← IGCSE Computer Science Paper 1

Logical left shift

Moves every bit one place left. The leftmost bit falls off and is lost, and a 0 fills the empty place on the right. In 8 bits, 00000101 becomes 00001010.

Part of: IGCSE Computer Science 0478 — 1.1 Number Systems — Module 24

Understanding Logical left shift in IGCSE Computer Science Paper 1

What it does: The bits move left and a 0 fills the right-hand end, so 00010111 becomes 00101110 and ends in 0. The leading 1 of 10110001 falls off the left end and is lost, leaving 0110001 with a 0 added on the right. Zeros always come in on the right during a logical left shift. Left shift means each bit moves one place towards the left end. The leftmost 1 is lost and a 0 fills the right end, giving 10000110, not 10000111.

Common mistakes: An 8-bit register holds exactly 8 bits. 0000101 has only 7 bits. The register should be 00000101. << is the left shift operator. >> shifts right. The line should be x = x << 1.

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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

After a logical left shift, what fills the empty place on the right?

A 0A 1

Matches

00000101 << 1 = 10000010

↓

This is a right shift result. In a left shift the bits move left and a 0 fills the right end, so the answer should be 00001010.

Applying

Meanings

Term

logical left shift

Meaning

Moves every bit one place left. The leftmost bit falls off and is lost, and a 0 fills the empty place on the right. In 8 bits, 00000101 becomes 00001010.

Practice

Before: 11000101

Testing

Gaps

Logical shift: 00000101 -> 00001010

Check Answer

Spots

1Register: 0000101

2Shift left by 1 place

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Quick Questions

1. After a logical left shift, what fills the empty place on the right?

A. A 1

B. A 0 ✓

Zeros always come in on the right during a logical left shift.

2. Which direction does every bit move in a logical left shift?

A. One place to the right

B. One place to the left ✓

Left shift means each bit moves one place towards the left end.

3. What happens to the leftmost bit when you do a logical left shift?

A. It falls off the end and is lost ✓

B. It moves round to the right end

Logical shifts do not wrap bits around. The bit that falls off the left end is lost.

4. What is 00000101 after one logical left shift in 8 bits?

A. 00001010 ✓

B. 00000010

Every bit moves one place left and a 0 enters on the right: 00000101 becomes 00001010.

5. What is 11000011 after one logical left shift in 8 bits?

A. 10000110 ✓

B. 10000111

The leftmost 1 is lost and a 0 fills the right end, giving 10000110, not 10000111.

Match the Pairs

00000101 << 1 = 00001010 → A logical left shift by one place: every bit moves one place left and a 0 fills the empty right end.

11000011 << 1 = 10000110 → The leftmost 1 falls off the end and is lost, and a 0 enters on the right.

Zeros come in on the right: after a left shift, the empty right-hand place is filled with 0. → 00010111 << 1 = 00101110

Bits shifted off the left end are lost: the leftmost bit is dropped and cannot be recovered. → 10110001 << 1 = 01100010

00000101 << 1 = 10000010 → This is a right shift result. In a left shift the bits move left and a 0 fills the right end, so the answer should be 00001010.

00110011 << 1 = 01100111 → The empty place on the right was filled with 1. A logical left shift always fills it with 0, so the answer should be 01100110.

Key Term

logical left shift

Moves every bit one place left. The leftmost bit falls off and is lost, and a 0 fills the empty place on the right. In 8 bits, 00000101 becomes 00001010.

Fill in the Blanks

1. Logical left shift: 00000101 -> 00001010

Hint: Type the direction the bits move in this shift, as the example shows the value getting bigger.

2. 00000101 << 1 = 00001010

Hint: Type the two-character operator that shifts bits to the left.

3. 11000011 << 1 = 10000110

Hint: Type the single bit that fills the empty place on the right.

4. 00000011 << 2 = 00001100

Hint: Type the number of places the bits are shifted. Compare how far the 11 has moved.

5. 10000001 << 1 = 00000010

Hint: Type the 8-bit result. The leftmost 1 falls off and is lost, and a 0 enters on the right.

Spot the Error

Question 1

A Register: 0000101

B Shift left by 1 place

C Drop the leftmost bit: 0

D Add 0 on the right

E Result: 00001010

Line A — An 8-bit register holds exactly 8 bits. 0000101 has only 7 bits. The register should be 00000101.

Question 2

A x = 00000011

B x = x >> 1

C # bits move one place left

D # 0 enters on the right

E # x is now 00000110

Line B — << is the left shift operator. >> shifts right. The line should be x = x << 1.

Question 3

A Start: 00010110

B Shift left 1 place

C Leftmost bit 0 goes to right

D New right bit is 0

E Result: 00101100

Line C — In a logical left shift the leftmost bit is dropped and lost. It does not wrap round to the right. The line should say the leftmost bit 0 is dropped.

Question 4

A Start: 11000001

B Shift left 1 place

C Leftmost 1 is dropped

D New right bit is 1

E Result: 10000010

Line D — A logical left shift always fills the empty right-hand place with 0, never 1. The line should say the new right bit is 0.

Question 5

A n = 00000100

B n = n << 2

C print(n)

D # shifted left by 1, 00001000

Line B — Shifting 00000100 left by 1 place gives 00001000. << 2 would shift two places and give 00010000. The line should be n = n << 1.

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