neo homebrew sample

helloworld.c

#include <stdint.h>
#include <stdbool.h>

// Reg defines, to extract to some header

#define REG_VRAMADDR (*((volatile uint16_t *)0x3c0000))
#define REG_VRAMDATA (*((volatile uint16_t *)0x3c0002))
#define REG_VRAMINC (*((volatile uint16_t *)0x3c0004))
#define REG_LSPCMODE (*((volatile uint16_t *)0x3c0006))

#define REG_INT_TIMER (*((volatile uint32_t *)0x3c0008))
#define REG_INT_TIMER_HI (*((volatile uint16_t *)0x3c0008))
#define REG_INT_TIMER_LO (*((volatile uint16_t *)0x3c000a))

#define REG_INT_TIMER_PAUSE (*((volatile uint16_t *)0x3c000e))

// ---

#define REG_WATCHDOG (*((volatile uint8_t *)0x300001))

// ---

#define PALETTE ((volatile uint16_t *)0x400000)

// ---

// Quick and dirty prog headers
// These would mostly be handled by the linker script rather than being fully defined here

typedef struct M68kVectors {
    uint32_t reset_sp;
    uint32_t irq_vectors[63];
} __attribute__((packed)) M68kVectors;

void user_entry(void);

typedef struct ProgHeader {
    uint8_t recognition_code_1[7];
    uint8_t system_type;
    uint16_t ngh;
    uint32_t prog_size;

    uint32_t wram_backup;
    uint16_t wram_backup_size;

    uint8_t eyectacher_config;
    uint8_t eyectacher_sprite_bank;

    const uint8_t *jp_softdips;
    const uint8_t *us_softdips;
    const uint8_t *eu_softdips;

    uint16_t user_jmp_op;
    const void *user_entry;

    // ...
    uint8_t padding[0x5a];
    // ...

    const void *recognition_code_2;
} __attribute__((packed)) ProgHeader;

__attribute__((used))
__attribute__ ((section (".vectors")))
static const M68kVectors vectors = {
    .reset_sp = 0x010f300,
    .irq_vectors = {
        // BERR
        // ...
        // VBL etc...
        // ...
    }
};

static const uint32_t recognition_code_2[] = {
    0x76004A6D, 0x0A146600, 0x003C206D, 0x0A043E2D,
    0x0A0813C0, 0x00300001, 0x32100C01, 0x00FF671A,
    0x30280002, 0xB02D0ACE, 0x66103028, 0x0004B02D,
    0x0ACF6606, 0xB22D0AD0, 0x67085088, 0x51CFFFD4,
    0x36074E75, 0x206D0A04, 0x3E2D0A08, 0x3210E049,
    0x0C0100FF, 0x671A3010, 0xB02D0ACE, 0x66123028,
    0x0002E048, 0xB02D0ACF, 0x6606B22D, 0x0AD06708,
    0x588851CF, 0xFFD83607,
    0x4e750000
};

__attribute__((used))
__attribute__ ((section (".prog_header")))
static const ProgHeader header = {
    .recognition_code_1 = { 'N', 'E', 'O', '-', 'G', 'E', 'O'},
    .system_type = 0,
    .ngh = 0x4040,
    .recognition_code_2 = recognition_code_2,

    .eyectacher_config = 2,

    .user_jmp_op = 0x4ef9,
    .user_entry = user_entry
};

// ---

void vram_clear(uint16_t address, uint16_t length, uint16_t data) {
    REG_WATCHDOG = 0;

    REG_VRAMADDR = address;
    REG_VRAMINC = 1;
    for (uint16_t i = 0; i < length; i++) {
        REG_VRAMDATA = data;
    }

    REG_WATCHDOG = 0;
}

void fix_clear() {
    vram_clear(0x7000, 0x500, ' ');
}

void pal_clear() {
    REG_WATCHDOG = 0;

    for (uint16_t i = 0; i < 0x1000; i++) {
        PALETTE[i] = 0x8000;
    }

    REG_WATCHDOG = 0;
}

void pal_write(uint8_t palette, uint8_t index, uint16_t data) {
    PALETTE[palette * 0x10 + index] = data;
}

uint16_t fix_location_vram(uint8_t x, uint8_t y) {
    return 0x7000 + x * 0x20 + y;
}

void fix_write(uint8_t x, uint8_t y, const char *string) {
    REG_VRAMINC = 0x20;

    char c = 0;
    const char *source = string;

    // Top half of 8x16 glyph
    REG_VRAMADDR = fix_location_vram(x, y);

    while (c = (*source++)) {
        REG_VRAMDATA = c | 0x100;
    }

    // Bottom half of 8x16 glyph
    REG_VRAMADDR = fix_location_vram(x, y + 1);
    source = string;

    while (c = (*source++)) {
        REG_VRAMDATA = c | 0x200;
    }
}

__attribute__((noreturn))
void user_entry() {
    vram_clear(0x0000, 0x8000, 0x0000);
    fix_clear();
    pal_clear();

    pal_write(0, 1, 0x7fff);
    pal_write(0, 2, 0x8000);

    const char *greeting = "Hello world!";
    const uint8_t x = 14;
    const uint8_t y = 14;
    fix_write(x, y, greeting);

    while (true) {
        REG_WATCHDOG = 0;
    }

    __builtin_unreachable();
}