Ready-to-use computing activity
Binary Overflow and Shifts
Teacher overview
Prior knowledge: Learners should be able to add small binary values and recognise 8-bit place values.
Learning objective: Predict and test binary addition, carry, overflow and logical shifts.
Starter
Ask learners to predict what happens to 00001100 after a logical left shift.
Main task
- Add 00000101 + 00000011 and record each carry.
- Add 11111111 + 00000001 and explain why the 8-bit result overflows.
- Predict the result of shifting 00010110 left once and right once.
- Use Binary Lab to test the predictions and correct the trace.
- Write one sentence explaining why a left shift often doubles a positive unsigned value.
Support prompts
- Line up the bits before adding.
- A carry leaves the current column and moves left.
- Overflow means the answer needs more bits than are available.
Challenge
Compare unsigned overflow with signed overflow for 01111111 + 00000001.
Plenary or exit question
Why is overflow a computer problem rather than just a maths problem?