path: root/1_hello_world/main.c

// Rewritten from https://github.com/BigEvilCorporation/megadrive_samples/blob/master/1_hello_world/hello.asm

#include <stdint.h>
#include <stddef.h>

#define VDP_DATA                ((volatile uint16_t*)0x00C00000L)
#define VDP_CONTROL             ((volatile uint16_t*)0x00C00004L)
#define VDP_CMD_VRAM_WRITE      ((uint32_t)0x40000000L)
#define VDP_CMD_CRAM_WRITE      ((uint32_t)0xC0000000L)
#define VRAM_ADDR_TILES         (0x0000)
#define VRAM_ADDR_PLANE_A       (0xC000)
#define VRAM_ADDR_PLANE_B       (0xE000)
#define VDP_SCREEN_WIDTH        (0x0140)
#define VDP_SCREEN_HEIGHT       (0x00F0)
#define VDP_PLANE_WIDTH         (0x40)
#define VDP_PLANE_HEIGHT        (0x20)
#define HARDWARE_VER_ADDRESS    ((uint8_t*)0x00A10001L)
#define TMSS_ADDRESS            ((uint32_t*)0x00A14000L)
#define TMSS_SIGNATURE          (('S' << 24) | ('E' << 16) | ('G' << 8) | 'A')
#define SIZE_WORD               (2)
#define SIZE_LONG               (4)
#define SIZE_PALETTE_B          (0x20)
#define SIZE_PALETTE_W          (SIZE_PALETTE_B/SIZE_WORD)
#define SIZE_PALETTE_L          (SIZE_PALETTE_B/SIZE_LONG)
#define SIZE_TILE_B             (0x20)
#define SIZE_TILE_W             (SIZE_TILE_B/SIZE_WORD)
#define SIZE_TILE_L             (SIZE_TILE_B/SIZE_LONG)
#define TEXT_POS_X              (0x08)
#define TEXT_POS_Y              (0x04)

// ==============================================================
//  TILE IDs
// ==============================================================
//  The indices of each tile above. Once the tiles have been
//  written to VRAM, the VDP refers to each tile by its index.
// ==============================================================
#define TILE_ID_SPACE (0x0)
#define TILE_ID_H     (0x1)
#define TILE_ID_E     (0x2)
#define TILE_ID_L     (0x3)
#define TILE_ID_O     (0x4)
#define TILE_ID_W     (0x5)
#define TILE_ID_R     (0x6)
#define TILE_ID_D     (0x7)
#define TILE_COUNT    (0x8) // Last entry is just the count

static uint8_t vdp_registers[] = {
    0x14, // 0x00:  H interrupt on, palettes on
    0x74, // 0x01:  V interrupt on, display on, DMA on, Genesis mode on
    0x30, // 0x02:  Pattern table for Scroll Plane A at VRAM 0xC000 (bits 3-5 = bits 13-15)
    0x00, // 0x03:  Pattern table for Window Plane at VRAM 0x0000 (disabled) (bits 1-5 = bits 11-15)
    0x07, // 0x04:  Pattern table for Scroll Plane B at VRAM 0xE000 (bits 0-2 = bits 11-15)
    0x78, // 0x05:  Sprite table at VRAM 0xF000 (bits 0-6 = bits 9-15)
    0x00, // 0x06:  Unused
    0x00, // 0x07:  Background colour: bits 0-3 = colour, bits 4-5 = palette
    0x00, // 0x08:  Unused
    0x00, // 0x09:  Unused
    0x08, // 0x0A: Frequency of Horiz. interrupt in Rasters (number of lines travelled by the beam)
    0x00, // 0x0B: External interrupts off, V scroll fullscreen, H scroll fullscreen
    0x81, // 0x0C: Shadows and highlights off, interlace off, H40 mode (320 x 224 screen res)
    0x3F, // 0x0D: Horiz. scroll table at VRAM 0xFC00 (bits 0-5)
    0x00, // 0x0E: Unused
    0x02, // 0x0F: Autoincrement 2 bytes
    0x01, // 0x10: Scroll plane size: 64x32 tiles
    0x00, // 0x11: Window Plane X pos 0 left (pos in bits 0-4, left/right in bit 7)
    0x00, // 0x12: Window Plane Y pos 0 up (pos in bits 0-4, up/down in bit 7)
    0xFF, // 0x13: DMA length lo byte
    0xFF, // 0x14: DMA length hi byte
    0x00, // 0x15: DMA source address lo byte
    0x00, // 0x16: DMA source address mid byte
    0x80, // 0x17: DMA source address hi byte, memory-to-VRAM mode (bits 6-7)
};

