yemu/src/6502/6502.c

#include <yemu/6502.h>
#include <stdio.h>
#include <stdlib.h>

struct CPU cpu;
struct MEMORY mem;

void memory_init () {
	for (int i = 0; i < MAX_MEMORY; i++){
		mem.memory[i] = 0;
	}
}
void reset () {
        cpu.PC = 0x0000; // FIXME //0xFFFC; 
	cpu.SP = 0x0000; //FIXME
	cpu.A = cpu.X = cpu.Y = 0;
	cpu.C = cpu.Z = cpu.I = cpu.D = cpu.B = cpu.V = cpu.N = 0;
	memory_init(mem);
}

void dump_cpu(struct CPU cpu) {
	printf("PC: %04X\n", cpu.PC);
	printf("SP: %04X\n", cpu.SP);
	printf("A: %04X\n", cpu.A);
	printf("X: %04X\n", cpu.X);
	printf("Y: %04X\n", cpu.Y);
	printf("Flags: C: %X, Z: %X, I: %X, ",  cpu.C, cpu.Z, cpu.I);
	printf("D: %X, V: %X, N: %X\n", cpu.D, cpu.V, cpu.N);
}

byte fetch_byte() {
	byte data = mem.memory[cpu.PC];
	cpu.PC++;
	return data;
}

word fetch_word() {
	word data = mem.memory[cpu.PC];
	cpu.PC++;

	data |= (mem.memory[cpu.PC] << 8);
	cpu.PC++;

	return data;
}

byte read_byte(word addr) {
	byte data = mem.memory[addr];
	cpu.PC++;
	return data;
}

word read_word(word addr) {
	word data = mem.memory[addr];
	data |= (mem.memory[addr+1] << 8);
	cpu.PC++;
	cpu.PC++;
	return data;
}

void write_byte(byte addr, byte value) {
	mem.memory[addr] 	= value;
}

void write_word(byte addr, word value) {
	mem.memory[addr] 	= value & 0xFF;
	mem.memory[addr+1] 	= (value >> 8);
}

void execute() {
	while (1) {
		byte instruction = fetch_byte();
		byte value;
		byte zero_page_addr;
		byte zero_page_x_addr;
		byte zero_page_y_addr;
		word jsr_addr;
		switch(instruction) {
			case INS_LDA_IM: 
				value = fetch_byte();
				cpu.A = value;
				cpu.Z = (cpu.A == 0);
				cpu.N = (cpu.A & 0x40) > 0; // 0x40 means 0b1000000 (fixed to not receive a warning from compiler)
				break;
			case INS_LDA_ZP: 
				zero_page_addr = fetch_byte();
				cpu.A = read_byte(zero_page_addr);
				cpu.Z = (cpu.A == 0);
				cpu.N = (cpu.A & 0x40) > 0;
				break;
			case INS_LDA_ZPX: 
				zero_page_x_addr = cpu.X + fetch_byte();
				cpu.A = read_byte(zero_page_x_addr);	
				cpu.Z = (cpu.A == 0);
				cpu.N = (cpu.A & 0x40) > 0;
				break;

			case INS_LDX_IM:
				value = fetch_byte();
				cpu.X = value;
				cpu.Z = (cpu.X == 0);
				cpu.N = (cpu.X & 0x40) > 0;
				break;
			case INS_LDX_ZP: 
				zero_page_addr = fetch_byte();
				cpu.X = read_byte(zero_page_addr);
				cpu.Z = (cpu.X == 0);
				cpu.N = (cpu.X & 0x40) > 0;
				break;
			case INS_LDX_ZPY:
				zero_page_y_addr = cpu.Y + fetch_byte();
				cpu.X = read_byte(zero_page_y_addr);	
				cpu.Z = (cpu.X == 0);
				cpu.N = (cpu.X & 0x40) > 0;
				break;

			case INS_LDY_IM:
				value = fetch_byte();
				cpu.Y = value;
				cpu.Z = (cpu.Y == 0);
				cpu.N = (cpu.Y & 0x40) > 0;
				break;
			case INS_LDY_ZP: 
				zero_page_addr = fetch_byte();
				cpu.Y = read_byte(zero_page_addr);
				cpu.Z = (cpu.Y == 0);
				cpu.N = (cpu.Y & 0x40) > 0;
				break;
			case INS_LDY_ZPX:
				zero_page_x_addr = cpu.Y + fetch_byte();
				cpu.Y = read_byte(zero_page_x_addr);	
				cpu.Z = (cpu.Y == 0);
				cpu.N = (cpu.Y & 0x40) > 0;
				break;
				
		        case INS_TAX:
			        cpu.X = cpu.A;
				cpu.Z = (cpu.X == 0);
				cpu.N = (cpu.X & 0x40) > 0;
				cpu.PC++;
			        break;
		        case INS_TAY:
			        cpu.Y = cpu.A;
			        cpu.Z = (cpu.Y == 0);
				cpu.N = (cpu.Y & 0x40) > 0;
				cpu.PC++;
			        break;

		        case INS_TSX:
			        cpu.X = cpu.SP;
			        cpu.Z = (cpu.X == 0);
				cpu.N = (cpu.X & 0x40) > 0;
				cpu.PC++;
			        break;
          		case INS_TXA:
			        cpu.A = cpu.X;
				cpu.Z = (cpu.A == 0);
				cpu.N = (cpu.A & 0x40) > 0;
				cpu.PC++;
			        break;
		        case INS_TXS:
			        cpu.SP = cpu.X;
				cpu.PC++;
			        break;
         		case INS_TYA:
			        cpu.A = cpu.Y;
				cpu.Z = (cpu.A == 0);
				cpu.N = (cpu.A & 0x40) > 0;
				cpu.PC++;
			        break;		        
				
	   		case INS_JSR:
				jsr_addr = fetch_word();
				write_word(cpu.PC - 1, cpu.SP);
				cpu.SP++;
				cpu.PC = jsr_addr;
				break;
			case INS_RTS:
				// TODO
				break;
		        case INS_6502_NOP:
			        break;
				
         		case INS_INX:
			        cpu.X++;
			        cpu.Z = (cpu.X == 0);
				cpu.N = (cpu.X & 0x40) > 0;
			        break;
		        case INS_INY:
			        cpu.Y++;
			        cpu.Z = (cpu.Y == 0);
				cpu.N = (cpu.Y & 0x40) > 0;
			        break;

		        case INS_DEX:
			        cpu.X--;
			        cpu.Z = (cpu.X == 0);
				cpu.N = (cpu.X & 0x40) > 0;
			        break;
		        case INS_DEY:
			        cpu.Y--;
			        cpu.Z = (cpu.Y == 0);
				cpu.N = (cpu.Y & 0x40) > 0;
			        break;
		        case 0x00: // EOF
			        return;
			default: 
				printf("%04X: Unhandled instruction: %04X\n", cpu.PC, instruction);
				return;
		}
	}
}
void init6502(FILE *infile) {
	reset();
	char ch;
	int pos = 0;
	while (1) { // FIXME
	  ch = fgetc(infile);
	  if (ch == EOF)
	    break;
	  mem.memory[pos] = ch;
	  pos++;
	}
	mem.memory[pos] = EOF;
        
	execute();
	dump_cpu(cpu);
	return;
}