8-bit-computer

A turing complete Von-Neumann 8-bit computer build entirely in Logisim Evolution.

High overview of the architecture

On the diagramm below the spatial positon of different modules inside the computer can be seen. Thick lines represent the data flow and thin lines show the clock propagation paths.

Usage

In order to be able to experiment and test the 8-bit-compuer the Logisim Evoltuion is required. It is simple:

1. Install Logisim Evoltuion on your system.
2. Open the 8_bit_computer.circ.

The standard Logisim might work, though this has never been tested.

Assembly instructions overview

This computer has a total of 16 byte of RAM and has the following set of assembly instructions:

• NOP -> 0000 -> No operation
• LDA X -> 0001 -> Load a value from RAM at the address X into the A rigister
• ADD X -> 0010 -> Add a value from RAM at the address X to theA rigister and store it in the A rigister
• SUB X -> 0011 -> Subtruct a value from RAM at the address X from A rigister and store it in the A rigister
• STA X -> 0100 -> Load a value from the A rigister into RAM at the address X
• LDI X -> 0101 -> Load X into the A rigister
• JMP X -> 0110 -> Jump to the instruction X
• JC X -> 0111 -> Jump to the instruction X if the carry flag is set
• JZ X -> 1000 -> Jump to the instruction X if the zero flag is set
• OUT -> 1110 -> Output the contents of the A rigister
• HLT -> 1111 -> Halt (Not implemented yet)

Programming the 8-bit-computer

Programming the computer using assembly-compiler:

1. Create a file containing your valid assembly instruction ( please think of the the limitaions of the 16-byte RAM)
2. Compile it using the assembly-compiler:
   • Make sure you have RUST installed on your machine
   • Navigate to the assembly-compiler folder
   • Compile using the following command cargo run -- -i <your_assembly_file> -o <output_file_path> -l
3. Copy the contents of the output file into the RAM module:
   • Right click on the PRAM
   • Select Edit Contents....
   • Select the fist address.
   • Paste the contens of your compiled code using v.3.0 hex , words and addressed as settings.
   • Click Ok.
4. Set PRAM pin to 1/High in order to switch to programmable memory.
5. Start the clock (cmd + k)

Programming each instruction manually:

1. Set the MANUAL pin to 1/High
2. Set the RI pin 1/High
3. Set the address using the pins A0 - A3
4. Set the assembly opcode (and the X if needed) using D0 - D7
5. Puls the clock High and Low (cmd + t)
6. Repeat for all the other instruction After writing all the instructions and data to the RAM:
7. Set the MANUAL pin back to 0/Low
8. Reset the computer using the RESET button
9. Start the clock (cmd + k)

Programming the Control Unit ROM

For Control Unit to work correctly it has to be programmed. The code for generatin of the ROM microinstruction is written in C and produces a file called control_unit which has the microcode in it. In most cases one does not have to reprogramm the ROM. In case if it has to be done:

1. Modify the microcode_gen.c if needed.
2. Compile and run the program.
3. Open 8_bit_computer.circ in Logisim Evolution.
4. Right click on the Control Unit ROM.
5. Select Edit Contents....
6. Select the fist address.
7. Paste the contens of generated control_unit using v.3.0 hex , words and addressed as settings.
8. Click Ok.

Picture of the Direct Implementation

Acknowledgements

This project was inspited by the works of Ben Eater and his tutorial series on building an 8-bit breadboard computer!. Some of the designs were take directly from his videos and some were modified. Also some parts of microcode_gen.c were taken from the eeprom-programmer of Ben Eater.