Intermediate Operating Systems

MODULE 5

OPERATING SYSTEM
Intermediate Level

© Cutajar & Cutajar 2009

WHAT IS AN OPERATING SYSTEM

 An OPERATING SYSTEM (OS) is set of programs that
manage all the hardware resources of a computer system,
thus isolating the user from the complexities of direct use
of the computer hardware.
 The operating system manages resources such as:
 input (keyboard, mouse, ),
 processing (CPU, … ),
 storage (main memory, hard discs, floppy discs, … )
 output ( monitor, printer, … ).
 It controls the way in which these resources are put to
work.

Operating Systems 2

ROLE OF THE OPERATING SYSTEM

User Interface

Hardware

User

OS Function Call Interface

Operating Systems 3

COMPUTER SYSTEM HIERARCHY

 The computer system hierarchy :

User 1 User 2  User n

Compiler database word 
System processor

System and application programs
Operating System
Computer Hardware

Operating Systems 4

LOADING AN OPERATING SYSTEM

 The operating system is usually held on disk and has to be loaded into
main memory once the computer is switched on before any other
program can be run. Exception: In very compact systems, the OS may
be held in ROM.
 The process of loading the operating system is called BOOTING the
system.
 The BOOTSTRAP LOADER is an initial small program that executes
itself once the system is powered on. This loader is held in ROM. It tells
the computer where to look for the operating system and gives
instructions to load at least part of it into memory.
 Once part of the OS has been loaded, more instructions can be executed
to load the rest of the OS kernel (nucleus).
 The operating system then waits for some event to occur.

Operating Systems 5

OBJECTIVES OF AN OPERATING SYSTEM

1. To make it possible for the resources of the computer to be
used efficiently.
 Efficient resource utilisation is made especially difficult by the fact that
some devices of a computer work much more quickly than others. Part of
the task of the operating system is to ensure that fast devices such as
processors, are not held up by slower peripheral devices. Thus the
operating system is responsible for allocating and controlling use of all
resources of a computer system.

2. To conceal the difficulties of dealing directly with the
hardware of the computer.
 To simplify the use of the hardware of a computer, the OS creates a
VIRTUAL MACHINE i.e. it takes care of the hardware difficulties presented
by the different component devices of the computer system and provides
the user with a simplified computer environment. Thus a computer user
need only know about (and how to operate) the „virtual machine‟.
Operating Systems 6

DESIRABLE FEATURES OF OPERATING SYSTEMS

 Efficiency
 An OS must carry out its tasks promptly. Time spent on operating system
functions is productive time wasted.
 Reliability
 Failure in an OS can render the host computer useless. An OS program
should do what it has to do correctly. Ideally the OS should be error free
and able to handle all contingencies - robustness.
 Maintainability
 Clearly written, well-structured (operating system) programs are essential
for any later modifications. This is a very complex set of programs to
develop; it is costly to produce (hence well-structuring and modularity
encourage the reusability of code). Also remember that maintenance is the
most costly part of any software existence cycle, hence having features
which enhance maintainability help keep maintenance costs down.
 Small Size
 Space used to hold the OS whether in main memory or on backing store is
wasted as far as productive computing is concerned.

Operating Systems 7

TYPES OF OPERATING SYSTEMS

 Operational mode is the form of operational organisation
which determines the way in which the computer system is
being used. This is determined by the operating system
which is currently controlling the computer.
 Batch
 Online
 Multiprogramming (multitasking)
 Time-sharing (multi-access)
 Single-user
 Real-time
o Hard real-time (process control)
o Soft real-time (transaction processing)

Operating Systems 8

BATCH

 A number of programs are batched together and run as a group.
Although the programs are run one at a time, input and output from
various programs can overlap to some extent.
 Programs are usually queued up for batch processing and the
operating system starts the next program in the queue as soon as
sufficient computing resources are available for it. Some application
area examples are payroll, processing of utility bills and master file
updating process.
 The main characteristic of batch processing is the lack of interaction
between the user and the job while the job is executing.
 Note that the above set up with the interleaving of batch jobs is
better known as multiprogrammed batched system as against
the simpler batch type where jobs are run one after the other.
Multiprogrammed batch systems were facilitated by technological
development. This set-up ensures better utilisation of the resources,
the CPU in particular.
Operating Systems 9

