Design and Implementation of a 4-Bit Breadboard CPU Using Digital Integrated Circuits

Abstract

This paper presents the design and hardware realization of a simple 4-bit CPU built using discrete TTL integrated circuits on a breadboard. The CPU employs a 4-bit common data bus with tri-state buffers for device arbitration, three 4-bit registers (Register A, Register B, and an Output Register) implemented with 74LS175 quad D flip-flops, and a 4-bit ALU for addition/subtraction using a 74LS83 adder and XOR gates. Manual control signals (load, en- able, add/sub, etc.) and DIP switch inputs allow step-by-step operation under a hand-driven clock pulse. The system was first verified in simulation and then tested on a physical proto- type. Experiments confirmed correct execution of register load, register-to-register transfer, add, and subtract operations across multiple test cases, with results validated against expected values on the LED output array. Beyond the specific hardware result, the project is framed as a self-contained pedagogical case study: every design decision, from the choice of a shared tri-state bus to the reuse of a single adder for both addition and subtraction, is made deliber- ately visible and traceable to a single hand-clocked step, so that a learner can connect abstract register-transfer-level concepts directly to measurable electrical behaviour. The reported test cases, spanning ordinary operation, zero operands, and deliberate overflow/underflow condi- tions, are intended to serve as a compact, reproducible verification checklist for similar low- cost, breadboard-scale processor builds, and the resulting datapath is explicitly positioned as a reusable foundation for the automated, control-unit-driven extension discussed later in the paper.