GCSE · Computer Science · AQA · Spec 8525

Binary shifts

Core vocabulary — binary shifts

Get these terms clear first — they come up throughout this topic.

Stop and predict

Commit to an answer before you reveal — this is the moment learning sticks.

An 8-bit register holds 10000000 (decimal 128). A student left-shifts it by 1 place. What will the decimal result be?

Left shift vs Right shift

Left shift (<<)vsRight shift (>>)

Same mechanism, opposite effects — know which is which before the exam.

Focus

Effect on value

Left shift (<<)

Multiplies by 2 per place shifted, provided no 1-bits are shifted beyond the register width

Right shift (>>)

Divides by 2 per place shifted (integer)

The insight

Direction of shift directly determines whether you multiply or divide — left = larger, right = smaller.

Vacated positions filled with

Left shift (<<)

0s on the right

Right shift (>>)

0s on the left

Risk of data loss

Left shift (<<)

Significant bits shifted beyond the left edge are discarded

Right shift (>>)

Truncation — the remainder of division is silently dropped

Equivalent arithmetic

Left shift (<<)

× 2ⁿ (where n = number of places), provided no 1-bits are shifted beyond the register width

Right shift (>>)

÷ 2ⁿ (integer division, floor result)

Example (8-bit)

Left shift (<<)

00000011 (3) << 2 → 00001100 (12)

Right shift (>>)

00001100 (12) >> 2 → 00000011 (3)

Cross-subject · Exam skill

Mark scheme practice — Binary shifts

Read the question, study the model answer, then compare it with your own.

Question

Explain the effect of performing a left shift on a binary number.

Student answer

A left shift moves all bits to the left by one or more places. Each place shifted multiplies the value by 2, provided no 1-bits are shifted beyond the register width. Zeros fill the vacated positions on the right.

Method marks0/2

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How moving bits left or right multiplies or divides — and what happens when they fall off the edge

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