README.md

Overview

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

Development Environment

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"

Useful Websites

As the processor is based on the GameBoy, these assembly tutorials and hardware documentation proved useful:

• Pan Docs on GB Dev
• GB ASM Tutorial