ALU 101 Cheat Sheet 2026

The 30 highest-yield ALU 101 facts, distilled from real exam questions. Print it, save it as a PDF, or study it here — free, no sign-up.

1. What is the function of the status register (flags register) in an ALU? → To record condition outcomes such as zero, carry, and overflow
2. What does the NOP instruction do in an ALU-based processor? → Performs no operation and advances the program counter
3. What logic gate is formed by combining an AND gate followed by a NOT gate? → NAND
4. What problem does the Gray code solve in ALU-related digital counter circuits? → Multiple bit transitions causing glitches when incrementing
5. Which gate can implement addition without carry at the bit level? → XOR
6. What does 'silent NaN' vs 'signaling NaN' mean in IEEE 754? → Signaling NaN raises a floating-point exception when used; quiet NaN propagates silently
7. In carry lookahead logic, what does the 'generate' signal (G = A·B) indicate? → The stage will always produce a carry-out regardless of the carry-in
8. What is the purpose of using a testbench in ALU simulation? → To define the testing environment and stimuli
9. What is the purpose of the carry save adder (CSA)? → To speed up multi-operand addition by deferring carry propagation
10. Which component of an ALU is responsible for selecting between arithmetic and logic operations? → Multiplexer
11. Which of the following gates produces a HIGH output only when all inputs are LOW? → NOR
12. What key technique does a carry lookahead adder (CLA) use to improve speed? → Computes all carry bits in parallel using generate and propagate signals
13. What is the effect of the INTO instruction in x86 architecture? → It generates an overflow exception trap (INT 4) if the Overflow flag is set
14. For a 1-bit stage with inputs A=1 and B=1, what are the values of the generate (G) and propagate (P) signals? → G=1, P=0
15. Which hardware component stores intermediate values between ALU operations in a typical CPU datapath? → General-purpose registers
16. Which Boolean identity represents the expression 𝐴 ⋅¯A? → 0
17. Which gate is the complement of the AND gate? → NAND
18. How many gates are required to build a full adder using basic logic gates? → 5
19. What do ALU control lines (operation select lines) determine? → Which arithmetic or logic operation the ALU performs on its inputs
20. What is the primary purpose of the accumulator register in an ALU? → To store the result of arithmetic and logic operations
21. What is the result of the Boolean expression A+A⋅B? → A
22. Which flag is set when the result of an ALU operation is exactly zero? → Zero flag
23. What is the maximum sum when adding two 4-bit unsigned numbers? → 30
24. In a ripple-carry adder, what causes the main performance limitation? → Carry propagation delay through each stage
25. Which number system uses the digits 0–7? → Octal
26. How does the ALU detect and flag an underflow condition in floating-point arithmetic? → When the result is too small to be represented as a normalized number
27. What happens in most hardware ALUs when integer overflow is not handled explicitly? → The result wraps around modulo 2^n, potentially producing an incorrect silent error
28. Overflow in signed binary arithmetic occurs when: → The result cannot be represented in the given number of bits
29. If 𝐴⋅𝐵=1, what can be concluded about A and B? → Both A and B are 1
30. A NOR gate with inputs A=0 and B=0 outputs: → 1