Circuit operation Arithmetic logic unit

the combinational logic circuitry of 74181 integrated circuit, simple four-bit alu

an alu combinational logic circuit, meaning outputs change asynchronously in response input changes. in normal operation, stable signals applied of alu inputs and, when enough time (known propagation delay ) has passed signals propagate through alu circuitry, result of alu operation appears @ alu outputs. external circuitry connected alu responsible ensuring stability of alu input signals throughout operation, , allowing sufficient time signals propagate through alu before sampling alu result.

in general, external circuitry controls alu applying signals inputs. typically, external circuitry employs sequential logic control alu operation, paced clock signal of sufficiently low frequency ensure enough time alu outputs settle under worst-case conditions.

for example, cpu begins alu addition operation routing operands sources (which registers) alu s operand inputs, while control unit simultaneously applies value alu s opcode input, configuring perform addition. @ same time, cpu routes alu result output destination register receive sum. alu s input signals, held stable until next clock, allowed propagate through alu , destination register while cpu waits next clock. when next clock arrives, destination register stores alu result and, since alu operation has completed, alu inputs may set next alu operation.