This is a hello world example that showcases a simple 8 bit CPU architecture and assembler written in Python. The architechure is loosely inspired by the 8080 processor and the SM83 processor, the processor used in the GameBoy and GameBoy Color. I made this to showcase my current understanding of CPU architecture and assembly programming. I'm using this project as a beginning stepping stone for what I wish to accomplish as a Computer Engineer.
Within the Logisim simulator, this program prints "Hello World!" to the simulated display three times, then prints "End program."
See a video demo and explanation
For developing this, I used my preferred code editor, Kate, as it has assembly syntax highlighting built in. For the simulated hardware I used Logisim Evolution, an open source circuit simulator.
The programming language used is a custom assembly instruction set:
| Opcode | Description | Usage |
|---|---|---|
| nop | No operation. | nop |
| ld | Load an integer from ROM into a register. | ld r0, 11 |
| ldc | Load an ASCII character from ROM into a register. | ldc r1, "c" |
| mov | Move a value from one register to another. | mov r0, r1 |
| disp | Display a value stored in a register. | disp r4 |
| dispd | Display a value or character stored in ROM. | dispd "@" |
| xor | Preform an XOR operation and store the result in the first argument. | xor r0, r1, r2 |
| or | Preform an OR operation and store the result in the first argument. | or r0, r1, r2 |
| add | Preform an ADD operation and store the result in the first argument. | add r7, re, rf |
| lsr | Preform an logical shift right and store the result in the first argument. | lsr r0, r1 |
| lsl | Preform an logical shift left and store the result in the first argument. | lsl r2, r2 |
| com | Preform a bit-wise complement and store the result in the first argument. | com r2, r6 |
| sub | Preform an SUB operation and store the result in the first argument. | sub r3, r2, r1 |
| inc | Increment register value by one. | inc r2 |
| dec | Decrement register value by one. | sub r1 |
| cp | Compare values in two registers. Sets compare flag if they are equal. | cp r0, r5 |
| jmp | Jump to an address stored in a register. | jmp rf |
| jp | Jump to an address stored in ROM. Supports labels. | jp Start |
| jpc | Jump to an address stored in ROM if compare flag is set. | jpc Branch |
| jpnc | Jump to an address stored in ROM if compare flag is not set. | jpnc Loop |
| disp_str | Macro for displaying multi-character strings using dispd. | disp_str "Wow" |
As the processor is based on the GameBoy, these assembly tutorials and hardware documentation proved useful: