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UNIT 1
CS1252-OPERATING SYSTEM UNIT I
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What is an Operating System?
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A program that acts as an intermediary between a
user of a computer and the computer hardware.
Operating system goals:
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Execute user programs and make solving user problems
easier.
Make the computer system convenient to use.
Use the computer hardware in an efficient manner.
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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).
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Abstract View of System
Components
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Operating System Definitions
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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).
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Mainframe Systems
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Reduce setup time by batching similar jobs
Automatic job sequencing – automatically transfers
control from one job to another. First rudimentary
operating system.
Resident monitor
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initial control in monitor
control transfers to job
when job completes control transfers pack to monitor
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Memory Layout for a Simple Batch System
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Multi - programmed Batch Systems
Several jobs are kept in main memory at the same time, and the
CPU is multiplexed among them.
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OS Features Needed for
Multiprogramming
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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.
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Time-Sharing Systems–Interactive
Computing
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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.
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Desktop Systems
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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)
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Parallel Systems
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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:
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Increased throughput
Economical
Increased reliability
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graceful degradation
fail-soft systems
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Parallel Systems (Cont.)
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Symmetric multiprocessing (SMP)
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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
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Each processor is assigned a specific task; master processor
schedules and allocated work to slave processors.
More common in extremely large systems
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Symmetric Multiprocessing Architecture
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Distributed Systems
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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.
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Resources Sharing
Computation speed up – load sharing
Reliability
Communications
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Distributed Systems (cont)
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Requires networking infrastructure.
Local area networks (LAN) or Wide area networks
(WAN)
May be either client-server or peer-to-peer systems.
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General Structure of Client-Server
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Clustered Systems
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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.
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Real-Time Systems
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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 realtime.
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Real-Time Systems (Cont.)
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Hard real-time:
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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
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Limited utility in industrial control of robotics
Useful in applications (multimedia, virtual reality) requiring
advanced operating-system features.
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Handheld Systems
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Personal Digital Assistants (PDAs)
Cellular telephones
Issues:
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Limited memory
Slow processors
Small display screens.
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Hardware Protection
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Dual-Mode Operation
I/O Protection
Memory Protection
CPU Protection
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Dual-Mode Operation
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Sharing system resources requires operating system to ensure
that an incorrect program cannot cause other programs to
execute incorrectly.
Provide hardware support to differentiate between at least two
modes of operations.
1. User mode – execution done on behalf of a user.
2. Monitor mode (also kernel mode or system mode) – execution done on
behalf of operating system.
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Dual-Mode Operation (Cont.)
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Mode bit added to computer hardware to indicate the
current mode: monitor (0) or user (1).
When an interrupt or fault occurs hardware switches
to monitor mode.Interrupt/fault
monitor
user
set user mode
Privileged instructions can be issued only in monitor mode.
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I/O Protection
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All I/O instructions are privileged instructions.
Must ensure that a user program could never gain control of
the computer in monitor mode (I.e., a user program that, as
part of its execution, stores a new address in the interrupt
vector).
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Use of A System Call to Perform I/O
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Memory Protection
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Must provide memory protection at least for the interrupt
vector and the interrupt service routines.
In order to have memory protection, add two registers that
determine the range of legal addresses a program may access:
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Base register – holds the smallest legal physical memory address.
Limit register – contains the size of the range
Memory outside the defined range is protected.
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Use of A Base and Limit Register
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Hardware Address Protection
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Hardware Protection
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When executing in monitor mode, the operating system has
unrestricted access to both monitor and user’s memory.
The load instructions for the base and limit registers are
privileged instructions.
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CPU Protection
