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https://github.com/vascofazza/8bit-cpu

Schematics & code for my 74LS-based 8-bit MK1 CPU
https://github.com/vascofazza/8bit-cpu

74ls 8bit 8bit-cpu cpu electronics homemade mk1 mk1-computer mk1-cpu programming-board schematics

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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:



[Hackaday.com](https://hackaday.com/2020/12/31/diy-8-bit-computer-knows-all-the-tricks/)



[Hackster.io](https://www.hackster.io/news/see-inside-of-a-cpu-with-the-mk1-8bit-computer-14fa9e313c73)



[InformaticaLab.com](https://blog.informaticalab.com/viaggio-allinterno-di-una-cpu-con-il-mk1-8bit-computer/)



## 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)**



[![MK1 Computer - home-made Programmable 8bit CPU](https://img.youtube.com/vi/qSviFkpLFKI/0.jpg)](https://www.youtube.com/watch?v=qSviFkpLFKI)



## 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](https://github.com/vascofazza/8bit-cpu/tree/master/MK1_CPU/programs/lib/mk1.cpu)**) 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 **A**rithmetic **L**ogic **U**nit 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](https://github.com/vascofazza/8bit-cpu/blob/master/MK1_CPU/code/microcode.txt)**) 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