static uint16_t palette[SIZE_PALETTE_W] = {
    0x0000, // Colour 0 = Transparent
    0x0000, // Colour 1 = Black
    0x0EEE, // Colour 2 = White
    0x000E, // Colour 3 = Red
    0x00E0, // Colour 4 = Blue
    0x0E00, // Colour 5 = Green
    0x0E0E, // Colour 6 = Pink
    0x0000, // Leave the rest black...
    0x0000,
    0x0000,
    0x0000,
    0x0000,
    0x0000,
    0x0000,
    0x0000,
    0x0000,
};

static uint32_t characters[TILE_COUNT * SIZE_TILE_L] = {
    // Space
    0x00000000L,
    0x00000000L,
    0x00000000L,
    0x00000000L,
    0x00000000L,
    0x00000000L,
    0x00000000L,
    0x00000000L,
    // H
    0x22000220L,
    0x22000220L,
    0x22000220L,
    0x22222220L,
    0x22000220L,
    0x22000220L,
    0x22000220L,
    0x00000000L,
    // E
    0x22222220L,
    0x22000000L,
    0x22000000L,
    0x22222220L,
    0x22000000L,
    0x22000000L,
    0x22222220L,
    0x00000000L,
    // L
    0x22000000L,
    0x22000000L,
    0x22000000L,
    0x22000000L,
    0x22000000L,
    0x22000000L,
    0x22222220L,
    0x00000000L,
    // O
    0x22222220L,
    0x22000220L,
    0x22000220L,
    0x22000220L,
    0x22000220L,
    0x22000220L,
    0x22222220L,
    0x00000000L,
    // W
    0x22000220L,
    0x22000220L,
    0x22000220L,
    0x22020220L,
    0x22020220L,
    0x22020220L,
    0x22222220L,
    0x00000000L,
    // R
    0x22222200L,
    0x22000220L,
    0x22000220L,
    0x22222200L,
    0x22022000L,
    0x22002200L,
    0x22000220L,
    0x00000000L,
    // D
    0x22222200L,
    0x22002220L,
    0x22000220L,
    0x22000220L,
    0x22000220L,
    0x22002220L,
    0x22222200L,
    0x00000000L,
};

// Set the VRAM (video RAM) address to write to next
static inline void SetVRAMWrite(uint16_t addr)
{
    *(volatile uint32_t*)VDP_CONTROL = VDP_CMD_VRAM_WRITE | ((addr & 0x3FFF) << 16) | (addr >> 14);
}

// Set the CRAM (colour RAM) address to write to next
static inline void SetCRAMWrite(uint16_t addr)
{
    *(volatile uint32_t*)VDP_CONTROL = VDP_CMD_CRAM_WRITE | ((addr & 0x3FFF) << 16) | (addr >> 14);
}

static void VDP_WriteTMSS(void)
{
    // The TMSS (Trademark Security System) locks up the VDP if we don't
    // write the string 'SEGA' to a special address. This was to discourage
    // unlicensed developers, since doing this displays the "LICENSED BY SEGA
    // ENTERPRISES LTD" message to screen (on Mega Drive models 1 and higher).
    //
    // First, we need to check if we're running on a model 1+, then write
    // 'SEGA' to hardware address 0xA14000.
    const uint8_t ver = (*HARDWARE_VER_ADDRESS) & 0x0f;
    if (ver != 0) {
        *TMSS_ADDRESS = TMSS_SIGNATURE;
    }

    // Check VDP
    // Read VDP status register (hangs if no access)
    *VDP_CONTROL;
}

static void VDP_LoadRegisters(void)
{
    // To initialise the VDP, we write all of its initial register values from
    // the table at the top of the file, using a loop.
    //
    // To write a register, we write a word to the control port.
    // The top bit must be set to 1 (so 0x8000), bits 8-12 specify the register
    // number to write to, and the bottom byte is the value to set.
    //
    // In binary:
    //   100X XXXX YYYY YYYY
    //   X = register number
    //   Y = value to write

    // Set VDP registers
    for (size_t i = 0; i < sizeof(vdp_registers) / sizeof(*vdp_registers); i++) {
        const uint16_t cmd = 0x8000; // 'Set register 0' command
        const uint16_t reg_num = i << 8;
        *VDP_CONTROL = cmd | reg_num | vdp_registers[i];
    }
}

int main(void)
{
    // ==============================================================
    //  Initialise the Mega Drive
    // ==============================================================

