GBCartRead/arduino/source/cart.c

#include "cart.h"

#include <stdint.h>
#include <avr/io.h>
#include <util/delay.h>

#include "utils.h"
#include "pins.h"
#include "spi.h"
#include "rom.h"


// Most of the pulses described here are arbitrarily chosen,
//   to satisfy my idiotic UX requirements with the leds.
// Here's a pretty cool bus analysis:
// https://dhole.github.io/post/gameboy_cartridge_emu_1/

static void cart_address(const uint16_t address)
{
    spi_transfer(address >> 8);
    spi_transfer(address & 0xFF);
    writepin(PIN_STORE, HIGH);
    __asm__ volatile ("nop");
    writepin(PIN_STORE, LOW);
}

uint8_t cart_read(const uint16_t address)
{
    cart_address(address);
    __asm__ volatile ("nop");
    return PINS_DAT;
}

void cart_write(const uint16_t address, const uint8_t byte)
{
    cart_address(address);
    PORTB = (PORTB & ~PINS_DATB) | (byte >> 6);
    PORTD = (PORTD & ~PINS_DATD) | (byte << 2);
    writepin(PIN_WRITE, LOW);
    __asm__ volatile ("nop");
    writepin(PIN_WRITE, HIGH);
}

void cart_set_read(void)
{
    // Set the data pins as pull-up inputs
    DDRB &= ~PINS_DATB;
    DDRD &= ~PINS_DATD;
    PORTB |= PINS_DATB;
    PORTD |= PINS_DATD;

    writepin(PIN_READ, LOW);
    writepin(PIN_WRITE, HIGH);
}

void cart_set_write(void)
{
    // Set the data pins as low outputs
    DDRB |= PINS_DATB;
    DDRD |= PINS_DATD;
    PORTB &= ~PINS_DATB;
    PORTD &= ~PINS_DATD;

    writepin(PIN_READ, HIGH);
    writepin(PIN_WRITE, HIGH);
}

void cart_set_standby(void)
{
    // Set the data pins as pull-up inputs
    DDRB &= ~PINS_DATB;
    DDRD &= ~PINS_DATD;
    PORTB |= PINS_DATB;
    PORTD |= PINS_DATD;

    writepin(PIN_READ, HIGH);
    writepin(PIN_WRITE, HIGH);
}

void cart_select_rom(void)
{
    writepin(PIN_CS, HIGH);
}

void cart_select_ram(void)
{
    writepin(PIN_CS, LOW);
}

void cart_init(void)
{
    // Set the appropriate pins as output
    DDRB |= pin(PIN_STORE);
    DDRC |= pin(PIN_READ) | pin(PIN_WRITE) | pin(PIN_CS);

    // Initialize the cart as standby
    cart_set_standby();
    cart_select_rom();

    // Initialize the shift register
    writepin(PIN_STORE, LOW);
    spi_init(SPI_DIV8);
    cart_address(0);
}

void cart_off(void)
{
    cart_address(0);
    spi_off();
    PORTC &= ~pin(PIN_READ) & ~pin(PIN_CS) & ~pin(PIN_WRITE);
    writepin(PIN_STORE, LOW);
}

int cart_boot(struct cart_info *info)
{
    cart_set_read();

    // "simulate" the GB power-up process, by reading the logo once, before comparing.
    // This might be compatible with carts that hijack the logo on bootup.
    // There is no point in simulating this more accurately (i.e. timing)
    //   if we don't even simulate the RD/WR pulses properly.
    // http://gbdev.gg8.se/wiki/articles/Gameboy_Bootstrap_ROM

    // Read the logo once (to display on the screen)
    for (unsigned i = rom_offset(logo);
            i < rom_offset_end(logo); i++) cart_read(i);

    // Read the logo again (to verify it)
    for (unsigned i = rom_offset(logo); i < rom_offset_end(logo); i++) {
        if (rom_logo[i - rom_offset(logo)] != cart_read(i)) {
            cart_set_standby();
            return 1;
        }
    }

