Program Counter

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Table of Contents

1. About The Project
2. Pictures
3. Technical Details
4. Directory Structure
5. To-Do
6. Acknowledgments

About The Project

AOS16 is a self-made 16-bit Reduced Instruction Set Computer (RISC) on PCBs, complete with a homebrew GPU for screen output. It is fully planned, designed and fabricated from scratch. A Turing-complete, Harvard-style computer with a completely custom architecture, at a blistering fast 10MHz.

The project is divided into smaller subsytems on PCBs, designed to interconnect via a central bus.

Features

• 16-bit address and data buses
• Hardware double buffering with parallel CPU and GPU operation
• 10 MHz clock
• 6 general-purpose registers, a stack pointer and a flags register
• 65 Kb of general purpose RAM
• 131 Kb of framebuffer (double buffered, so 65Kb usable)
• 131 Kb of program memory

Why?

This project was a learning effort for me. I started this project wanting to learn more about computer architectures and gain hands-on experience with electrical engineering. Also, building a computer, from scratch, is the coolest project I've seen around. Some inspiration from amazing engineers including Ben Eater and jdh were more than enough to convince me to try my hand at this project.

How's it different?

Most homebrew computers on the internet are 8-bit computers built on breadboards. AOS16 is novel in its capabilities, being a 16-bit computer built on PCBs with a much higher clock speed, with display support directly integrated.

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Pictures

A simulation of the computer in Digital displaying the color palette on the screen

GPU displaying the pillars of creation

The Arithmetic Logic Unit in KiCAD

The logic simulation in Digital.

The Reg4x16 PCB

The Double Buffer PCB

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Technical Details

Details about the design decisions, design of the instruction word, etc. can be found in the Design Doc.

Directory Structure

• assembler: Contains the source code for the AOS16 assembler
• sim: Logic simulation files written for Digital
• microcode: Microcode implementations written in MiLa
• tests: some basic tests for ICs and the simulation
• common: basic KiCAD schematics (registers, transceivers) which are shared across subsystems

Each of the remaining folders represents a subsystem, built on a separate PCB. Each folder will contain a KiCAD schematic and PCB file.

To-do

Main Phases of the project:

• Finalize project parameters and design goals
• Hardware:
  • Logic Simulation and Design
  • PCB Design
  • PCB Fabrication & integration
• Software:
  • Microcode (with MiLa)
  • Assembly language design
  • Assembler
  • ROM programming software

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Acknowledgments

• Ben Eater for his excellent series on the breadboard computer
• jdh for the inspiration
• The Elements of Computing Systems and the accompanying Nand2Tetris course by Shimon Schocken and Noam Nisan
• r/homebrewcomputer and r/PrintedCircuitBoard commmunities for PCB reviews and design checks
• Digital, the logic simulator

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