4-Bit CPU
By Michell Cronberg - for use in training and presentations

A 4-bit CPU is the simplest form of a Central Processing Unit. This fictional CPU-4 chip shows the essential pins needed for a minimal processor.

The chip uses a standard 18-pin DIP (Dual In-line Package) format with 9 pins on each side.

Pin Groups

Power (2 pins)
  • GND (Pin 1) - Ground reference (0V)
  • VCC (Pin 18) - Power supply (+5V)
Data Bus (4 pins)
  • D0-D3 (Pins 2-5) - Bidirectional data bus
  • Used to read instructions and data from memory
  • Also used to write results back to memory
Address Bus (4 pins)
  • A0-A3 (Pins 6-9) - Memory address
  • 4 bits = 16 possible addresses (0-15)
  • CPU outputs which memory location to access
Control Inputs (3 pins)
  • CLK (Pin 17) - Clock signal
  • RST (Pin 16) - Reset (active low)
  • IRQ (Pin 14) - Interrupt request
Control Outputs (1 pin)
  • R/W (Pin 15) - Read/Write signal
  • HIGH = CPU is reading from memory
  • LOW = CPU is writing to memory
Output Port (4 pins)
  • O0-O3 (Pins 10-13) - General output
  • Can drive LEDs, displays, or other devices
  • 4 bits = values 0-15

How It Works

Fetch: CPU puts address on A0-A3, sets R/W=1, reads instruction from D0-D3
Decode: CPU interprets the instruction (e.g., ADD, LOAD, STORE)
Execute: CPU performs the operation, may read/write more data
Repeat: Move to next instruction (controlled by CLK)

Specifications

Data Width 4 bits
Address Space 16 bytes (4-bit address)
Package 18-pin DIP
Power +5V DC

Historical Context

The first commercial 4-bit CPU was the Intel 4004 (1971) with 16 pins. It had 2,300 transistors and ran at 740 kHz. Our fictional CPU-4 is inspired by this design but simplified for educational purposes.

(c) Michell Cronberg 2026 - MIT license