Operating SystemsCS208

What is Operating System? It is a program. It is the first piece of software to run after

the system boots. It coordinates the execution of all other

software. It provides various common services

needed by users and applications.

Operating System Definition Short definition:

a program that manages a computer’s resources and acts as an intermediary between a user and those resources

Application

Hardware

Operating system

Computer

Application

Application

User

User

Operating System (OS) Functions

For the User: Provides a user interface to let the user access hardware and software resources.

For the System: Manages all the system tasks and resources to provide security, and fair, efficient use.

Shell

Kernel

Hardware

Operating

System

Applications

Operating System Goals

Execute user programs and make solving user problems easier.

Make the computer system convenient to use.

Use the computer hardware in an efficient manner.

Parts of an Operating System User Interface (Shell)

MenuCommand LanguageGraphical User Interface (GUI)

System SecurityLogin control

Process ManagementCPU Scheduler

Parts of an Operating System Memory Management

Swapper/Pager Temporarily unused pages are stored on

disk (swapped out) When they are needed again, they are

brought back into the memory (swapped in)Garbage Collection

Resource Management File ManagementDevice Management

Operating System Terminology

KernelSoftware component that controls the

hardware directly, and implements the core privileged OS functions.

ProcessAn executing program.

Time Sharing Allows multiple processes to run on the same

computer, seemingly at the same time. CPU is multiplexed among several processes that

are kept in memory (the CPU is allocated to a process only if the process is in memory). A process is swapped in and out of memory to

the disk as needed. OS must manage process scheduling and provide

memory protection to keep one program from crashing the system or corrupting other programs.

CPU Time Slicing for Time Sharing

Only ONE process can be running on the CPU at a time.Each process is allocated a “slice” of time

in the CPU.When the time runs out, the process is

interrupted, and another process is loaded into the CPU.

The act of giving each process a small slice of time to run is called time slicing.

CPU Time Slicing

Allocate CPU to Process 1

CPU

Process 1

Process 3

Main Memory

Process 2CurrentProcess

Allocate CPU to Process 2

Allocate CPU to Process 3

Repeat until all processes have completed.

CPU execution modes CPUs supports (at least) 2 execution modes:

User mode

The code of the user programs

Kernel (supervisor, privileged, monitor, system) mode

The code of OS

The execution mode is indicated by a bit in the processor status word (PSW) (a register in the CPU)

Protecting Kernel mode OS code executes in the Kernel mode

Called via interrupts and system calls

Only the OS code is allowed to be executed in the Kernel mode

The user code must never be executed in the Kernel mode

The program counter (PC) is only set to point to the OS code when the CPU goes to the Kernel mode

Interrupts

An interrupt is the way by which the hardware informs the OS of special conditions that require OS attention

Interrupts cause the CPU not to execute the next instruction

Instead, the control is passed to OS

System Calls A System Call is used by a process to

request a service from the OS

Typical system calls

Open/read/write/close the file

Get the current time

Create a new process

Request more memory

Parallel Systems Multi-processor systems with more than one CPU in

close communication.

Tightly coupled system – processors share memory and a clock; communication usually takes place through the shared memory.

Advantages of parallel system:

Economical

Increased throughput

Increased reliability

Parallel Systems Symmetric multiprocessing (SMP)

Each processor runs an identical copy of the operating system.

Many processes can run at once without performance deterioration.

Most modern operating systems support SMP Asymmetric multiprocessing

Each processor is assigned a specific task; master processor schedules and allocates work to slave processors.

More common in extremely large systems

Distributed Systems Distribute the computation among several physical computers.

Each has its own CPU, local memory, stable storage, I/O paths connecting to the environment

Interconnections

Loosely coupled system – each processor has its own local memory; processors communicate with one another through various communications lines, such as high-speed buses or telephone lines.

Systems cooperate to maintain shared state and coordinate global information

Distributed Systems Advantages of distributed systems.

Inherent distribution Speedup - improved performance due to load sharing Fault tolerance/Reliability Resource Sharing Scalability Flexibility

Distributed Systems Network Operating System

provides file sharing provides communication scheme runs independently from other computers on the

network Distributed Operating System

less autonomy between computers gives the impression there is a single operating

system controlling the network.