Software interrupts

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Quick Questions

Shade in the A or B circle for each question.

1. A program tries to calculate 10 / 0. What type of interrupt is generated?

A Software interrupt

B Hardware interrupt

2. Which one is a software interrupt?

A Two processes try to access the same memory location

B A user presses a key on the keyboard

3. Who or what generates a software interrupt?

A A peripheral device such as a mouse

B A program or the processor itself

4. A program contains an invalid instruction that the CPU cannot decode. What happens?

A A software interrupt is generated so the error can be handled

B Nothing happens and the CPU skips it silently

5. A program needs the operating system to save a file for it. How does it ask?

A It sends a hardware interrupt through the file's storage device

B It generates a software interrupt to request an OS service

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1result = 10 / 0		AA key press is a hardware interrupt, because it comes from a hardware device. A software interrupt comes from a program or the processor.
2open("notes.txt")		BProcess A writes to address 5000 while Process B writes to address 5000
3Two processes trying to use the same memory location at once cause a software interrupt		CCPU fetches the instruction 11111111 (not in the instruction set)
4The processor raises a software interrupt when it meets an instruction it cannot run		DA running program divides by zero, so a software interrupt is generated
5A software interrupt is sent by a keyboard when a key is pressed		EA program asks the operating system for a service, which causes a software interrupt
6A software interrupt means the program keeps running and the error is ignored		FA software interrupt makes the CPU pause the process so the error or request can be handled first.

Definitions

software interrupt

Fill in the Blanks

Complete each line of code by filling in the missing parts.

result ← 10 / _ // CPU sends a software interrupt

IF divisor = 0 THEN // CPU sends a ________ interrupt ENDIF

CALL PrintFile(report) // Program requests a service from the __

// Process A writes to address 500 // Process B writes to address 500 // Same ______ location causes an interrupt

instruction ← "ZZZ" // The CPU cannot run an _______ instruction

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A INPUT divisor

B total ← 100

C IF divisor > 0 THEN

D SEND interrupt TO CPU

E ENDIF

Question 2

A FUNCTION safeDivide(a, b)

B IF b = 1 THEN

C RAISE DivideByZero

D ENDIF

E RETURN a / b

F ENDFUNCTION

Question 3

A PROCESS A writes address 500

B PROCESS B writes address 900

C CPU detects memory conflict

D CPU generates software interrupt

Question 4

A instruction ← FETCH(PC)

B opcode ← DECODE(instruction)

C IF opcode NOT valid THEN

D GENERATE hardware interrupt

E ENDIF

Question 5

A program asks OS to save a file

B CPU pauses the program

C ISR deletes the program

D program resumes afterwards

Answer Key

Software interrupts

Section A: Quick Questions

A — Software interrupt

A — Two processes try to access the same memory location

B — A program or the processor itself

A — A software interrupt is generated so the error can be handled

B — It generates a software interrupt to request an OS service

Section B: Match the Pairs

result = 10 / 0 → A running program divides by zero, so a software interrupt is generated

open("notes.txt") → A program asks the operating system for a service, which causes a software interrupt

Two processes trying to use the same memory location at once cause a software interrupt → Process A writes to address 5000 while Process B writes to address 5000

The processor raises a software interrupt when it meets an instruction it cannot run → CPU fetches the instruction 11111111 (not in the instruction set)

A software interrupt is sent by a keyboard when a key is pressed → A key press is a hardware interrupt, because it comes from a hardware device. A software interrupt comes from a program or the processor.

A software interrupt means the program keeps running and the error is ignored → A software interrupt makes the CPU pause the process so the error or request can be handled first.

Section C: Definitions

software interrupt — A signal sent to the CPU by a program or the processor itself when a process hits an error (e.g. divide by zero, invalid instruction) or needs an OS service, so the CPU pauses it.

Section D: Fill in the Blanks

result ← 10 / 0
// CPU sends a software interrupt

IF divisor = 0 THEN
   // CPU sends a software interrupt
ENDIF

CALL PrintFile(report)
// Program requests a service from the OS

// Process A writes to address 500
// Process B writes to address 500
// Same memory location causes an interrupt

instruction ← "ZZZ"
// The CPU cannot run an invalid instruction

Section E: Spot the Error

C — A divide by zero error happens when the divisor equals 0, so the check must be IF divisor = 0 THEN. Testing for a value greater than 0 would raise the software interrupt for valid divisors.

B — Dividing by 1 is fine. The error (and the software interrupt) happens when the divisor is 0, so the line should be IF b = 0 THEN.

B — The software interrupt is caused by two processes trying to use the same memory location. If B writes to address 900 there is no conflict. It should be address 500.

D — An invalid instruction is spotted by the processor itself while running a program, so it generates a software interrupt, not a hardware interrupt from a device.

C — When a program requests an OS service with a software interrupt, the ISR must carry out that service (saving the file). It should not delete the program.