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Timer – interrupts computer after specified period to ensure
operating system maintains control.
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Timer is decremented every clock tick.
When timer reaches the value 0, an interrupt occurs.
Timer commonly used to implement time sharing.
Time also used to compute the current time.
Load-timer is a privileged instruction.
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Common System Components
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Process Management
Main Memory Management
File Management
I/O System Management
Secondary Management
Networking
Protection System
Command-Interpreter System
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Contd…
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PROCESS MANAGEMENT
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A process is a program in execution: (A program is passive, a process active.)
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A process has resources (CPU time, files) and attributes that must be managed.
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Management of processes includes:
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Process Scheduling (priority, time management, . . . )
Creation/termination
Block/Unblock (suspension/resumption )
Synchronization
Communication
Deadlock handling
Debugging
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Contd…
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MAIN MEMORY MANAGEMENT
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Allocation/de-allocation for processes, files, I/O.
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Maintenance of several processes at a time
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Keep track of who's using what memory
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Movement of process memory to/from secondary storage.
SECONDARY STORAGE MANAGEMENT
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Disks, tapes, optical, ...
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Free space management ( paging/swapping )
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Storage allocation ( what data goes where on disk )
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Disk scheduling
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DEVICE MANAGEMENT
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Buffer caching system
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Generic device driver code
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Drivers for each device - translate read/write requests into disk position
commands.
FILE MANAGEMENT
Keep track of what's on secondary storage.
 file == logical entity,
 disk == physical entity
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Map logical file locations onto physical disk locations.
May involve management of a file structure ( directory hierarchy )
·
Does file/directory creation/deletion
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File manipulation ( rename, move, append )
·
File backup
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PROTECTION
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Of files, memory, CPU, etc.
Means controlling of access
Depends on the attributes of the file and user
COMMUNICATION
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Communication system between distributed processors.
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Getting information about files/processes/etc. on a remote machine.
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Can use either a message passing or a shared memory model.
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Operating System Services
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Program execution – system capability to load a program into
memory and to run it.
I/O operations – since user programs cannot execute I/O
operations directly, the operating system must provide some
means to perform I/O.
File-system manipulation – program capability to read, write,
create, and delete files.
Communications – exchange of information between processes
executing either on the same computer or on different systems
tied together by a network. Implemented via shared memory or
message passing.
Error detection – ensure correct computing by detecting errors
in the CPU and memory hardware, in I/O devices, or in user
programs.
These are discussed in detail below
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Operating System Services
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One set of operating-system services provides
functions that are helpful to the user:
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User interface - Almost all operating systems have a user interface (UI)
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Varies between Command-Line (CLI), Graphics User Interface (GUI), Batch
Program execution - The system must be able to load a program into
memory and to run that program, end execution, either normally or
abnormally (indicating error)
I/O operations - A running program may require I/O, which may
involve a file or an I/O device.
File-system manipulation - The file system is of particular interest.
Obviously, programs need to read and write files and directories, create
and delete them, search them, list file Information, permission
management.
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Operating System Services (Cont.)
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One set of operating-system services provides
functions that are helpful to the user (Cont):
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Communications – Processes may exchange information, on the same
computer or between computers over a network
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Communications may be via shared memory or through message passing
(packets moved by the OS)
Error detection – OS needs to be constantly aware of possible errors
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May occur in the CPU and memory hardware, in I/O devices, in user
program
For each type of error, OS should take the appropriate action to ensure
correct and consistent computing
Debugging facilities can greatly enhance the user’s and programmer’s
abilities to efficiently use the system
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Operating System Services (Cont.)
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Another set of OS functions exists for ensuring the efficient operation
of the system itself via resource sharing
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Resource allocation - When multiple users or multiple jobs running
concurrently, resources must be allocated to each of them
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Many types of resources - Some (such as CPU cycles,mainmemory, and file
storage) may have special allocation code, others (such as I/O devices) may
have general request and release code.
Accounting - To keep track of which users use how much and what kinds
of computer resources
Protection and security - The owners of information stored in a
multiuser or networked computer system may want to control use of that
information, concurrent processes should not interfere with each other
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Protection involves ensuring that all access to system resources is controlled
Security of the system from outsiders requires user authentication, extends to
defending external I/O devices from invalid access attempts
If a system is to be protected and secure, precautions must be instituted
