An arithmetic logic unit, or ALU, combines operations such as addition, subtraction, AND and OR behind a selection input. Its output is the result of the selected operation. Flags such as zero, carry and signed overflow provide additional information under a specified convention.
Worked example
Imagine a four-bit ALU where opcode 00 selects addition, 01 selects subtraction, 10 selects AND and 11 selects OR. With A = 1010 and B = 1100, opcode 10 produces 1000, while opcode 11 produces 1110.
What to watch for
An opcode assignment is a design choice, not a universal standard. Define whether each flag is meaningful for every operation, especially logical operations.
Try it yourself
For the example ALU, what does opcode 00 produce for A = 0011 and B = 0010?
Answer: Addition gives 0101, or decimal 5. The zero flag is clear because the result is nonzero.
How to study this topic
State the operand width and signedness first. Work out a decimal answer, then compare it with the binary result. Keep result bits, carry, borrow and signed overflow distinct when interpreting the circuit's output.
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Digital Electronics