Unit 6 Software T3 Interrupts

Objectives

• Understand how hardware, firmware and an operating

system are required to run applications

•Understand the role of the bootloader

• Describe the role and operation of interrupts including:

•How an interrupt is generated

•How it is handled using an interrupt service routine

•What happens as a result of the interrupts

• Be able to describe examples of software and

hardware interrupts

Interrupts

Unit 6 Software

Starter

• ROM on a computer stores programs that cannot be

changed as they are stored on Read Only Memory

• Are these programs software or hardware?

• Discuss your thoughts with a partner

Open Ended

ROM on a computer stores programs that cannot be changed as they are stored on Read Only Memory

Are these programs software or hardware?

Interrupts

Unit 6 Software

Firmware

• Hardware is named after the fact it is physically hard

• The term is used for all physical components of a computer or

peripherals

• Software isn’t a physical object, so it is given the opposite

name (soft rather than hard)

• Firmware (between hard and soft) contains

programs that are stored permanently in hardware

• The BIOS stored on a computer

is stored on a flash ROM

• It can only be updated by flashing

the ROM

• Other peripherals may have their own firmware

Interrupts

Unit 6 Software

Bootloader

• When you first turn on a computer it starts running

firmware stored in the ROM

• These instructions then load the

first instructions from the hard disk
telling the computer what tasks
to do

• The part of the hard disk that stores

these instructions is the boot sector

• Turning on a computer is

known as ‘booting’ after
“pulling yourself up by
your bootstraps”

Reorder

Reorder the following

Bootloader

• When you first turn on a computer it starts running firmware stored in

• the ROM. These instructions then load the first instructions from the hard

• disk telling the computer what tasks to do. The part of the hard disk that stores

• these instructions is the boot sector. Turning on a computer is known

• as ‘booting’ after “pulling yourself up by your bootstraps”.

1

2

3

4

5

• 1
• 2
• 3
• 4
• 5

Interrupts

Unit 6 Software

Running applications

• Applications run on the computer’s

operating system

• If they need to access hardware, they do

this via ‘calls’ to the operating system or
device drivers

• The operating system itself runs on

top of the firmware

• The bootloader and other firmware runs

on the physical hardware itself

Applications

Operating system

Firmware

Hardware

Match

Match the following

Applications run on the computer’s operating system

If they need to access hardware, they do this via ‘calls’ to the operating system or device drivers

• Top Level

• Second Level

• Third level

• Bottom Level

Applications

Operating system

Firmware

Hardware

• Applications

• Operating system

• Firmware

• Hardware

Interrupts

Unit 6 Software

Interrupts

• When you need to get the

attention of someone who is busy
you need to interrupt them

• In the same way, peripheral devices

also need to get the attention of the CPU

• They do this by sending an interrupt

which leads to the relevant
interrupt service routine (ISR) being run

• What happens to the current process

that is running on the processor?

Interrupts

Unit 6 Software

Interrupts

• The current process will need to pause, but it will

need to be restarted later

• The value stored in the program counter (PC) is saved

in a location in RAM

• No more instructions of the following task are performed (they

will be later)

Multiple Select

Peripheral devices also need to get the attention of the CPU

They do this by sending an interrupt which leads to the relevant interrupt service routine (ISR) being run

What happens to the current process that is running on the processor?

[more than 1]

• Processor for hardware devices send interrupt signals to the CPU

• No more instructions of the following task are performed (they will be later)

• The value stored in the program counter (PC) is saved in a location in RAM

• The current process will need to pause, but it will need to be restarted later

• ROM on a computer stores programs that cannot be changed as they are stored on Read Only Memory

Interrupts

Unit 6 Software

Interrupt Service Routine (ISR)

• Once the CPU has been interrupted, it needs to

process the interrupt

• It runs the interrupts program code known as an Interrupt

Service Routine (ISR)

• This is also known as an interrupt handler

• The ISR needs to execute as quickly as possible as it has

interrupted the CPU from the task it was working on

• For instance, if a key is pressed, the ISR may save the key

that was pressed. Later, a word-processor task can process it
further

Interrupt service

routine (ISR)
Interrupt
generated

Processor returns to

previous task

Multiple Select

Which of these is correct for the Interrupt Service Routine (ISR)