MULTIPROGRAMMING (MULTITASKING)

 At any one time, a number of programs
are on the computer at various stages of
completion. (Remember multi-programmed
batch systems!). UNI CPU idle
 Resources are allocated to programs
according to the requirements of the
programs and in order to maximise the
usage of the resources of the computer.
 Having multiple jobs running concurrently
on the same system implies more
demanding operating system functions MULTI
such as process scheduling, memory
management and system security and
protection.
 It also implies better utilisation of CPU idle
computer resources.
time
Operating Systems 10

TIME-SHARING (MULTI-ACCESS)

 Time-sharing systems are a logical extension of multiprogramming
systems. The operating system allows many users to use the
computer at apparently the same time.
 Each job on the computer is given a quantum of processing time. The
CPU switches from one job to another so quickly that each user is
given the impression that s/he has her own computer, whereas
actually one computer is being shared by several users.
 Time-sharing systems make possible the interactive use of multi-
programmed computer systems. In this environment, the user gives
instructions to the computer and immediately receives a response.
 Most operating systems provide both batch processing and time-
sharing (although their basic design is either one or the other).


REAL-TIME SYSTEM

 A real-time system is used when there are rigid time requirements on
the operation of a processor or the flow of data such as systems that
control scientific experiments, medical imaging systems, industrial
control systems, some display systems, home-appliance controllers,
weapon systems.
 They can be classified in:
 Hard real-time (Real-time process control)
 Soft real-time (Real-time transaction processing)
BOOM!
HARD

SOFT
deadline

time

REAL-TIME PROCESS CONTROL

 Real-time process control systems require that all delays are
bounded, from the retrieval of stored data to the time that it takes the
operating system to finish any request made of it.
 Such time constraints dictate the facilities that are available in hard
real-time systems - secondary storage is either missing or very limited,
virtual memory is non-existent, advanced OS features separating user
from hardware are absent.

 Hard real-time systems cannot
be mixed with the operation of
time-sharing systems (and any
general purpose operating
system) because of conflicting
features.


REAL-TIME ONLINE TRANSACTION PROCESSING

 Online Transaction Processing (OLTP) systems are
less restrictive. A critical real-time task gets priority over
other tasks and retains that priority until it completes. This
type of real-time system may be mixed with other types of
systems. (Application areas include interactive database
applications, multimedia presentations, virtual reality, )


PARALLEL (MULTIPROCESSOR) SYSTEM

 Computer systems having more than one processor enjoy increased
throughput (more work is done in a shorter period of time), increased
reliability (failure of one processor will not halt the system - the
ability to continue providing service proportional to the level of
surviving hardware is called graceful degradation – systems designed
for graceful degradation are called fault tolerant)

 Processors share the computer bus,
the clock, memory and peripheral
devices and are referred to as
tightly coupled systems.


DISTRIBUTED SYSTEM

 In this environment, computation is distributed among several processors. In
contrast to tightly coupled systems, processors do not share memory or a clock;
each processor has its own local memory. Processors communicate with one
another through various communication lines (high speed buses, telephone line).
 Hence they are usually referred to as loosely coupled systems. Processors in a
distributed system may vary in size and function – small microprocessors,
workstations, minicomputers, and large general-purpose computer systems.
 Reasons for building distributed systems may include:
 Resource sharing: a user may use resources residing at another site,
 Computation speedup: partitioning a computation across a number of sites or
moving a computation to another more lightly loaded site (load sharing),
 Reliability: if one site fails other sites can continue operating given that the system
has sufficient inbuilt redundancy (in both hardware and data),
 Communication: Processes at different sites have the possibility of exchanging
information; besides users may initiate file transfers or communicate with each other
via email.
 Note that a distributed system implies a Network system. A network OS is one
that supports the interconnection of computers that communicate with each
other and are connected together according to a predefined topology.