    // Read and verify the checksum (and save some info from the header)
    uint8_t checksum = rom_offset(header_checksum) - rom_offset(title);
    for (unsigned i = rom_offset(title);
            i < rom_offset_end(header_checksum); i++) {
        uint8_t c = cart_read(i);
        checksum += c;

        if (i == rom_offset(type)) info->type = c;
        if (i == rom_offset(rom_size)) info->rom_size = c;
        if (i == rom_offset(ram_size)) info->ram_size = c;
    }

    cart_set_standby();

    return checksum;
}

// https://ia801906.us.archive.org/19/items/GameBoyProgManVer1.1/GameBoyProgManVer1.1.pdf (page 215)

// NOTE: These functions only support MBC1, MBC2, MBC3 and MBC5.
// Thankfully, that covers most of the licensed carts.

void cart_rom_bank(__attribute__((unused)) struct cart_info *info, const unsigned bank)
{
    // MBC1: 0x2000-0x3FFF (bit 0-4) 0x4000-0x5FFF (bit 5-6, if 0x6000-0x7FFF is set to 0)
    // MBC2: 0x2100-0x21FF (bit 0-3)
    // MBC3: 0x2000-0x3FFF (bit 0-6)
    // MBC5: 0x2000-0x2FFF (bit 0-7) 0x3000-0x3FFF (bit 8)
    cart_set_write();

    if (info->type >= ROM_TYPE_MBC1 &&
            info->type <= ROM_TYPE_MBC1_RAM_BATTERY) {
        cart_write(0x2000, bank & 0x1F);
        cart_write(0x6000, 0);
        cart_write(0x4000, bank >> 5);
        return;
    }

    cart_write(0x2100, bank);
    if (info->type >= ROM_TYPE_MBC5 &&
            info->type <= ROM_TYPE_MBC5_RUMBLE_RAM_BATTERY) {
        cart_write(0x3000, bank >> 8);
    }
}

void cart_ram_enable(__attribute__((unused)) struct cart_info *info)
{
    // MBC1: 0x0000-0x1FFF = 0x0A
    // MBC2: 0x0000-0x0FFF = 0x0A
    // MBC3: 0x0000-0x1FFF = 0x0A
    // MBC5: 0x0000-0x1FFF = 0x0A
    cart_set_write();
    cart_write(0x0000, 0x0A);
    cart_select_ram();
}

void cart_ram_disable(__attribute__((unused)) struct cart_info *info)
{
    // MBC1: 0x0000-0x1FFF = 0x00
    // MBC2: 0x0000-0x0FFF = 0x00
    // MBC3: 0x0000-0x1FFF = 0x00
    // MBC5: 0x0000-0x1FFF = 0x00
    cart_set_write();
    cart_select_rom();
    cart_write(0x0000, 0x00);
}

void cart_ram_bank(__attribute__((unused)) struct cart_info *info, const unsigned bank)
{
    // MBC1: 0x4000-0x5FFF (bit 0-1, if 0x6000-0x7FFF is set to 1)
    // MBC2: None, it has only one bank. (Writing to 0x4000-0x5FFF should have no effect)
    // MBC3: 0x4000-0x5FFF (bit 0-1) NOTE: Writing values 0x08-0x0C opens up possible problems with the RTC!
    // MBC5: 0x4000-0x5FFF (bit 0-3)
    cart_set_write();

    if (info->type >= ROM_TYPE_MBC1 &&
            info->type <= ROM_TYPE_MBC1_RAM_BATTERY) {
        cart_write(0x6000, 1);
    }

    // Since writing 0x08-0x0C here opens up the timer,
    //   it opens up the possibility for corruption.
    // As such, it's a good idea to check for it...
    if (info->type >= ROM_TYPE_MBC3_TIMER_BATTERY &&
            info->type <= ROM_TYPE_MBC3_RAM_BATTERY) {
        cart_write(0x4000, bank & 0x07);
        return;
    }

    cart_write(0x4000, bank);
}