Method and circuit for fast generation of zero flag condition code in a microprocessor-based computer

摘要

An adder circuit in parallel with a zero flag generation circuit. In a preferred embodiment, an arithmetic logic unit (ALU) circuit in a microprocessor based computer system includes an adder circuit preferably adapted to receive first and second operands. The preferred adder circuit is further adapted to produce a result equal to the sum of the first and second operands. The ALU circuit further includes a zero flag generation circuit. The zero flag generation circuit is adapted to receive the first and second operands in parallel with the adder circuit and to produce a zero flag signal in response to the operands. The zero flag signal is indicative of whether the sum of the operands is equal to zero. In one embodiment, the zero flag generation circuit includes N half adders in parallel wherein each adder receives a bit from the first operand and a corresponding bit from the second operand. Each half adder produces a sum bit and a carry bit in response to the inputs. Preferably, the zero flag generation circuit further includes N-1 Exclusive OR (EXOR) gates. Each of the N-1 EXOR gates receives one bit of the N sum bits and a corresponding bit of the N carry bits as inputs. The N-1 outputs from the EXOR gates, together with an inverted least significant sum bit, are routed to a logic circuit. The logic circuit functions as an AND gate, producing an output signal indicative of whether each input signal is equal to 1.