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Schematics & code for my 74LS-based 8-bit MK1 CPU

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MK1 8bit Computer

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

Articles about this project:


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:


MK1 Computer - home-made Programmable 8bit CPU


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 (
    ) 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 (
    ) and zero (
    ) 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.


  • MK1 CPU/:
    • 8bit-computer/: KiCad project, schematics and PCB design of the MK1 CPU.
    • code/:
    • generates the binary EEPROM microcode.
    • generates the binary output display EEPROM code.
    • uploads a binary MK1 program to the CPU.
    • mk1computeruploader/: 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
  • start9programminginterface/:
    • programming_interface/: KiCad project, schematics and PCB design of the Start9 programming board.
    • code/start9programminginterface/: Arduino code for the programming interface.
  • helixdisplayinterface/:
    • display_interface/: KiCad project, schematics and PCB design of the Helix display interface board.
    • code/helixdisplayinterface/: 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.


  • minor hardware revision
  • revisited PCB design
  • new memory architecture, code memory section is read-only, stack and data live on separated spaces
  • minor hardware revision
  • Variable-length instructions (1 or 2 bytes)
  • new custom assembler, thanks to
  • few new instructions
  • revisited microcode and instruction-set
  • 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
  • Bugfixes.
  • HL
    address signals available in the
  • Initial release

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