COMPUTER OPERATION TERMINOLOGY

 On-line operation: this is processing carried out on devices under the
control of the central processor, while the user remains in
communication with the computer.
 Example: OS sending a command to a printer to change its present printing
font.
 Off-line operation: this is processing carried out by devices not under
the immediate control of the central processing.
 Example: pressing the printer Line Feed button while the printer is off-line.
 Remote Access: It is a form of access where the computer terminal is
physically separated from the central computer installation.
 Example: work from home through a modem using Telnet.


FUNCTIONS OF AN OPERATING SYSTEM

 Main functions of an operating system include:
 Process Management : allowing processes to co-exist in the
system, allocating job priorities, scheduling processor time
 Memory Management: allocating main memory space to each
process, protecting processes from one another, enabling different
processes to share the same memory.
 Input/output control: Channelling data to and from peripherals of
the computer
 File Management: allocating space on secondary storage media,
keep track of data stored on secondary storage, control file access
rights and privileges
 Error handling and protection: The detection and reporting of
errors and minimising their effect. Also it involves protection against
errors and against deliberate abuse of the system.

STRUCTURE OF OPERATING SYSTEM

 The program structure of an operating system is presented as a
number of modules:

ACCOUNTING

PROTECTION

RESOURCE ALLOCATION

BACKING STORE MANAGEMENT

I/O CONTROL

MEMORY MANAGEMENT

NUCLEUS
HARDWARE

NUCLEUS (KERNEL)

 This is the lowest level of an OS. It has direct interaction with the
hardware. It provides a number of services to the other modules:
 Handling of interrupts
 An interrupt is a signal generated by a source such as some input or
output device or a system software routine, which causes a break in the
execution of the current routine. Control passes to another routine in such
a way that afterwards the original routine may be resumed.
o Examples of interrupts are information transfer requests, errors
 Co-ordinates the way in which different processes can exist together.
 The OS provides the mechanism for switching the processor from one job
to another. The OS shares the processor time amongst the different tasks
in the system in a way so as to maximise processor utilisation
 Allocation of work to the processor
 In many cases more than one job exists in the system at the same time.
The OS provides the mechanism for switching from one job to another

MEMORY MANAGEMENT

 The main memory of most computers is too small to handle all programs
and data all at one time. This part of the OS allocates main memory,
normally RAM, to programs or parts of programs that need it most.
Everything else is kept on backing store.
 Virtual memory is a technique used by this layer of the OS. Virtual
memory is when the computer appears to have more main memory than
it physically has.
 Caching is a technique used to enhance
system performance. Some very fast
memory called Cache memory,
usually static RAM, is reserved to store
sections of the executing routines in
order to take advantage of the short
access time hence reducing the fetch-
execute cycle time.

I/O CONTROL

 I/O devices have different characteristics and run at different speeds.
The OS makes these problems transparent to the user. From a user‟s
point of view all devices have the same characteristics and are instructed
in the same way. Thus I/O is DEVICE DEPENDANT from a user‟s point of
view.
 The OS deals with the physical aspects (e.g. Blocks on disk) of the data
transfer leaving the programmer free to concentrate on the logical
aspects of the data structure transferred.
 A very useful output technique (usually used for printing) is SPOOLING.
Data to be output is held on a spool queue on backing store until the
output device is ready for it.

User Printer
Program Output (Output)
Output Queue Output
added to removed
queue from
queue

BACKING STORE MANAGEMENT

 The OS has the responsibility of how programs and data are stored in
secondary memory and ensure that this is being done efficiently.
 Data and programs in backing store are kept in FILES. The OS must
take care of the creation, deletion and updating of these files.
 Some files are shared; others are
private. The OS has the
responsibility of ensuring that
users respect these ACCESS
PRIVELEGES.
 Note that this layer of the OS co-
operates with memory
management during data transfer
to/from backing store.


RESOURCE ALLOCATION AND SCHEDULING

 Resources (say, processor, shared files, discs, printers) are usually
shared by a number of users. The demand for resources is usually
greater than their availability. Thus some policy must be adopted as to
which user is to be serviced – resource allocation.
 A FIRST IN FIRST OUT (FIFO) policy is simplest but this could easily
result in DEADLOCK. Deadlock is when two or more programs prevent
each other from continuing because each claims a resource that is
possessed by another. Various policies are enforced to prevent deadlock
or to recover from it once it occurs.
 Scheduling is mainly concerned with enforcing a policy whereby the
processor is used efficiently. Its task is to allocate processor time to
programs. The scheduling algorithm may be as simple as a round-robin
which deals with each user equally in turn, or as complex as a scheme of
priorities distinguishing between users and between tasks.


