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Diffstat (limited to '1_hello_world/main.c')
| -rw-r--r-- | 1_hello_world/main.c | 332 |
1 files changed, 332 insertions, 0 deletions
diff --git a/1_hello_world/main.c b/1_hello_world/main.c new file mode 100644 index 0000000..5852765 --- /dev/null +++ b/1_hello_world/main.c @@ -0,0 +1,332 @@ +// 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]; + } +} + +void __start(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); +} |