throughout it. A chain is only as strong as its weakest link.
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User Operating System Interface CLI
CLI allows direct command entry
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Sometimes implemented in kernel, sometimes by systems program
Sometimes multiple flavors implemented – shells
Primarily fetches a command from user and executes it
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Sometimes commands built-in, sometimes just names of programs
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If the latter, adding new features doesn’t require shell modification
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User Operating System Interface GUI
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User-friendly desktop metaphor interface
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Usually mouse, keyboard, and monitor
Icons represent files, programs, actions, etc
Various mouse buttons over objects in the interface cause various
actions (provide information, options, execute function, open directory
(known as a folder)
Invented at Xerox PARC
Many systems now include both CLI and GUI interfaces
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Microsoft Windows is GUI with CLI “command” shell
Apple Mac OS X as “Aqua” GUI interface with UNIX kernel
underneath and shells available
Solaris is CLI with optional GUI interfaces (Java Desktop, KDE)
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Additional Operating System Functions
Additional functions exist not for helping the user, but rather for
ensuring efficient system operations.
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Resource allocation – allocating resources to multiple users or multiple
jobs running at the same time.
Accounting – keep track of and record which users use how much and
what kinds of computer resources for account billing or for
accumulating usage statistics.
Protection – ensuring that all access to system resources is controlled.
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System Calls
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System calls provide the interface between a running
program and the operating system.
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Generally available as assembly-language instructions.
Languages defined to replace assembly language for
systems programming allow system calls to be made
directly (e.g., C, C++)
Three general methods are used to pass parameters
between a running program and the operating
system.
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Pass parameters in registers.
Store the parameters in a table in memory, and the table
address is passed as a parameter in a register.
Push (store) the parameters onto the stack by the program,
and pop off the stack by operating system.
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Example of System Calls
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System call sequence to copy the contents of one file to
another file
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Example of Standard API
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Consider the ReadFile() function in the
Win32 API—a function for reading from a file
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A description of the parameters passed to ReadFile()
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HANDLE file—the file to be read
LPVOID buffer—a buffer where the data will be read into and written from
DWORD bytesToRead—the number of bytes to be read into the buffer
LPDWORD bytesRead—the number of bytes read during the last read
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LPOVERLAPPED ovl—indicates
if overlapped
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System Call Implementation
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Typically, a number associated with each system call
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System-call interface maintains a table indexed according to these
numbers
The system call interface invokes intended system call in OS
kernel and returns status of the system call and any return
values
The caller need know nothing about how the system call is
implemented
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Just needs to obey API and understand what OS will do as a result call
Most details of OS interface hidden from programmer by API
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Managed by run-time support library (set of functions built into libraries
included with compiler)
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API – System Call – OS Relationship
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Standard C Library Example
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C program invoking printf() library call, which calls
write() system call
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System Call Parameter Passing
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Often, more information is required than simply identity of
desired system call
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Exact type and amount of information vary according to OS and call
Three general methods used to pass parameters to the OS
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Simplest: pass the parameters in registers
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Parameters stored in a block, or table, in memory, and address of block
passed as a parameter in a register
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In some cases, may be more parameters than registers
This approach taken by Linux and Solaris
Parameters placed, or pushed, onto the stack by the program and
popped off the stack by the operating system
Block and stack methods do not limit the number or length of
parameters being passed
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Passing of Parameters As A Table
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Types of System Calls
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Process control
File management
Device management
Information maintenance
Communications
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MS-DOS Execution
At System Start-up
Running a Program
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UNIX Running Multiple Programs
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Communication Models
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Communication may take place using either message
passing or shared memory.
Msg Passing
Shared Memory
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System Programs
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System programs provide a convenient environment for
program development and execution. The can be divided
into:
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File manipulation
Status information
File modification
Programming language support
Program loading and execution
Communications
Application programs
Most users’ view of the operation system is defined by
system programs, not the actual system calls.
This is told in detail as below
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System Programs
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Provide a convenient environment for program development and execution
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Some of them are simply user interfaces to system calls; others are considerably
more complex
File management - Create, delete, copy, rename, print, dump, list, and
generally manipulate files and directories