PROTECTION

 The OS should protect the system against errors and
system malfunction and system abuse.
 The OS should detect and diagnose errors in application
programs as soon as possible and limit their effects.
 The OS should also protect the software from system
malfunction. This is achieved by a periodic dump of certain
files into a suitable backup medium. Also power failure is
immediately recognised and essential contents of CPU are
saved on disk before the power has fallen to a level where
the system has to be shut down. This would only take a
few milliseconds.
 Protection against abuse is more difficult to deal with.
Protection mechanisms designed to prevent unauthorised
activities from succeeding are not always completely
foolproof.

ACCOUNTING

 An operating system must keep record of
the changes incurred by each user program
such as processor time, use of backing
store, and printer paper.
 Information is passed to the accounting
module from the scheduler, to establish how
much processor time a program has used.
 Most multi-user OS‟s take note of the
number of times a user logs on to/off from a
system. Some OS s keep track of all actions
of a user, by logging all his commands to a
file.

COMMUNICATION WITH
THE OPERATING SYSTEM

Human communication with the operating system may be of
two types:
 User mode of operation - between computer and user
 Users communicate with the OS in order to run application
programs (execute programs) and manage their data files
(perform housekeeping tasks).
 Supervisor mode of operation -between computer and
operator
 In supervisor mode, the computer system operator is in ultimate
control of the system and may override many functions of the OS
such as switching processor between processes, accessing
registers used by memory protection hardware, reorganising
scheduling and resource allocation.


TYPES OF USER INTERFACE

 Different types of user
interface used by
different operating CLI
systems.
C:> dir *.exe
 User interfaces may be:
 Command line interface
(CLI)
 Job-Control Language
 Graphical User Interface
(GUI)

GUI


COMMAND LINE INTERFACE (CLI)

 An interactive terminal allows the system to prompt and the user to
type a command to initiate program execution or housekeeping tasks
 e.g. C:tpuser1>copy myprog1.pas a:
 The user interface module contains a command-line interface (CLI)
which performs the actual task of identifying and executing the
command.


GRAPHICAL USER INTERFACE (GUI)

 Using a GUI, the user is presented with pictorial representations
(ICONS) of the different tasks. The user uses the GUI in conjunction
with a mouse. Hence initialising a task involves clicking the mouse on
the required task icon.
 The environment is said to be of the WIMP (Window, Icon, Mouse,
Pointer) type.
 The main advantages of GUI are:
 ease of use
 user oriented.
 The main disadvantages of GUI are:
 excessive use of main and secondary storage
 slower in executing commands because of much required interpretation
 (require a powerful processor and better graphics display)


JOB-CONTROL LANGUAGE

E.g. JCL program to
 In this type of environment, the user compile and execute a
has no direct interaction with the COBOL program :
computer system. The user prepares a
series of instructions off-line using a $JOB 123
JCL to describe to the system the $PRIORITY 2
requirements of his/her task. $COBOL
 When the user‟s job is eventually $INPUT PROG1 (DISK 1)
executed, the execution is guided by $LIST LP
the JCL-prepared description. The $IF ERROR THEN END
results are then made available at a $RUN
later time via some off-line medium say $MEMORY 250K
line printer paper. $TIME 5
 A job control language is more $FILES „PAYFILE‟
complex and powerful than a command $IF ERROR THEN DUMP
language. As from the example above,
$END
it is used to write the job description
i.e. all job control information.

MULTI-PROCESSING

 A program is a sequential set of commands which are
executed in order by a processor.
 The execution of a program on a processor is called a
process. Thus if the same program is run twice on the
same processor, it will be one program but two different
processes.
 When a machine can have more then one process running
at the same time, the system is called a multiprocessing
system.
 A system can have more than one processor, but the
number of processes running on a machine is generally
greater then the number of processors.


