make up a typical system.

Any of these devices can cause an electrical interrupt that grabs the attention of the CPU.

OVERVIEW

Hardware

Support

This is O.S. Bookkeeping. These structures are necessary to keep track of IO in progress.

Very fast storage is very expensive. So the Operating System manages a hierarchy of storage devices in order to make the best use of resources. In fact, considerable effort goes into this support.

OVERVIEW

Storage

Hierarchy

Fast and Expensive

Slow an Cheap

Caching:

• Important principle, performed at many levels in a computer (in hardware,

operating system, software)

• Information in use copied from slower to faster storage temporarily
• Faster storage (cache) checked first to determine if information is there
  • If it is, information used directly from the cache (fast)
  • If not, data copied to cache and used there
• Cache smaller than storage being cached
  • Cache management important design problem
  • Cache size and replacement policy

OVERVIEW

Storage

Hierarchy

The goal is protecting the Operating System and others from malicious or ignorant users.

The User/Supervisor Mode and privileged instructions.

Concurrent threads might interfere with others. This leads to protection of resources by user/supervisor mode. These resources include:

I/O Define I/O instructions as privileged; they can be executed only in Supervisor mode. System calls get us from user to supervisor mode.

OVERVIEW

Protection

CPU A clock prevents programs from using all the CPU time. This

clock causes an interrupt that causes the operating system to gain control

from a user program.

OVERVIEW

Protection

For machines connected together, this protection must extend across:

Shared resources,

Multiprocessor Architectures,

Clustered Systems

The practice of this is called “distributed operating systems”.

WRAPUP

We’ve completed our first overview of an Operating System – this was

the equivalent of a Satellite picture.

The next view will be at the level of a high flying plane.

After that, we’ll be at ground level, looking at pieces in detail.

OPERATING SYSTEM OVERVIEW