Download Operating Systems CS208

Operating
Systems
CS208
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
Computer
User
Application
Operating system
Application
User
Hardware
Application
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.
Hardware
Kernel
Shell
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)
 Menu
 Command
Language
 Graphical User Interface (GUI)

System Security
 Login

control
Process Management
 CPU
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 Management
 Device Management
Operating System Terminology

Kernel
 Software
component that controls the
hardware directly, and implements the
core privileged OS functions.

Process
 An
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
CPU
Main Memory
Allocate CPU
to Process 1
Current
Process
Allocate CPU
to Process 2
Allocate CPU
to Process 3
Repeat until all processes have completed.
Process 1
Process 2
Process 3
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
 Get
the file
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
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.
 Loosely
 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.