8bit computer

MK1 8bit Computer

Schematics and code for my home-made 8-bit CPU and its companion boards.

Articles about this project:

Hackaday.com

Hackster.io

InformaticaLab.com

Overview

During the past month, I designed and built a programmable 8-bit CPU from scratch, out of basic series 74LS logic ICs.

This repository contains pictures, schematics, and code for this project and its companion boards.

• V 1.0 assembled:

• V 2.0b assembled:

• Helix Display Interface in action:

• V 1.0 in action:

DEMO VIDEO (YouTube)

Architecture

The MK1 CPU is composed of several modules, all connected trough a common 8-bit BUS, the status of each module is shown by dedicated LEDs.

• The clock module is designed to allow step-by-step execution; in automatic mode the clock speed can be adjusted from 1Hz up to 32KHz.
• The computer programs are stored in RAM and the CPU can be programmed both manually, by inserting binary machine code through dip-switches, and automatically via a USB PC interface.
  • The Programming interface is designed to be used in conjunction with an Arduino Nano or the Start9 programming board.
  • The Start9 programming board allows the loading of multiple programs stored on an on-board flash memory without the aid of an external computer device.
• The Addressable memory space is 1024 byte, data, stack and code spaces are separated, the code address space is not writable.
• The instructions are variable-length (see instruction-set here) 1 or 2 bytes long (first byte for the opcode, the second one for the argument), there are 4 general purpose registers (A, B, C, D) and a stack pointer register for subroutine calls.
• The Arithmetic Logic Unit has a dedicated register for the second operand and supports the following operations:
  • Addition
  • Subtraction
  • OR
  • AND
  • NOT
  • Left/Right Shift
  • Left/Right Rotation
• The Control Unit combinatory logic is implemented using EEPROMs (see microcode here) whilst each instruction is realized through a variable number of micro-instruction for a maximum of 6 micro-steps per instruction, including the fetch cycle. The instruction-set supports both direct and indirect memory access as well as absolute and conditional jumps on carry (CF) and zero (ZF) ALU flags.
• The computation output can be visualized on a 4-digit display, with a dedicated register, able to represent positive and 2-complement negative numbers both in decimal and hexadecimal format.
• The CPU can be extended thanks to the external BUS interface capable of handling up to 2 peripheral. The communication is bidirectional, the devices can send interrupts to the CPU to notify when new data is available. Interrupts are cleared once the data has been processed.
  • The only available peripheral at the moment is the Helix display interface, an ATmega328-driven 2x16 LCD output display.

Structure

• MK1 CPU/:
  • 8bit-computer/: KiCad project, schematics and PCB design of the MK1 CPU.
  • code/:
    • microcode.py: generates the binary EEPROM microcode.
    • out_display.py: generates the binary output display EEPROM code.
    • uploader.py: uploads a binary MK1 program to the CPU.
    • mk1_computer_uploader/: Arduino programmer interface sketch.
  • assembler/: fork of the hlorenzi's assembler, improved and customized for the MK1
  • programs/: a collection of programs for the MK1 CPU plus the assembler definition
• start9_programming_interface/:
  • programming_interface/: KiCad project, schematics and PCB design of the Start9 programming board.
  • code/start9_programming_interface/: Arduino code for the programming interface.
• helix_display_interface/:
  • display_interface/: KiCad project, schematics and PCB design of the Helix display interface board.
  • code/helix_display_interface/: Arduino code for the display interface.
• eeprom_programmer/: KiCad project, schematics and PCB design of a simple Arduino-based eeprom programmer
• bus_breakout/: KiCad project, schematics and PCB design of the external bus connector breakout board.

Changelog

V2.0d:

• minor hardware revision
• revisited PCB design
• new memory architecture, code memory section is read-only, stack and data live on separated spaces

V2.0b:

• minor hardware revision
• Variable-length instructions (1 or 2 bytes)
• new custom assembler, thanks to https://github.com/hlorenzi/customasm
• few new instructions
• revisited microcode and instruction-set

V2.0:

• 4 general purposes registers (A, B, C, D)
• Stack Pointer implemented as an up-down counter
• External interface and interrupt handling
• Output display decodes HEX and ASCII values
• Clock speed multipliers
• Control Unit extended with 4 EEPROMs
• Variable step microcode counter length (each instruction uses the minimum amount of micro-steps)
• revisited microcode and instruction-set

V1.0:

• Bugfixes.
• HL and STK address signals available in the MAR dip-switch.

V0.1:

• Initial release