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Diffstat (limited to 'main.c')
| -rw-r--r-- | main.c | 332 |
1 files changed, 0 insertions, 332 deletions
@@ -1,332 +0,0 @@ -// 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); -} |