Once the CPU has been interrupted, it needs to process the interrupt

• It runs the interrupts program code known as an Interrupt Service Routine (ISR

• This is also known as an Harrington interrupt handler

• Interrupt generated -> Interrupt service routine (ISR) -> Processor returns to previous task

• For instance, if a key is pressed, the ISR may save the key that was pressed. Later, a word-processor task can process it further

• The ISR needs to execute as slowly as possible as it has interrupted the CPU from the task it was working on

Interrupts

Unit 6 Software

Hardware interrupts

• Hardware devices send interrupt signals to the CPU

• The interrupts are sent via Interrupt Request Lines (IRQ)

• For instance, if a key on a keyboard is pressed, an interrupt

will be sent (actually an interrupt is sent when the key is
pressed and another is sent when it is released)

• When a mouse is moved, constant

interrupts are sent to the
computer so that it
is able to update
the location of
the pointer

Reorder

Reorder the following about Hardware interrupts

• Hardware devices send interrupt signals to the CPU. The interrupts are sent

• via Interrupt Request Lines (IRQ). For instance, if a key on a keyboard is pressed,

• an interrupt will be sent (actually an interrupt is sent when the key is pressed

• and another is sent when it is released). When a mouse is moved, constant interrupts

• are sent to the computer so that it is able to update the location of the pointer.

1

2

3

4

5

• 1
• 2
• 3
• 4
• 5

Interrupts

Unit 6 Software

Software interrupts

• Software interrupts are generated by programs that

are running

• For instance, if a program tries to divide by zero, a ‘division by

zero’ interrupt will need
to be handled

• In this case, an

exception is
handled by the OS

Interrupts

Unit 6 Software

Software interrupts

• Software interrupts deal with faults such as:

• Page faults – this is when the data isn’t in RAM, but in

virtual memory and needs to be loaded back into RAM

• Segmentation faults – this is when the one program tries to

make an illegal memory location, such as the memory used
by another program

• When a fault occurs

an interrupt service
routine is called
which handles
the error

Multiple Select

Software interrupts deal with:

• Segmentation faults – this is when the one program tries to make an illegal memory location, such as the memory used by another program

• exceptions caused by programs executing invalid maths

• Page faults – this is when the data isn’t in RAM, but in virtual memory and needs to be loaded back into RAM

• If a key is pressed, the ISR may save the key that was pressed. Later, a word-processor task can process it further

• When a mouse is moved, constant interrupts are sent to the computer

Interrupts

Unit 6 Software

Plenary

• Take turns to explain to a partner each of the

following terms

Bootloader

Firmware

Interrupt

Interrupt service

routine (ISR)
Software interrupt

Hardware interrupt

Multiple Choice

What is this describing?

the first instructions a computer runs that tell it where to find the boot sector on the hard drive. The bootloader is stored in the ROM’s firmware

• Hardware interrupt

• Interrupt

• Firmware

• Bootloader

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Interrupts

Unit 6 Software

Multiple Choice

What is this describing?

programs stored in ROM

• Hardware interrupt

• Interrupt

• Firmware

• Bootloader

Multiple Choice

What is this describing?

a short set of instructions that is executed when an interrupt occurs – also known as an interrupt handler

• Hardware interrupt

• Interrupt

• Software interrupt

• Interrupt service routine

Multiple Choice

What is this describing?

interrupts generated by software, such as an attempted division by zero or an attempt to access a memory location currently being used by another program

• Hardware interrupt

• Interrupt

• Software interrupt

• Interrupt service routine

Multiple Choice

What is this describing?

A signal sent to a processor causing it to pause the current task and handle the interrupt

• Hardware interrupt

• Interrupt

• Software interrupt

• Firmware

Multiple Choice

What is this describing?

interrupts generated by hardware, such as a key press or mouse movement

• Hardware interrupt

• Interrupt

• Software interrupt

• Firmware

Interrupts

Unit 6 Software

Worksheet 3

• Complete Task 1 on Worksheet 3