ALU#

The Arithmetic Logic Unit is where the building blocks converge. It is a single combinational block that performs a selected operation — add, subtract, AND, OR, XOR, compare, shift — on its two operands, with function-select inputs choosing which. It is the computational core of a processor’s datapath, and it is assembled almost entirely from the primitives covered elsewhere in this section.

What It Combines#

An ALU is less an invention than an arrangement:

• An add/subtract unit — the adder with its controllable input inversion — provides the arithmetic.
• Banks of gates (AND, OR, XOR, NOT across the operands) provide the logic operations.
• A multiplexer driven by the function-select lines picks which of those results actually appears at the output.
• Status flags come out alongside the result: zero (the result is all 0s), carry, signed overflow, and negative/sign — the bits a program branches on.

Feed it two operands and a function code, and it produces the result and the flags in one combinational sweep. It computes nothing sequential on its own; the sequencing around it is somebody else’s job.

The 74181 and What Came After#

The landmark part is the 74181, a 4-bit ALU on a single chip offering sixteen arithmetic and sixteen logic functions selected by its mode and function inputs. It was the first complete ALU available as one MSI package, and it sat at the center of many 1970s minicomputers — several 74181s cascaded to build a wider word.

That is exactly the through-line the book keeps returning to. The ALU is the point where the adder, the logic gates, and a selecting mux come together under control; wrap it with registers to hold the operands and result and a sequencer to feed it a stream of operations, and the result is the execution core of a CPU. The ALU inside any modern microcontroller or processor is this same arrangement, no longer a chip of its own but a region of a much larger die — the functional building blocks, doing math.