How Simple Logic Gates Build a CPU: From Diodes to 4‑Bit Adders

Semiconductor Basics

A semiconductor is a material whose conductivity lies between that of a conductor and an insulator; diodes are a common example, allowing current to flow in one direction while blocking it in the opposite direction.

Diodes as Logic Elements

When the anode (A) is at a higher voltage than the cathode (C), the diode conducts; otherwise it behaves as an open circuit. This property can be interpreted as a binary "1" (conducting) or "0" (non‑conducting).

Logic Gates Using Diodes

By arranging diodes, we can implement basic gates:

AND gate : output is low only when both inputs are high.

OR gate : output is low only when both inputs are low.

NOT gate : inverts the input voltage.

XOR gate : output is high when exactly one input is high.

These gates can be drawn with simplified symbols for easier analysis.

Building Simple Adders

A half‑adder adds two single‑bit numbers, producing a sum (S) and a carry (C). Extending this, a full‑adder adds three bits (two operands plus a carry‑in) and yields a sum and a carry‑out. By chaining four full‑adders we obtain a 4‑bit adder capable of adding numbers up to 15 (binary 1111).

Multiplication by Shifting

Multiplying a binary number by two is equivalent to shifting it left by one position and inserting a zero at the least‑significant bit. Multiplying by three can be achieved by a left shift (×2) followed by an addition of the original number, and similarly for other small constants.

Flip‑Flops and Registers

A flip‑flop (FF) stores a single bit of data. The RS‑type FF has two inputs: Reset (R) clears the stored bit to 0, and Set (S) stores a 1. By connecting multiple FFs in parallel we form a register capable of holding multi‑bit values (e.g., 4‑bit or 8‑bit registers).

Multiplexers (MUX)

A multiplexer selects one of several input lines based on a selector signal (sel). For a 2‑to‑1 MUX, sel=0 routes input i0 to the output, while sel=1 routes input i1. Larger MUXes can select among more inputs, such as a 4‑to‑1 selector.

Designing a Minimal CPU

The prototype CPU uses eight input pins: four for the instruction word and four for data. Three instructions are defined: 0100 – Load data into the register. 0001 – Add the data to the register and store the result. 0010 – Shift the register left by one bit (multiply by 2).

Each instruction activates the corresponding module (register, adder, or shifter) via an enable pin. Execution proceeds in three stages: fetch the instruction, perform the operation, and write back to the register.

Instruction Execution Steps

A simple CPU follows the three‑step cycle described above, whereas a classic RISC pipeline expands this to five stages: Instruction Fetch (IF), Instruction Decode (ID), Execute (EX), Memory Access (MEM), and Write‑Back (WB). Real‑world x86 CPUs may involve up to twenty micro‑operations for a single instruction.

Historical Note

In 1970, the collaboration between Japan’s BUSICOM and Intel produced the world’s first microprocessor, the Intel 4004, which demonstrated that a CPU could be implemented as a programmable, reusable chip rather than a bespoke, hard‑wired circuit.

Putting It All Together

By combining the described components—diode‑based gates, adders, shifters, flip‑flops, registers, and a small instruction set—we can assemble a functional 4‑bit CPU capable of executing expressions such as (1+4)×2+3. The binary program for this computation is: 01000001000101000010000000010011 Loading this sequence into the CPU demonstrates how machine code (a series of 0s and 1s) directly controls hardware to perform arithmetic operations.