APPLICATION EXAMPLE

 Consider an application program running on a system with one
processor, one disk and one printer. Part of the program‟s task
involves reading some data from disk, performing some computation
on the data, saving the results and printing the result.
Printer
Disk

OS Routine

Program

t
Request data Performs Saves results Prints Program
from disk Computation to disk Results Ends


CONCURRENCY

 With reference to the example (on previous slide), a lot of
time is wasted when neither the OS nor the program are
executing and the processor is free to perform other
tasks.
 This time can be fruitfully used thanks to concurrency
where several activities can exist in parallel.
 Concurrency raises problems of:
 Switching from one activity to another.
 Protecting one activity from the effects of another.
 Synchronizing activities which are mutually dependent.
 Control over shared resources.
 Communication between two activities.


CPU SHARING IN CONCURRENT SYSTEMS

 Here several processes share the same CPU and other
common resources.

Process 1 is said to be preempted
Process P1

Process P2

Process P3

Processor
Context Switching Process 3 is said to be dispatched time


PROCESS STATES

 A process can be in one of three states:
 RUNNING (or EXECUTING),
 RUNNABLE (or READY),
 UNRUNNABLE (or BLOCKED)

Suspended dispatched Suspended

Activation Termination
READY EXECUTING

preempted

SIGNAL WAIT
resource free BLOCKED resource busy


SCHEDULING

 Scheduling involves the timing of when to introduce new processes into
the system ( Activation ) and the order in which processes should run
or be stopped ( dispatching and preemption ).

Ready Queue
Activation dispatching Execution Termination
... P3 P2 P1
on CPU

preemption
P1 
P2 
Scheduler P3 
P4 


TYPES OF SCHEDULING

 There are various types of scheduling :
 Preemptive: The running process can be interrupted at any time
to assign the processor to another process
 Non-Preemptive: A process, once started, can continue running
until its end.
 Static: scheduling is decided on fixed parameters before the
scheduling of a process occurs.
 Dynamic: Scheduling decisions are based on dynamic parameters
and can be changed during program execution.
 Real Time: There are other scheduling techniques used for real
time operating systems in which the most important factor
effecting the scheduling is the time constraint.


CRITICAL REGIONS

 When a process tries to access a shared
resource, which must be used in mutual P1 P2
exclusion, the process is said to execute a
critical region (CR).
 For example suppose that Process P1 and
Process P2 need to use the same (shared) CR
memory location at some time during their CR
execution.
 If a process tries to access a resource which is
already occupied by some other preempted
process, the scheduler blocks that process and
sends some other process in execution until the
shared resource is free. Thus a process stopped
on a shared resource is said to be blocked.


SEMAPHORES

 A hardware or software flag. In
multitasking systems, a semaphore is a
variable with a value that indicates the
status of a common resource. It's used to
lock the resource that is being used.
 A process needing the resource checks the
semaphore to determine the resource's
status and then decides how to proceed.
 For implementation reasons the lock
(WAIT) and unlock (SIGNAL) must be
atomic.
 There are various types of semaphores –
binary, readers/writers … etc.


MUTUAL EXCLUSION

 Non-shareable resources, whether peripherals, files, or data in
memory, can be protected from simultaneous access by several
processes by preventing the processes from concurrently executing
the pieces of program through which access is made.
 Exclusion can be achieved by the simple expedient of enclosing each
critical section by wait and signal operations on a single semaphore
whose initial value is 1. Thus each critical section is programmed as

wait (mutex);
critical section
signal(mutex);

where mutex is the name of a semaphore.


PROCESS STATE CONTROL

 Thus a process moves in one of the indicated directions:
Partially completed processes (preempted)

Job queue
New Termination
Processes READY C.P.U.
Process
(Activation) dispatched
Runnable
Running

Blocked
wait
signal Semaphore
Queues


PROCESSOR SCHEDULING ALGORITHMS

 Given a set of processes ( or tasks ) = { P1, P2, P3 …Pn}, a
scheduling algorithm involves a plan for assigning the CPU
to each process. Among the most popular scheduling
algorithms which exist are:
 First Come First Served (FCFS)
 Shortest Job First (SJF)
 Priority Scheduling
 Round Robin (RR)
 Multilevel Scheduling


FIRST COME FIRST SERVED

 Simplest algorithm implemented by keeping a simple FIFO
queue, with new processes inserted at the tail of the queue.
 Characteristics of this algorithm:
 Non Preemptive.
 Used mainly for batch non-interactive processes.
 Not used in modern operating system since it offers no real
concurrent processing.

