Logic Gates: One-Bit ALU


Bit 31

Full 1-bit ALU

Handles AND, OR, and binary addition. Handles binary subtraction by negating b and doing addition. (Flip all the bits and set CarryIn0 to 1 to compute two's complement.) Can also negate a to implement NOR operation. To determine whether a < b, calculate (a - b) and determine whether result is negative (bit 31 is 1).

Overflow detection uses carry31 (overflow if unsigned), or uses comparison of bit 31 of a and b (same sign?) and comparison of that against bit 31 of the sum (different sign?).

Stop and Think: How does the ALU execute the NOR operation? What control lines would be turned on?

Alyce Brady, Kalamazoo College