← IGCSE Computer Science Paper 1

Logical right shift

Moves every bit one place right. The bit at the right end is lost and a 0 fills the empty left end. For a positive integer this halves the value (rounded down).

Part of: IGCSE Computer Science 0478 — 1.1 Number Systems — Module 25

Understanding Logical right shift in IGCSE Computer Science Paper 1

What it does: The 1 moves one place right and a 0 fills the empty left end, giving 01000000. 00001001 is 9. After the shift it is 00000100, which is 4, so 9 halved and rounded down. The lost 1 on the right explains the rounding. The final 1 is lost and a 0 comes in on the left, so the result is 00001011. A 1 never enters on the left in a logical shift. One bit leaves on the right but a 0 enters on the left, so the register still holds 8 bits. Bits shifted off the end are discarded. Only the 8 bits remaining in the register are kept.

Common mistakes: 00101100 is 32 + 8 + 4 = 44, not 42. The start value must be 44 so that shifting right gives 22 (half of 44). In a logical right shift the empty place on the left is always filled with 0, not 1. It should read 'Fill on left: 0'.

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This module's key terms are also in a printable crossword (Modules 18–27) and word search (Modules 3–27).

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Learning

Duo

Logical right shift of 8-bit 11001100 by one place gives:

0110011010011001

Matches

00110100 >> 1 = 10011010

↓

A 1 was put in the left end. A logical right shift always fills the empty left end with 0, so the answer should be 00011010.

Applying

Meanings

Term

logical right shift

Meaning

Moves every bit one place right. The bit at the right end is lost and a 0 fills the empty left end. For a positive integer this halves the value (rounded down).

Practice

Before: 00101101

Testing

Gaps

After:

Check Answer

Spots

1Start: 00101100 (42)

2Shift right 1 place

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Quick Questions

1. Logical right shift of 8-bit 11001100 by one place gives:

A. 01100110 ✓

B. 10011001

Every bit moves one place right, the right-hand 0 is lost and a 0 enters on the left: 01100110.

2. What fills the empty space on the left after a logical right shift?

A. A copy of the bit that fell off

B. A 0 ✓

In a logical right shift, the empty left end is always filled with 0.

3. What happens to the bit that falls off the right end during a logical right shift?

A. It is lost ✓

B. It wraps round to the left end

A logical shift does not wrap bits around. The bit that leaves the right end is lost.

4. Logical right shift of 00010111 by one place gives:

A. 10001011

B. 00001011 ✓

The final 1 is lost and a 0 comes in on the left, so the result is 00001011. A 1 never enters on the left in a logical shift.

5. Which statement describes a logical right shift of one place on a positive 8-bit integer?

A. Its value is halved, rounded down ✓

B. Its value is doubled

Each bit moves to a place worth half as much, so the value halves. Any 1 lost from the units place means the result is rounded down.

Match the Pairs

00101100 >> 1 = 00010110 → A logical right shift of one place on an 8-bit value: every bit moves right and a 0 enters on the left.

00010111 >> 1 = 00001011 → The right-most 1 is lost off the end, and a 0 is added at the left.

Zeros come in on the left: shifting 10000000 right by one place gives a leading 0 at the left → 10000000 >> 1 = 01000000

Shifting a positive 8-bit integer right by one place halves its value, rounded down → 00001001 >> 1 = 00000100

00110100 >> 1 = 10011010 → A 1 was put in the left end. A logical right shift always fills the empty left end with 0, so the answer should be 00011010.

00001010 >> 1 = 00000101 and the 0 on the right is stored in a spare box → The bit that falls off the right end is lost and not stored or kept anywhere.

Key Term

logical right shift

Moves every bit one place right. The bit at the right end is lost and a 0 fills the empty left end. For a positive integer this halves the value (rounded down).

Fill in the Blanks

1. Before: 00001100 Logical right shift by 1 place After: 00000110

Hint: Type the 8-bit result after every bit moves one place right and a 0 enters on the left.

2. Before: 10110000 Logical right shift by 1 place After: 00011000

Hint: Type the bit that fills the empty left end after a logical right shift.

3. Before: 00001101 After: 00000110 Bit lost off the right end: 1

Hint: Type the bit that fell off the right end and was lost.

4. 00001100 is 12 in denary After one right shift: 00000110 New denary value: 6

Hint: Type the denary value, which is half of the original.

5. A logical right shift halves a positive integer, rounding down.

Hint: Type the direction of the shift that divides the value by 2.

Spot the Error

Question 1

A Start: 00101100 (42)

B Shift right 1 place

C Bit lost: 0

D Result: 00010110 (22)

Line A — 00101100 is 32 + 8 + 4 = 44, not 42. The start value must be 44 so that shifting right gives 22 (half of 44).

Question 2

A Start: 00001101 (13)

B Logical right shift by 1

C Bit lost: 1

D Fill on left: 1

E Result: 00000110 (6)

Line D — In a logical right shift the empty place on the left is always filled with 0, not 1. It should read 'Fill on left: 0'.

Question 3

A Start: 10000000 (128)

B Shift left 1 place

C New left bit: 0

D Result: 01000000 (64)

Line B — The question is about a right shift. The bits move one place right, so the line should be 'Shift right 1 place'. A left shift would give 00000000 and lose the 1.

Question 4

A Start: 00000111 (7)

B Shift right 1 place

C Result: 00001110 (14)

D Bit lost: 1

Line C — 00001110 is what a left shift gives (it doubles the value). A right shift moves the bits right to give 00000011, which is 3.

Question 5

A Start: 00110100 (52)

B Shift right 2 places

C Bits lost: 00

D Result: 00001101 (26)

Line D — 00001101 is 8 + 4 + 1 = 13. Shifting right 2 places divides 52 by 4, which gives 13, not 26. 26 would be a shift of only 1 place.

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