FCFS Queue
Activation Dispatch Termination
P3 P2 P1 CPU


EXAMPLE FCFS

 Suppose we have 3 processes to be scheduled

Process Activation time Execution time
P1 3 4
P2 0 5
P3 2 2

Process P1

Process P2

Process P3

0 1 2 3 4 5 6 7 8 9 10 11 Processor
time

ROUND ROBIN

 This is particularly suitable for timesharing systems
 Each process is allowed execution for a certain quantum
of time Q,
 Q must be accurately chosen, in such a way that it is not too
small to be close to the process commutation time, and not to
large to leave processes not dispatched for a long time.

Activation Dispatch
P3 P1 P2 CPU Termination

Preemptive after a time quantum Q


ROUND-ROBIN


Process Activation time Execution time
P1 2 4
P2 0 5
P3 4 2

Process P1

Process P2

Process P3

0 1 2 3 4 5 6 7 8 9 10 11 Processor
time

PRIORITY SCHEDULING

 Each process is assigned a priority and the process with the highest
priority is dispatched first. Processes with the same priority are
dispatched on a FCFS bases.
 One problem in this type of scheduling is starvation, i.e. a process
with a low priority can wait to be dispatched for an indefinite long
time, due to higher priority processes.
 One method to avoid starvation is by using aging in which the
priority of a process is increased dynamically while waiting in the
queue.
FCFS Queues
Activation
P7 P4 P2
Higher Activation Termination
P6 P5 P1 CPU
Priority
Activation
P9 P8 P3


EXAMPLE PRIORITY SCHEDULING
(NON-PREEMTIVE)


Process Priority Activation time Execution time
P1 1 (highest) 2 4
P2 2 0 5
P3 3 (lowest) 4 2

Process P1

Process P2

Process P3

0 1 2 3 4 5 6 7 8 9 10 11 Processor
time

EXAMPLE PRIORITY SCHEDULING
(PREEMTIVE)


Process Priority Activation time Execution time
P1 1 (highest) 2 4
P2 2 0 5
P3 3 (lowest) 4 2

Process P1

Process P2

Process P3

0 1 2 3 4 5 6 7 8 9 10 11 Processor
time

RELOCATION

 The translator does not know where the program will be placed in
memory when it is executed
 Furthermore, a process may be swapped in and out of memory, so it
will occupy different physical locations at different times.
 Consequently, the translator cannot assign physical addresses to the
instructions and data - it must assign logical (or virtual) addresses.
 On the other hand, once programs are loaded, the processor and OS
must be able to reference the actual physical addresses at which the
code and data are stored.
 Relocation is the process of converting a logical address to a physical
address. Its purpose is to let a program be executed from different
areas of main memory at different times.


ADDRESS TYPES

 A physical address or absolute address
refers to a physical location in main
memory.
 A logical address is a reference to a 00000
memory location independent of the
physical structure or organization of b
a
memory. 03000
 Compilers produce code in which all r
memory references are expressed as 07000 PROCESS
logical addresses.
 A relative address is a kind of logical
address in which the address is expressed
relative to some known point in the
program. Usually, the first location is
address zero and all other addresses are
offsets from this address.


ADDRESS TRANSLATION

 Programs are loaded in main memory with all memory references in
relative form
 Physical addresses are calculated “on the fly” as the instructions are
executed. This process is called address translation, or address
mapping.
 For adequate performance, the translation from relative to physical
address must by done by hardware.

LOGICAL ADDRESS PHYSICAL
ADDRESS TRANSLATION ADDRESS


BASE-LIMIT REGISTER TECHNIQUE

 When a process is dispatched, a base
register (in the CPU) is loaded with the
physical start address of the process
and a bound register is loaded with 03000
the process‟s ending physical address. base
 When a relative address is r PROCESS 1
encountered, add it to the contents of
the base register to get the actual 09000
physical address. bound
 The physical address is then compared PROCESS 2
to the bound register to make sure the
process isn‟t referencing a location
outside its own address space.
 The hardware mechanism provides
translation and protection.


