9691 Computing
Paper 3
Section 3.1
The functions of Operating Systems
Functions of Operating Systems (OS)
 All Operating Systems (OS) have three main functions:
 Controlling and Managing Hardware
 Providing An Interface (Human-Machine and Machine-Software)
 Facilitating Application Software To Run
 Operating systems must:
 Provide and manage hardware resources
 Provide an interface between the user and the machine
 Provide an interface between application software and the
machine
 Provide security for data on the system
 Provide utility software to allow maintenance to be done
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Features of OS
1. User interface: the two main types of user interfaces
are command line and graphical user interface.
2. Device drivers (Routines which control hardware).
Example: Keyboard driver
3. Multitasking capability which enables the computer
to run more than one program at one time.
(Example: Running a word processing program, spreadsheet
program and a database program at the same time in the
computer.)
4. Spooling: directs jobs and data files to a queue on a
backing store before sending them to their intended
peripheral device.
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Features of OS (continued)
5. Security: ensures that users can keep their files
confidential
6. Scheduler: allocates job priorities and find and
resolve deadlocks using scheduling algorithms
7. Memory manager: allocates memory to jobs
and data
8. Interrupt handler: routine in the operating
system which puts interrupts in a queue until
they are processed by the processor.
9. Translators: converts source code of application
programs into object code or to machine code
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Interrupts

An interrupt is a signal from a hardware device or an
instruction from a software program indicating the processor
the need for a change in execution.

The processor processes program instructions stored in the
memory. It fetches instructions one after the other and
executes them.

If a problem occurs (anywhere in the system) while the
processor is busy processing, the processor must respond to
the problem as soon as possible in order to avoid damage to
data and/or hardware.

This requires the processor to stop whatever it is doing and
give its attention to the problem.