Status information
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Some ask the system for info - date, time, amount of available memory, disk
space, number of users
Others provide detailed performance, logging, and debugging information
Typically, these programs format and print the output to the terminal or other
output devices
Some systems implement a registry - used to store and retrieve configuration
information
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System Programs (cont’d)
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File modification
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Text editors to create and modify files
Special commands to search contents of files or perform
transformations of the text
Programming-language support - Compilers, assemblers,
debuggers and interpreters sometimes provided
Program loading and execution- Absolute loaders, relocatable
loaders, linkage editors, and overlay-loaders, debugging
systems for higher-level and machine language
Communications - Provide the mechanism for creating virtual
connections among processes, users, and computer systems
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Allow users to send messages to one another’s screens, browse web
pages, send electronic-mail messages, log in remotely, transfer files
from one machine to another
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Process Concept
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An operating system executes a variety of programs:
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Batch system – jobs
Time-shared systems – user programs or tasks
Textbook uses the terms job and process almost
interchangeably.
Process – a program in execution; process execution
must progress in sequential fashion.
A process includes:
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Text section
program counter
stack
data section
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Process State
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As a process executes, it changes state
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new: The process is being created.
running: Instructions are being executed.
waiting: The process is waiting for some event to occur.
ready: The process is waiting to be assigned to a process.
terminated: The process has finished execution.
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Diagram of Process State
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Process Control Block (PCB)
Information associated with each process.
 Process state
 Program counter
 CPU registers
 CPU scheduling information
 Memory-management information
 Accounting information
 I/O status information
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Process Control Block (PCB)
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CPU Switch From Process to Process
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Process Scheduling Queues
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Job queue – set of all processes in the system.
Ready queue – set of all processes residing in main memory,
ready and waiting to execute.
Device queues – set of processes waiting for an I/O device.
Process migration between the various queues.
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Ready Queue And Various I/O Device Queues
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Representation of Process Scheduling
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Schedulers
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Long-term scheduler (or job scheduler) – selects which
processes should be brought into the ready queue.
Short-term scheduler (or CPU scheduler) – selects which
process should be executed next and allocates CPU.
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Addition of Medium Term
Scheduling
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Schedulers (Cont.)
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Short-term scheduler is invoked very frequently (milliseconds)
 (must be fast).
Long-term scheduler is invoked very infrequently (seconds,
minutes)  (may be slow).
The long-term scheduler controls the degree of
multiprogramming.
Processes can be described as either:
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I/O-bound process – spends more time doing I/O than computations,
many short CPU bursts.
CPU-bound process – spends more time doing computations; few very
long CPU bursts.
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Context Switch
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When CPU switches to another process, the system must save
the state of the old process and load the saved state for the new
process.
Context-switch time is overhead; the system does no useful
work while switching.
Time dependent on hardware support.
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Process Creation
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Parent process create children processes, which, in turn create
other processes, forming a tree of processes.
Resource sharing
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Parent and children share all resources.
Children share subset of parent’s resources.
Parent and child share no resources.
Execution
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Parent and children execute concurrently.
Parent waits until children terminate.
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Process Creation (Cont.)
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Address space
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Child duplicate of parent.
Child has a program loaded into it.
UNIX examples
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fork system call creates new process
exec system call used after a fork to replace the process’ memory space
with a new program.
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Processes Tree on a UNIX System
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Process Termination
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Process executes last statement and asks the operating system
to decide it (exit).
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Output data from child to parent (via wait).
Process’ resources are deallocated by operating system.
Parent may terminate execution of children processes (abort).
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Child has exceeded allocated resources.
Task assigned to child is no longer required.
Parent is exiting.
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Operating system does not allow child to continue if its parent terminates.
Cascading termination.
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Cooperating Processes
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Independent process cannot affect or be affected by the
execution of another process.
Cooperating process can affect or be affected by the execution
of another process
Advantages of process cooperation
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Information sharing
Computation speed-up
Modularity
Convenience
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Producer-Consumer Problem
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Paradigm for cooperating processes, producer process
produces information that is consumed by a consumer process.
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unbounded-buffer places no practical limit on the size of the buffer.
bounded-buffer assumes that there is a fixed buffer size.
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Bounded-Buffer – Shared-Memory Solution

