Harvard-8Bit-CPU

An 8 bit CPU designed in Logisim with a custom assembler. A modification of Ben Eater’s 8-bit CPU design that uses a Harvard Architecture.

Instruction Set

Register Operations

• LIA [imm8] - Load immediate into register A
• LIB [imm8] - Load immediate into register B
• LDA [addr8] - Load RAM into register A
• LDB [addr8] - Load RAM into register B
• STA [addr8] - Store register A's contents in RAM
• STB [addr8] - Store register B's contents in RAM
• AOUT - Display the contents of register A
• BOUT - Display the contents of register B

ALU Operations

• ADD [addr8] - Add registers A and B and store in RAM
• SAB [addr8] - Subtract register B from A and store in RAM
• SBA [addr8] - Subtract register A from B and store in RAM
• AND [addr8] - AND registers A and B and store in RAM
• XOR [addr8] - XOR registers A and B and store in RAM
• OR [addr8] - OR registers A and B and store in RAM

Jump / Conditional Operations

• JMP [addr8] - Jump in program
• JMZ [addr8] - Jump in program if ALU last gave zero
• JMV [addr8] - Jump in program if ALU last gave overflow
• JMN [addr8] - Jump in program if ALU last gave negative
• JMC [addr8] - Jump in program if ALU last gave carry

Miscellaneous

• OUT [addr8] - Display a value in RAM
• NOP - Do nothing
• HLT - Halt the program

Assembling / Running A Program

Run program_rom_programmer.py and put in the name of the desired assembly program. This will generate a [program_name]_rom.txt in the rom_content folder. In Logisim, load that ROM text file into the PROGRAM ROM, and finally enable ticks to run the program.