nesemu/cpu/cpu.c

//#include "log.h"
#include <assert.h>
#include <string.h>
#include "../include/cpu.h"
#include "cpu.h"
#include "memory.h"
#include "op.h"
#include "decoding.h"
#include "log.h"

/*
 * =====================================================================================
 *
 *       Filename:  cpu.c
 *
 *    Description:  6502 CPU emulator 
 *
 *        Version:  1.0
 *        Created:  2023-09-21 10:10:26 PM
 *       Revision:  none
 *       Compiler:  gcc
 *
 *         Author:  William Nolin, 
 *   Organization:  
 *
 * =====================================================================================
 */

void cpu_init(CPU *cpu) {
    cpu->program_counter = 0x8000;
    cpu->stack_pointer = 0xfd;
    cpu->accumulator = 0x00;
    cpu->x = 0x00;
    cpu->y = 0x00;
    cpu->status = 0x04;
    cpu->oam_dma_triggered = false;
    cpu->nmi_requested = false;
}

void print_registers(CPU cpu, byte op, unsigned long cycle_count) {
    log_debug("%#02x %#02x %s \t A:%#02x X:%#02x Y:%#02x F:%#02x SP:%#02x \t [%d]",
              cpu.program_counter,
              op,
              get_op_code_name(op),
              cpu.accumulator,
              cpu.x,
              cpu.y,
              cpu.status,
              cpu.stack_pointer,
              cycle_count);
}

void cpu_process_nmi(System *system) {
    cpu_stack_push_context(system);

    address handler_addr = mem_get_word(system, 0xfffa);
    log_debug("NMI %#04x", handler_addr);

    system->cpu.nmi_requested = false;
    system->cpu.program_counter = handler_addr;
}

void oam_dma_upload(System *system) {
    byte page_high_addr = *system->ppu.oam_dma_register;
    address page_addr = ((address) page_high_addr) << 8;
    byte n = 0xff;

    byte *ram_source = &system->ram[page_addr];
    byte *oam_destination = system->ppu.oam;

    memcpy(oam_destination, ram_source, n);

    log_debug("OAM DMA %#04x", page_addr);
    cpu_add_cycles(system, 513);    // TODO
}

void cpu_cycle(System *system) {
    if (system->cpu.nmi_requested) {
        cpu_process_nmi(system);
    }

    if (system->cpu.oam_dma_triggered) {
        oam_dma_upload(system);
        system->cpu.oam_dma_triggered = false;
        return;
    }

    CPU cpu = system->cpu;
    byte op = cpu_get_next_byte(system);

    print_registers(cpu, op, system->cycle_count);

    process_op_code(system, op);
}

void cpu_add_cycles(System *system, unsigned int cycle_count) {
    system->cycle_count += cycle_count;
}

// === Registers ===
bool system_get_flag(System *system, byte mask) {
    return cpu_get_flag(&system->cpu, mask);
}

void system_set_flag(System *system, byte mask, bool set) {
    if (set) {
        system->cpu.status |= mask;
    } else {
        system->cpu.status &= ~mask;
    }
}

byte cpu_get_next_byte(System *system) {
    byte next_byte = mem_get_byte(system, system->cpu.program_counter);
    system->cpu.program_counter++;
    return next_byte;
}

word cpu_get_next_word(System *system) {
    word next_word = mem_get_word(system, system->cpu.program_counter);
    system->cpu.program_counter += 2;
    return next_word;
}

void cpu_stack_push(System *system, byte value) {
    assert(system->cpu.stack_pointer > 0);

    address mem_addr = CPU_STACK_ADDR | system->cpu.stack_pointer;
    mem_set_byte(system, mem_addr, value);
    system->cpu.stack_pointer--;
}

byte cpu_stack_pop(System *system) {
    assert(system->cpu.stack_pointer < 0xff);

    system->cpu.stack_pointer++;
    address mem_addr = CPU_STACK_ADDR | system->cpu.stack_pointer;
    byte value = mem_get_byte(system, mem_addr);
    return value;
}

void cpu_stack_push_context(System *system) {
    cpu_stack_push(system, system->cpu.program_counter >> 8);
    cpu_stack_push(system, system->cpu.program_counter & 0xff);
    cpu_stack_push(system, system->cpu.status);
}

void cpu_stack_pop_context(System *system) {
    byte value = cpu_stack_pop(system);
    value &= 0xef;  // The B mask cannot be set as it is a CPU signal
    value |= 0x20;  // This value is always set
    system->cpu.status = value;

    byte lo = cpu_stack_pop(system);
    address pc = cpu_stack_pop(system) << 8;
    pc += lo;
    system->cpu.program_counter = pc;
}