    // Write the TMSS signature (if a model 1+ Mega Drive)
    VDP_WriteTMSS();

    // Load the initial VDP registers
    VDP_LoadRegisters();

    //==============================================================
    // Clear VRAM (video memory)
    //==============================================================

    // Setup the VDP to write to VRAM address 0x0000 (start of VRAM)
    SetVRAMWrite(0x0000);

    // Write 0's across all of VRAM
    const size_t count = (0x00010000 / SIZE_WORD); // Loop counter = 64kb, in words
    for (size_t i = 0; i < count; i++) {
        *VDP_DATA = 0x0; // Write a 0x0000 (word size) to VRAM
    }

    //==============================================================
    // Initialise status register and set interrupt level.
    // This begins firing vertical and horizontal interrupts.
    //==============================================================
    asm inline volatile ("  move.w #0x2300, %sr");

    //==============================================================
    // Write the palette to CRAM (colour memory)
    //==============================================================

    // Setup the VDP to write to CRAM address 0x0000 (first palette)
    SetCRAMWrite(0x0000);

    // Write the palette to CRAM
    for (size_t i = 0; i < SIZE_PALETTE_W; i++) {
        *VDP_DATA = palette[i]; // Write palette entry
    }

    //==============================================================
    // Write the font tiles to VRAM
    //==============================================================

    // Setup the VDP to write to VRAM address 0x0000 (the address of the first graphics tile, index 0)
    SetVRAMWrite(VRAM_ADDR_TILES);

    // Write the font glyph tiles to VRAM
    for (size_t i = 0; i < TILE_COUNT * SIZE_TILE_L; i++) {
        *(volatile uint32_t*)VDP_DATA = characters[i]; // Write palette entry
    }

    //==============================================================
    // Write the tile IDs of "HELLO WORLD" to Plane A's cell grid
    //==============================================================

    // Each scroll plane is made up of a 64x32 tile grid (this size is specified in VDP register 0x10),
    // with each cell specifying the index of each tile to draw, the palette to draw it with, and
    // some flags (for priority and flipping).
    //
    // Each plane cell is 1 word in size (2 bytes), in the binary format
    // ABBC DEEE EEEE EEEE, where:
    //
    //   A = Draw priority (1 bit)
    //   B = Palette index (2 bits, specifies palette 0, 1, 2, or 3)
    //   C = Flip tile horizontally (1 bit)
    //   D = Flip tile vertically (1 bit)
    //   E = Tile index to draw (11 bits, specifies tile index from 0 to 2047)
    //
    // Since we're using priority 0, palette 0, and no flipping, we
    // only need to write the tile ID and leave everything else zero.

    // Setup the VDP to write the tile ID at text_pos_x,text_pos_y in plane A's cell grid.
    // Plane A's cell grid starts at address 0xC000, which is specified in VDP register 0x2.
    //
    // Since each cell is 1 word in size, to compute a cell address within plane A:
    // ((y_pos * plane_width) + x_pos) * size_word
    SetVRAMWrite(VRAM_ADDR_PLANE_A + (((TEXT_POS_Y * VDP_PLANE_WIDTH) + TEXT_POS_X) * SIZE_WORD));

    // then move the tile ID for "H" to VRAM
    *VDP_DATA = TILE_ID_H;

    // Repeat for the remaining characters in the string.
    // We don't need to adjust the VRAM address each time, since the auto-increment
    // register (VDP register 0xF) is set to 2, so the destination address
    // will automatically increment by one word (conveniently the size of a cell)
    // after each write.
    *VDP_DATA = TILE_ID_E;
    *VDP_DATA = TILE_ID_L;
    *VDP_DATA = TILE_ID_L;
    *VDP_DATA = TILE_ID_O;
    *VDP_DATA = TILE_ID_SPACE;
    *VDP_DATA = TILE_ID_W;
    *VDP_DATA = TILE_ID_O;
    *VDP_DATA = TILE_ID_R;
    *VDP_DATA = TILE_ID_L;
    *VDP_DATA = TILE_ID_D;

    // Finished!

    //==============================================================
    // Loop forever
    //==============================================================
    // This loops forever, effectively ending our code. The VDP will
    // still continue to run (and fire vertical/horizontal interrupts)
    // of its own accord, so it will continue to render our Hello World
    // even though the CPU is stuck in this loop.
    while (1);
}