What Are the Minimal Building Blocks of a CPU? From Relays to Logic Gates

CPU Minimal Hardware Atoms

Modern CPUs consist of millions of transistors, but the earliest computers were built from thousands of relays and vacuum tubes. Early machines contained hundreds of thousands of components and kilometers of wiring, with control logic implemented using electromechanical relays. The transition to semiconductor devices dramatically reduced size and complexity.

Basic Boolean algebra combined relays into logic gates (AND, OR, NOT, XOR), forming the foundation for data control.

Logic Gates and Simple Circuits

Using transistors, a basic AND gate can be built, as can NOT and OR gates. The article shows diagrams of these gates and their truth tables.

The NOT gate inverts a single input, while the OR gate outputs low only when both inputs are low.

Constructing Adders and the ALU

A 1‑bit adder is formed by combining an XOR gate (producing the sum) with an AND gate (producing the carry). By chaining eight full‑adders, an 8‑bit adder is created, with carries propagating to the next stage.

The Arithmetic Logic Unit (ALU) contains two sub‑units: an arithmetic unit (addition, subtraction, increment) and a logic unit (zero‑detect, sign‑detect). Flags such as OVERFLOW, ZERO, and NEGATIVE indicate the result status.

Memory, Registers, and Latches

To retain results, the CPU uses memory cells. Simple 1‑bit latches are built from AND, OR, and NOT gates, with a SET input storing a 1 and a RESET input clearing it. The latch’s output reflects the stored bit until changed.

Instruction Fetch‑Decode‑Execute Cycle

CPU execution proceeds in three stages: fetch the instruction from RAM into the Instruction Register, decode the opcode, then execute. The article illustrates a simple 8‑bit machine code example 00101110, where the high nibble 0010 represents the LOAD_A operation and the low nibble 1110 specifies address 14.

During execution, the CPU copies the value at address 14 into register A, then increments the Program Counter to fetch the next instruction.

From Machine Code to Assembly Language

Early programmers wrote raw machine code or pseudo‑code on paper. Assemblers translate human‑readable mnemonics (e.g., LOAD_A 14) into binary opcodes, greatly simplifying programming.

Over time, high‑level languages such as FORTRAN, ALGOL, C, and modern languages like Python and Go have emerged, each compiled or interpreted down to machine code executed by the CPU.

The evolution from relays to transistors, from simple logic gates to complex ALUs, and from binary machine code to expressive programming languages illustrates the layered abstraction that makes modern computing possible.