Shared data
#define BUFFER_SIZE 10
Typedef struct {
...
} item;
item buffer[BUFFER_SIZE];
int in = 0;
int out = 0;

Solution is correct, but can only use BUFFER_SIZE1 elements
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Bounded-Buffer – Producer Process
item nextProduced;
while (1) {
while (((in + 1) % BUFFER_SIZE) == out)
; /* do nothing */
buffer[in] = nextProduced;
in = (in + 1) % BUFFER_SIZE;
}
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Bounded-Buffer – Consumer Process
item nextConsumed;
while (1) {
while (in == out)
; /* do nothing */
nextConsumed = buffer[out];
out = (out + 1) % BUFFER_SIZE;
}
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Interprocess Communication (IPC)



Mechanism for processes to communicate and to synchronize
their actions.
Message system – processes communicate with each other
without resorting to shared variables.
IPC facility provides two operations:



If P and Q wish to communicate, they need to:



send(message) – message size fixed or variable
receive(message)
establish a communication link between them
exchange messages via send/receive
Implementation of communication link


physical (e.g., shared memory,
hardware
bus)
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Implementation Questions






How are links established?
Can a link be associated with more than two processes?
How many links can there be between every pair of
communicating processes?
What is the capacity of a link?
Is the size of a message that the link can accommodate fixed or
variable?
Is a link unidirectional or bi-directional?
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Direct Communication

Processes must name each other explicitly:



send (P, message) – send a message to process P
receive(Q, message) – receive a message from process Q
Properties of communication link




Links are established automatically.
A link is associated with exactly one pair of communicating processes.
Between each pair there exists exactly one link.
The link may be unidirectional, but is usually bi-directional.
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Indirect Communication

Messages are directed and received from mailboxes
(also referred to as ports).



Each mailbox has a unique id.
Processes can communicate only if they share a mailbox.
Properties of communication link




Link established only if processes share a common mailbox
A link may be associated with many processes.
Each pair of processes may share several communication
links.
Link may be unidirectional or bi-directional.
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Indirect Communication

Operations




create a new mailbox
send and receive messages through mailbox
destroy a mailbox
Primitives are defined as:
send(A, message) – send a message to mailbox A
receive(A, message) – receive a message from
mailbox A
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Indirect Communication

Mailbox sharing




P1, P2, and P3 share mailbox A.
P1, sends; P2 and P3 receive.
Who gets the message?
Solutions



Allow a link to be associated with at most two processes.
Allow only one process at a time to execute a receive operation.
Allow the system to select arbitrarily the receiver. Sender is notified
who the receiver was.
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Synchronization




Message passing may be either blocking or non-blocking.
Blocking is considered synchronous
Non-blocking is considered asynchronous
send and receive primitives may be either blocking or nonblocking.
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Buffering

Queue of messages attached to the link; implemented in one of
three ways.
1. Zero capacity – 0 messages
Sender must wait for receiver (rendezvous).
2. Bounded capacity – finite length of n messages
Sender must wait if link full.
3. Unbounded capacity – infinite length
Sender never waits.
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END
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