STRENGTHS AND WEAKNESSES OF BASE-LIMIT REGISTER
TECHNIQUE

 The base-and-limit register approach supports dynamic
relocation, as well as protection and translation, since
each time the process is dispatched the registers can be
loaded with the appropriate values.
 However, it works only if the process image is loaded into
consecutive memory locations.

Bound

Base


NON-CONTIGUOUS STORAGE

 Relaxing the assumption that process address space will
be stored contiguously will let the memory management
system utilize memory better.
 If portions of processes can be loaded in separate areas
of memory, compaction will not be necessary.


PHYSICAL ORGANIZATION

 Primary memory is fast and expensive.
 Secondary memory is slower and cheaper.
 Consequently, computer systems usually have small amounts of main
memory and large amounts of disk storage.
 The OS must be responsible for moving information between the two
levels.
 This is a vital part of memory management.

SWAP OUT

MAIN
MEMORY DISK

SWAP IN


PAGING

 Paging assumes that memory is divided into
fixed-size chunks, called page frames.
 Process address space is divided into chunks
called pages.
 Pages are the same size as frames, so a page
can fit into any free space of memory.
 Each address is composed of 2 parts, a page
number and an offset. Page numbers are
concatenated with offset to form the complete
address.
 The term virtual ( paged ) memory is used to
describe this system.


EXAMPLE OF PROCESS LOADING

 Processes A, B, and C are loaded into memory in order, and assuming
that the memory is originally empty.


EXAMPLE … CONTINUED

 Now, when B finishes or is
swapped out a new
process (D) can be loaded.
 D needs 5 pages; it would
not fit into either of the
available free chunks, but
with paging, we can
accommodate it.


VIRTUAL AND REAL ADDRESSES

 The actual address in memory ( the actual bit pattern
going in the memory chip ) is called the real address and
the program address (the one specified by the program) is
called the virtual address
 A hardware mechanism is used to translate the virtual
page number to a real page number.
 This mechanism operates only on the pages in main memory.
 If the required page is not in main memory a software program
must be executed to:
o Find the required page in secondary memory,
o Transfer this page into the main memory,
o Provide space in main memory by choosing the best out block.


PAGED ADDRESS TRANSLATION

(Notice that only the
page# field changes)


PAGE SIZE CONSIDERATIONS

 Page size is typically 512 words.
 If a smaller page is chosen
o Time taken to transfer pages from main to secondary storage is
shorter
o A large selection of pages resides in main memory
o However it necessitates a large page table.
 For Larger pages:
o Time taken is longer.
o A small page table is needed
o The unused space at the end of each page is likely to increase
(This is known as internal fragmentation )
 The number of words per page chosen is a compromise.


PAGING & ADDRESS TRANSLATION REMARKS

 Address translation is transparent to the user.
 The OS is responsible for finding enough free frames for a process
and for creating and initializing the page table.
 Protection is provided by the page table. If the referenced page
isn‟t part of the process address space, there won‟t be a PT entry.


FILES

 Disk (and tape) provide permanent storage, as opposed to
the volatile storage in main memory.
 Files are an abstract data type, used to structure data
stored on a disk.
 Virtually all applications use files for input and output data
 In addition, files hold source code, executables, documents, ….


THE FILE MANAGEMENT SYSTEM

 The file management system provides support to help
users and applications create, delete, and use files.
 Objectives:
 Provide capability to create and access files.
 Protect validity of data, as much as possible.
 Optimize performance: throughput and response time.
 Interact with multiple users and devices.
 Provide device independent interface.


FILE SYSTEM ARCHITECTURE

 File systems extend/expand the layers in the I/O stack
 At the bottom are the device drivers
 The top level provides the access methods, or the interface between
user and file system.
 Intermediate levels:
o maintain directory,
o map logical file operations into physical addresses,
o allocate disk storage,
o provide access control,
o manage physical I/O.


