← IGCSE Computer Science Paper 1

Parity block check

Checks a block of bytes in two directions: a parity bit on every byte (row) and a parity byte for the columns, so the wrong row and column pinpoint the error.

Part of: IGCSE Computer Science 0478 — 2.2 Methods of Error Detection — Module 18

Understanding Parity block check in IGCSE Computer Science Paper 1

What it does: Three 1s plus a parity bit of 1 gives four 1s, which is even, so this row passes the even parity check. A parity bit checks each byte (row) and the parity byte checks each column, so the block is checked in two directions. The failing row and failing column meet at one bit. That bit is the one that was changed, so the error is pinpointed.

Common mistakes: This is an even parity block. The data 0111000 has three 1s, so the parity bit must be 1 to make the total even. The row should be 01110001. Row1 now has three 1s, which is odd, so it breaks even parity. The correct row is 01101010, which has four 1s.

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This module's key terms are also in a printable crossword (Modules 155–164) and word search (Modules 142–164).

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Learning
Duo
In how many directions does a parity block check test the data?
Two: across the rows and down the columnsOne: across the rows only
Matches
Block check = parity bit on each row only
↓
Without a parity byte for the columns, the block is only checked in one direction. The row is found but not the column, so the wrong bit cannot be pinpointed.
Applying
Meanings
Term
parity block check
Meaning
Checks a block of bytes in two directions: a parity bit on every byte (row) and a parity byte for the columns, so the wrong row and column pinpoint the error.
Practice
EVEN PARITY BLOCK
Testing
Gaps
Row 1: 1011001
Check Answer
Spots
2Row2: 11001100
3Row3: 01110000

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Quick Questions

1. In how many directions does a parity block check test the data?
A. One: across the rows only
B. Two: across the rows and down the columns ✓
A parity bit checks each byte (row) and the parity byte checks each column, so the block is checked in two directions.
2. What does the parity byte in a parity block hold?
A. One parity bit for each column of the block ✓
B. A copy of the first byte in the block
The parity byte is an extra row at the bottom. Each of its bits is the parity bit for the column above it.
3. Even parity block: the byte 10110010 is one row. Which parity bit does it need?
A. 0, because it already has four 1s ✓
B. 1, because it already has four 1s
Even parity means the total number of 1s, including the parity bit, must be even. Four 1s is already even, so the parity bit is 0.
4. In an even parity block, a column has 1, 1, 0, 1 in its data bytes. What is that column's bit in the parity byte?
A. 0, because the column already has an even number of 1s
B. 1, to make the number of 1s even ✓
The column has three 1s, which is odd. A parity bit of 1 makes four 1s, which is even.
5. After transmission, row 3 and column 5 both fail the even parity check. Where is the error?
A. At the single bit where row 3 and column 5 cross ✓
B. In every bit of row 3 and every bit of column 5
The failing row and failing column meet at one bit. That bit is the one that was changed, so the error is pinpointed.

Match the Pairs

Row 3 fails, column 5 fails → The error is in the bit where row 3 and column 5 cross, so that bit can be found and flipped
1011001 0 → An even parity row: data has four 1s, so a parity bit of 0 keeps the total even
Even parity row: the data 1101000 has three 1s, so the parity bit must be 1 to make the total even → 11010001
Parity byte: a column holding 1, 0, 1, 1 has three 1s, so its bit in the parity byte is 1 under even parity → Column 1,0,1,1 -> parity 1
Block check = parity bit on each row only → Without a parity byte for the columns, the block is only checked in one direction. The row is found but not the column, so the wrong bit cannot be pinpointed.
Even parity: 10110011 passes → The byte has five 1s, which is odd, so it fails an even parity check. Even parity needs an even number of 1s.

Key Term

parity block check
Checks a block of bytes in two directions: a parity bit on every byte (row) and a parity byte for the columns, so the wrong row and column pinpoint the error.

Fill in the Blanks

1. Row 1: 1011001 0
Hint: Type the even parity bit for this row of seven data bits.
2. Column bits 1,1,0,1 give parity bit 1
Hint: Count the 1s in this column and type the bit that makes the total even.
3. Parity block: a bit per row, a byte for columns
Hint: Type the direction that the parity byte checks, as opposed to rows.
4. Row 3 and column 5 fail: flip bit (3, 5)
Hint: Type the column number that meets the failed row at the error.
5. 0110 | 0 1011 | 1 1101 | 1
Hint: Type the even parity bit for the middle row of the block.

Spot the Error

Question 1
A Row1: 10100011
B Row2: 11001100
C Row3: 01110000
D Par: 00011110
Line C — This is an even parity block. The data 0111000 has three 1s, so the parity bit must be 1 to make the total even. The row should be 01110001.
Question 2
A Even parity block
B Row1: 01001010
C Row2: 10011100
D Row3: 11100111
E Par: 00010001
Line B — Row1 now has three 1s, which is odd, so it breaks even parity. The correct row is 01101010, which has four 1s.
Question 3
A Row1: 11110000
B Row2: 00011110
C Row3: 10000111
D Par: 01101011
Line D — The parity byte must make every column even. Column 7 has two 1s (rows 2 and 3), so its parity bit must be 0. The byte should be 01101001.
Question 4
A Row1: 00001111
B Row2: 10111010
C Row3: 01100110
D Row4: 11111100
E Par: 00111111
Line B — A bit in Row2 has changed, so the row now has five 1s (odd) and fails even parity. The row should be 10101010 with four 1s.
Question 5
A Receive the block of bytes
B Check each row's parity bit
C Skip the parity byte columns
D Error is where they cross
Line C — A parity block check works in two directions. The columns must be checked as well as the rows, otherwise the column of the error cannot be found. The line should say to check the parity byte columns.

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