← IGCSE Computer Science Paper 1

Parity bit

One extra bit added to a group of bits (such as 7 data bits) so the total number of 1s follows a rule, even or odd, agreed by sender and receiver. It is used to detect errors.

Part of: IGCSE Computer Science 0478 — 2.2 Methods of Error Detection — Module 11, ACARA Digital Technologies Years 7–8 — Wired and Wireless Networks — Module 62, ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 63, WA Computer Science ATAR Year 11 — Network Communications — Module 77

Understanding Parity bit in IGCSE Computer Science Paper 1

What it does: The data 1100101 has four 1s, which is already even, so the parity bit is 0 and the total stays at four 1s. Both devices must agree the rule beforehand. Otherwise the receiver cannot tell whether the byte is correct. The receiver counts the 1s. If the total breaks the agreed rule, it knows an error has happened. A parity bit does not speed up transmission. A parity bit is the one extra bit joined to the 7 data bits to make an 8-bit byte. The parity bit depends on how many 1s are in the data, so you count the 1s first.

Common mistakes: The counter for the number of 1s must start at 0. Starting at 1 means every count is one too high, so the parity bit would be wrong. It should be ones = 0. The sender and receiver must agree the same rule beforehand. If the sender uses even parity, the receiver must check for even parity too. It should be receiver rule = even.

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Learning

Duo

What is a parity bit?

A bit that stores the ASCII code of the characterOne extra bit added to a group of bits so the total number of 1s follows an agreed rule

Matches

Parity bit added to 7 data bits makes a 9-bit byte

↓

7 data bits plus 1 parity bit make 8 bits, not 9. The parity bit is only one extra bit.

Applying

Meanings

Term

parity bit

Meaning

One extra bit added to a group of bits (such as 7 data bits) so the total number of 1s follows a rule, even or odd, agreed by sender and receiver. It is used to detect errors.

Practice

// 7 data bits: 1011001

Testing

Gaps

Data bits 1011011 contain ones

Check Answer

Spots

1ones = 1

2FOR each bit in data

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Quick Questions

1. What is a parity bit?

A. One extra bit added to a group of bits so the total number of 1s follows an agreed rule ✓

B. A bit that stores the ASCII code of the character

A parity bit is one extra bit added to the data bits. It makes the total number of 1s follow a rule, either even or odd.

2. A byte holds 7 data bits plus a parity bit. How many bits are there in total?

A. 8 bits ✓

B. 7 bits

7 data bits + 1 parity bit = 8 bits, which is one byte. The parity bit takes the place of the 8th bit.

3. Which statement about the rule is correct?

A. The receiver works out which rule the sender used after the data arrives

B. The sender and receiver agree the parity rule (even or odd) before the data is sent ✓

Both devices must agree the rule beforehand. Otherwise the receiver cannot tell whether the byte is correct.

4. What does the parity bit make the total number of 1s in the byte follow?

A. A random pattern chosen by the receiver

B. An agreed rule: always even, or always odd ✓

The total number of 1s, including the parity bit, must match the agreed rule (even or odd).

5. Using even parity, the 7 data bits are 1011000. What should the parity bit be?

A. 1, because there are three 1s and one more makes four ✓

B. 0, because there are three 1s and that is already even

1011000 has three 1s, which is odd. Adding a parity bit of 1 gives four 1s, which is even.

Match the Pairs

1010001 + 1 → 10100011 → Seven data bits with a parity bit added to the end to make the byte; here the parity bit is 1

0110100 → three 1s → Counting the 1s in the data bits, which is the first step in deciding the parity bit

With an even parity rule, the parity bit makes the total number of 1s even → 1100101 + 0 → 11001010

With an odd parity rule, the parity bit makes the total number of 1s odd → 1100101 + 1 → 11001011

Parity bit added to 7 data bits makes a 9-bit byte → 7 data bits plus 1 parity bit make 8 bits, not 9. The parity bit is only one extra bit.

The sender picks even parity and the receiver picks odd parity → The sender and receiver must agree on the same rule beforehand. Otherwise good data would look like errors.

Key Term

parity bit

One extra bit added to a group of bits (such as 7 data bits) so the total number of 1s follows a rule, even or odd, agreed by sender and receiver. It is used to detect errors.

Fill in the Blanks

1. Data bits 1011011 contain 5 ones

Hint: Count how many 1s are in the seven data bits.

2. 7 data bits + 1 parity bit = 8 bits

Hint: Type the total number of bits sent for each character.

3. IF ones MOD 2 = 1 THEN parity ← 1 ELSE parity ← 0

Hint: Type the parity bit value that makes the total number of 1s even when the count so far is odd.

4. Data 1101100 with even parity: parity = 0

Hint: Count the 1s, then decide whether any extra 1 is needed to keep the total even.

5. IF ones MOD 2 = 0 THEN OUTPUT "OK" ELSE OUTPUT "Error"

Hint: Type the remainder you expect when the receiver counts 1s under the agreed even rule.

Spot the Error

Question 1

A ones = 1

B FOR each bit in data

C IF bit = 1 THEN

D ones = ones + 1

E ENDIF

F NEXT

Line A — The counter for the number of 1s must start at 0. Starting at 1 means every count is one too high, so the parity bit would be wrong. It should be ones = 0.

Question 2

A sender rule = even

B data = 1010100

C receiver rule = odd

D parity = 1

Line C — The sender and receiver must agree the same rule beforehand. If the sender uses even parity, the receiver must check for even parity too. It should be receiver rule = even.

Question 3

A data = 0110100

B ones = 4

C rule = odd

D parity = 0

Line B — 0110100 contains three 1s, not four. The count of 1s must be right before the parity bit is chosen. It should be ones = 3.

Question 4

A data = 0000001

B ones = 1

C rule = even

D parity = 0

Line D — There is one 1 in the data and the rule is even. The parity bit must be 1 to make the total two 1s. With parity = 0 the total stays odd. It should be parity = 1.

Question 5

A ones = 5

B parity = ones DIV 2

C total = ones + parity

D rule: total is even

Line B — For even parity the parity bit is the remainder when the number of 1s is divided by 2, so MOD 2. DIV 2 gives the wrong value (2 here). It should be parity = ones MOD 2.

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