Basic os-concepts

Introduction to Basic OS
Concepts

Introduction
 What is an Operating System?
 Mainframe Systems
 Desktop Systems
 Multiprocessor Systems
 Distributed Systems
 Clustered System
 Real -Time Systems
 Handheld Systems

What is an Operating System?
 A program that acts as an intermediary
between a user of a computer and the
computer hardware.
 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.

What is OS?
 Computer systems typically contain:
Hardware and Software
Hardware - electronic, mechanical,
optical devices
Software – programs
 Without support software, the computer
is of little use..

What is OS?
 An interface between Hardware and User
Programs
 An abstraction of the hardware for all the
(user) processes
 Hide the complexity of the underlying hardware
and give the user a better view of the computer
 => A MUST!

Computer System
Components
1. Hardware – provides basic computing resources (CPU,
memory, I/O devices).
2. Operating system – controls and coordinates the use of the
hardware among the various application programs for the
various users.
3. Applications programs – define the ways in which the system
resources are used to solve the computing problems of the
users (compilers, database systems, video games, business
programs).
4. Users (people, machines, other computers).

Abstract View of System
Components

The OS
. . .
CPU
memory
device device device
Operating system
utilities
applications
software
systems
software
hardware
components
. . .
CPU
memory
device device device
Operating system
utilities
applications
software
systems
software
hardware
components

Operating System Definitions
 Resource allocator – manages and
allocates resources.
 Control program – controls the
execution of user programs and
operations of I/O devices .
 Kernel – the one program running
at all times (all else being
application programs).

The Goals of an OS
 Let users run programs:
 Correctness

Memory boundaries, priorities, steady state
 Convenience

User should not handle the tiny details
(encapsulate/abstract), provide synchronization
primitives, system calls, file system, tools

The Goals of an OS
 Let users run programs:
 Efficiency

Resource Utilization, resource Sharing,
Multitasking
 Fairness (in resource allocation)

Among: users, tasks, resources

The tradeoff between efficiency and fairness

An OS is a Resource Allocator
“Mama says: It’s good to share!”
 Multiple users (?) get all computing resources
“simultaneously”:
 Cpu time
 Memory (ram, swap, working set, virtual,..)
 File system (storage space)
 I/O devices (display, printers, mouse,..)
 Clock
 The OS should give every user the illusion that
she is getting all resources to herself (not sharing!)

What an OS does for a living..
loop forever {
run the process for a while.
stop process and save its state.
load state of another process.
}

Virtual Continuity
 A process can get “switched in” or “switched
out”.
 OS should give the illusion for the process as
if it exists in the CPU continuously
=> Context Switching

 When an event occurs, the operating system saves
the state of the active process and restores the
state of the new process.
 This mechanism is called a Context Switch.
 What must get saved? Everything that the next
process could or will damage. For example:

Program counter (PC)

Program status word (PSW)

CPU registers (general purpose, floating-point)

File access pointer(s)

Memory (perhaps?)
Context switching

 A process can give up the cpu:
 A. by performing I/O (e.g. getchar())
 B. by entering a waiting state (e.g. semaphore)
 C. by entering a suspended state (e.g. sleep())
 Give up the CPU == switch out the current process
+
switch in another process
Scheduling and Context
switch

 There are OS’s where a process is forced to give up
the cpu (e.g. when stayed for too long).
 Such systems are implementing a “preemptive
scheduling” policy.
 Examples include Windows NT, Unix,
- BUT NOT - Windows prior to Win95 ! or Macintosh!
 Xinu?
Should a real-time system implement preemptive scheduling?
Preemptive Scheduling

 Most OS’s provide the priority mechanism
 Priorities are associated with processes
 Priority are used to help the OS to reach fairness
Can you think of processes (e.g. in Windows) for which you will
give especially high/low priority ??
Using Priorities

Process
 A process is a program in execution.
 The components of a process are:
 the program to be executed,
 the data on which the program will execute,
 the resources required by the program—such as
memory and file(s)—and
 the status of the execution.

Process Interleaving
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‫ציר‬
‫הזמן‬
C
B
A
D
‫עוקבים‬ ‫תהליכים‬

Mainframe Systems
 Reduce setup time by batching similar jobs
 Automatic job sequencing – automatically
transfers control from one job to another.
First rudimentary operating system.
 Resident monitor
 initial control in monitor
 control transfers to job
 when job completes control transfers pack to
monitor

Memory Layout for a Simple Batch
System

Multiprogrammed Batch
Systems
Several jobs are kept in main memory at the same time, and the
CPU is multiplexed among them.

OS Features Needed for
Multiprogramming
 I/O routine supplied by the system.
 Memory management – the system
must allocate the memory to
several jobs.
 CPU scheduling – the system must
choose among several jobs ready
to run.
 Allocation of devices.

Time-Sharing Systems–Interactive
Computing
 The CPU is multiplexed among several jobs that are
kept in memory and on disk (the CPU is allocated to
a job only if the job is in memory).
 A job swapped in and out of memory to the disk.
 On-line communication between the user and the
system is provided; when the operating system
finishes the execution of one command, it seeks the
next “control statement” from the user’s keyboard.
 On-line system must be available for users to
access data and code.

Desktop Systems
 Personal computers – computer system dedicated
to a single user.
 I/O devices – keyboards, mice, display screens,
small printers.
 User convenience and responsiveness.
 Can adopt technology developed for larger
operating system’ often individuals have sole use
of computer and do not need advanced CPU
utilization of protection features.
 May run several different types of operating
systems (Windows, MacOS, UNIX, Linux)

Parallel Systems
 Multiprocessor systems with more than on 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:
 Increased throughput
 Economical
 Increased reliability

graceful degradation

fail-soft systems

Parallel Systems (Cont.)
 Symmetric multiprocessing (SMP)
 Each processor runs and 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 allocated work to slave
processors.
 More common in extremely large systems

Symmetric Multiprocessing
Architecture

Distributed Systems
 Distribute the computation among several physical
processors.
 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.
 Advantages of distributed systems.
 Resources Sharing
 Computation speed up – load sharing
 Reliability
 Communications

Distributed Systems (cont)
 Requires networking infrastructure.
 Local area networks (LAN) or Wide
area networks (WAN)
 May be either client-server or peer-
to-peer systems.

General Structure of Client-
Server

Clustered Systems
 Clustering allows two or more systems
to share storage.
 Provides high reliability.
 Asymmetric clustering: one server runs
the application while other servers
standby.
 Symmetric clustering: all N hosts are
running the application.

Real-Time Systems
 Often used as a control device in a
dedicated application such as
controlling scientific experiments,
medical imaging systems, industrial
control systems, and some display
systems.
 Well-defined fixed-time constraints.
 Real-Time systems may be either
hard or soft real-time.

Real-Time Systems (Cont.)
 Hard real-time:
 Secondary storage limited or absent, data
stored in short term memory, or read-only
memory (ROM)
 Conflicts with time-sharing systems, not
supported by general-purpose operating
systems.
 Soft real-time
 Limited utility in industrial control of robotics
 Useful in applications (multimedia, virtual
reality) requiring advanced operating-system
features.

Handheld Systems
 Personal Digital Assistants (PDAs)
 Cellular telephones
 Issues:
 Limited memory
 Slow processors
 Small display screens.

Migration of Operating-System
Concepts and Features

Computing Environments
 Traditional computing
 Web-Based Computing
 Embedded Computing

The PC-XINU OS
Let’s fillin’ the bits..

Basic os-concepts

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