The device (hardware) or software, where the problem has
occurred generates a signal, called an “interrupt” to get
processor’s attention.
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This is a simple method a
computer uses to carry
out instructions.
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Types of Interrupt
• I/O interrupt
– Generated by an I/O device to signal that a job is
complete or an error has occurred.
• E.g.
– Printer is out of paper or is not connected.
• Timer interrupt
– Generated at fixed intervals.
– Allows for display refresh and to control access to
processor in multi-access or multi-programming system.
Types of Interrupt
• Hardware interrupt
– E.g.
• Power failure which indicates that the OS must close
down as safely as possible.
• Program interrupt
– Generated due to an error in a program such as
violation of memory use (trying to use part of the
memory reserved by the OS for other use) or an
attempt to execute an invalid instruction (such as
division by zero).
Types of Interrupts (Summary)
• Hardware: Generated by hardware devices. (Hardware
failure. Hardware going off-line)
• Software: Generated by software applications.
(Violation of memory space. Virus found, division by
zero)
• Timer: Generated by internal clock. (Scheduling of
processes; an event is due)
• I/O: Generated by an I/O device.
(e.g Printer out of paper; CD copying completed)
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Managing interrupts
• After execution of an
instruction, the
processor must see if an
interrupt has occurred.
• If yes, the OS services
that interrupt following
a new set of
instructions.
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What happens if the current job is more
important than the interrupt?
• The Priority of the interrupt is compared with
current job.
– If higher:
• An appropriate interrupt service routine (ISR),
depending on the interrupt's importance, is loaded and
run.
• The Interrupt is serviced by the processor.
– If lower:
• The interrupt is placed in a queue.
• The current job continues with next cycle.
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Resuming an interrupted program from
where it left off after interrupt has been
Serviced
• The state of the current job is saved.
• This is done by saving the contents of all the registers
in the processor so that the OS can use them to
service the interrupt.
• The OS can load the register contents back in the
registers to resume the job being interrupted once
the interrupt has been serviced
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What happens if several interrupts
occur at the same time?
• Place the interrupts in a queue and only allow
return to the originally interrupted program
when the queue is empty.
• However, if an interrupt is considered important
enough then an interrupt service routine (ISR) is
chosen, which will “mask” the interrupt, meaning
that any future interrupts are not considered
until the current interrupt is completed.
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What happens if some interrupts are
more important than others?
• The Interrupt is
allocated a position
in the job queue
according to
priorities.
– How this priority is
decided is called
scheduling
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Handling Interrupts - Summary
• The Current cycle is completed
• Then the priority of interrupt is compared with the
current job
• If higher:
– Contents of special registers are saved, the current job is
allocated a position in the job queue according to priorities
– Interrupt is serviced by OS.
– On completion, values of special registers from original
program are loaded and the original job is restored.
• If lower:
– Interrupt is allocated a position in the job queue according
to priorities.
– Current job continues with next cycle.
JOB SCHEDULING
• Scheduling is an OS process that starts and ends tasks
(programs), manages concurrently running processes, and
allocates system resources.
REASONS FOR SCHEDULING
 Maximize the use of the whole of the computer system
 Be fair to all users
 Provide a reasonable response time to all users, whether they
are on-line users or a batch processing user
 Prevent the system failing if it is becoming overloaded
 Make sure that the system is consistent by always giving similar
response times to similar activities from day to day
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Input Output (I/O) bound Vs Processor
bound jobs
• I/O bound jobs are those that require relatively
little processing but do need to use the
peripheral devices substantially.
– E.g. Printing wage slips for the employees of a large
company
• Processor bound jobs are those that require a
large amount of processor time and very little use
of the various peripheral devices.
– E.g. Analysing the annual, world-wide sales of the
company which has a turnover of many billions of
Shillings.
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Scheduling Example
• Job A is processor-bound and Job
B is I/O bound.
• I/O bound jobs are given more
time in order to allow them to
finish in reasonable time.
• Processor bound jobs are given
lesser time as they can do more
in less time and shouldn’t hold
the I/O bound jobs.
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STRATEGIES FOR SCHEDULING
• Scheduling of processes requires careful considerations. The
following are the most common:
 Priority: Giving some jobs priority over others.
 I/O or Processor Bound: Deciding which job is processor bound and
which is I/O bound.
 Type of Job: Batch processing, on-line and real-time jobs all require
different response times.
 Resource Requirements: The amount of time needed to complete the
job, the memory required, I/O and processor time.
 Resources Used So Far: The amount of processor time used so far,
how much I/O used so far.
 Waiting Time: The time the job has been waiting to use the system.
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Process handling
• A job can be in any of the
following states.
NOTE
– Ready [Ready to start]
– Running [on the system]
– Blocked [Waiting for a
peripheral]
• A job can only enter the running state from the ready state.
• The ready and blocked states are queues that may hold several jobs.
• On a standard single processor computer only one job can be in the
running state.
• All jobs entering the system normally enter via the ready state and
(normally) only leave the system from the running state.
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Schedulers
• High Level Scheduler (HLS) is responsible for
placing a job in the ready queue. The HLS makes
sure that the system is not over loaded.
• Mid-Level Scheduler (MLS) is responsible for
swapping jobs between main and secondary
memory.
• Low-Level Scheduler (LLS) is responsible for
Moving jobs in and out of the ready state.
– The LLS decides the order in which jobs are to be
placed in the running state.
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Scheduling policies
• There are many policies that may be used to
do scheduling, but they can all be placed in
one of two classes. These are pre-emptive and
non-pre-emptive policies.
• Pre-emptive policy: allows the LLS to put or
remove jobs from the RUNNING queue as and
when needed.
• Non Pre-emptive policy allows jobs to run
until they no longer need the processor.
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Common scheduling algorithms
• First come first Served (FCFS): Simply means that the first job to enter the
ready queue is the first to enter the running state. This favors long jobs. (Non
pre-emptive by design)
• Shortest Job first (SJF): Simply means sort jobs in the ready queue in ascending
order of times expected to be needed by each job. New jobs are added to the
queue in such a way as to preserve this order. (Non pre-emptive)
• Round Robin (RR): This gives each job a maximum length of processor time
(called a time slice) after which the job is put at the back of the ready queue
and the job at the front of the queue is given use of the processor. If a job is
completed before the maximum time is up it leaves the system.
• Shortest Remaining Time (SRT): The ready queue is sorted on the amount of
expected time still required by a job. This scheme favours short jobs even more
than SJF. Also there is a danger of long jobs being prevented from running.
• Priority Queues (PQ): Priority Queues involve queues of different priorities.
Jobs in higher priority queues are executed first.
• Multi-level Feedback queues (MFQ): Involves several queues of different
priorities with jobs migrating downwards.
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Other methods of allocating priorities:
All the policies discussed on previous slides are methods of
allocating priorities.
Here are some others:
• Amount of time already waited.
• Amount of processor time already given.
• Amount of peripheral time.
– Compares I/O jobs and asks “How much of a I/O job are
you?”
• Necessary response time.
– Compares real-time / on-line jobs and asks “How much of
a real-time / on-line jobs are you?” or “OK, you are a realtime / on-line job but how quickly do I need to respond to
you?”
Other methods of allocating priorities:
•
Importance of Job / Type of Job
–
Safety critical jobs will be given very high priority, on-line and real time
applications will also have to have high priorities.
• E.g.
1. A computer monitoring the temperature and pressure in a chemical
process whilst analysing results of readings taken over a period of
time must give the high priority to the control program.
– If the temperature or pressure goes out of a pre-defined range,
the control program must take over immediately.
2. A bank's computer is printing bank statements over night and
someone wishes to use a cash point, the cash point job must take
priority.
–
Also remember I/O bound jobs will also get priority as stated earlier.
–
Can be Pre-Emptive or Non-Pre-Emptive depending on if the queue is
checked after each cycle or not.
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Memory Management
• Memory Management is the process of managing
jobs and their data in the memory.
 When a job needs to be processed, it is stored in
the main memory along with its data.
 The job’s code and data must be managed and
protected from actions of other other.
 In most of the modern OS, more than one job can
be loaded into the memory at one time.
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