FILE MANAGEMENT FUNCTIONS

 User issues command (read, open, etc.) and the file
system must:
 Use a directory which describes the location of all files plus their
attributes
 On a shared system enforce user access controls
 Translate user commands into lower level commands, based on
access method

 Users access files on a byte
or record basis; I/O is done
on a block basis


FILE BLOCKING FUNCTIONS

 Blocking includes
 Allocation of files to free blocks on
disk
 Management of free storage lists
 scheduling block I/O requests
 Thus files must be divided into
blocks
 Blocks are the unit of storage
both on disk and in main-
memory buffers


FILE DIRECTORIES

 Directories provide a mapping between file names and the actual files.
 They contain lists of files and their attributes.
 name
 location (address)
 ownership
 Directories are just special files, owned by the OS.
 The structure of the directory must support at least the following set of
operations:
 search (to locate a file)
 create
 delete
 list (all or part of the directory)
 When a file is opened, directory information is usually copied from disk
into a main memory open-file table.


DIRECTORY STRUCTURE

 Single-level: a list of all files
 slow to search
 requires names to be unique across the entire system
 useful only when there are few files
 Two-level directory
 level two: personal directory for each user
 level one: master directory for entire system
 names unique only within the user directory
 Tree-structured directory
 One-level and two-level directories don‟t give the user any way to structure
files.
 Tree structured directories allow multiple hierarchical subdirectories in each
user directory (see Windows and UNIX)
 File names are unique only within a subdirectory
 Pathnames are used to locate files; as in any tree structure, search is fairly
efficient

FILE SHARING

 Most file systems allow users to share files, if the owner of
the file agrees

File sharing introduces
two issues:
 determining access rights:
who, besides the owner,
can modify, execute,
delete, etc.
 managing simultaneous
access: what if more than
one user is trying to access
a file at the same time?


ACCESS CONTROL

 Most common access rights are
 read: user can read and use the file, but can‟t change it
 write: user can modify, delete or add to the file
 execute:user can load and run the program, cannot change
or copy it
 Other access rights include:
 None
 Knowledge
 Append
 Change protections
 Delete
 Access can be granted to different classes of users:
 individuals
 groups (a set of users, such as class members)
 all (includes everybody with access to the system - e.g.,
public files) Operating Systems 73

ACCESS CONTROLS – PRIVILEGES

 A File Management System helps prevent loss, corruption
and unauthorized access to files.
 The operating system is used to identify and authenticate
users and their processes.
 The file access is authenticated through id‟s and
passwords.
 Example:
 UNIX defines three access control types:
o Read
o Write

o Execute


FILE SYSTEM RELIABILITY

 For efficiency, many file system structures (or parts of the
structures) are cached in memory.
 directory info, disk allocation table, etc.
 If the file system updates one of these tables but does not
save the change to disk, the most current information will
be lost in a crash.
 Fault Tolerance
 Describes methods of securing file content against hardware
failure.
 File backup, recovery, and transaction logging are forms or
protection against disk failure.


FILE BACKUP

 Storing files/directories to different storage device for
safekeeping purposes
 Protects against data loss, device failure, sabotage, etc.
 Can be automatic or manual
 Types of File Backup
 Full Backup:
o the FMS copies all files and directories for an entire storage
volume.
 Incremental Backup:
o only files that have been modified are archived.
 Differential Backup:
o only the changed portions of the files are archived.


TRANSACTION LOGGING

 A form of automated file backup.
 A transaction is any single change to
file contents or attributes.
 All changes are logged (when, by
whom, etc.).
 Transaction logging provides a high
degree of protection against data loss
due to program or hardware failure.
 Lost updates can be recovered from log
files.
 Imposes performance penalty (every
change requires two writes - one to file,
the other to the log).


FILE RECOVERY

 The file management
system maintains backup
logs to aid in locating
backup copies of lost or
damages files.
 The recovery utility
reconstructs as much of
the directory and storage
allocation data structures
as possible and makes a
consistency check.


MIRRORING

 A fault tolerance technique
in which all disk write
operations are made
simultaneously or
concurrently to two different
storage devices.
 Disk mirroring provides a
high degree of protection
against data loss with no
performance penalty if
implemented in hardware.


Intermediate Operating Systems

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Intermediate Operating Systems