Use mode 0 to render some background tiles and scroll around.

scratch/scratch.c

@@ -1,10 +1,56 @@
 #include <gba.h>
 
-static void fill_screen(u16 colour) {
-#define VID_BUF ((u16*)VRAM)
-  // memset() or Tonc's memset16() would be better?
-  for (int pix_idx = 0; pix_idx < SCREEN_WIDTH * SCREEN_HEIGHT; ++pix_idx) {
-    VID_BUF[pix_idx] = colour;
+
+void init_gfx() {
+  // Set the video mode to 0, enable bg 0, enable objects.
+  SetMode(MODE_0 | BG0_ENABLE | OBJ_ENABLE);
+
+  // Initialised bg0 parameters:
+  // - char block 0
+  // - screen block 28
+  // - 64x64 tiles, 512x512 px
+#define SCR_BASE_BLOCK 28
+  BGCTRL[0] = CHAR_BASE(0) | SCREEN_BASE(SCR_BASE_BLOCK) | BG_SIZE_3;
+
+  // Init bg0 scroll to the origin.
+  REG_BG0HOFS = 0;
+  REG_BG0VOFS = 0;
+
+  // (1) Create 2 4bpp tiles: 1x basic tile and 1x cross
+  const u32 tiles[2][8] = {
+    { 0x11111111,
+      0x01111111,
+      0x01111111,
+      0x01111111,
+      0x01111111,
+      0x01111111,
+      0x01111111,
+      0x00000001 },
+
+    { 0x00000000,
+      0x00100100,
+      0x01100110,
+      0x00011000,
+      0x00011000,
+      0x01100110,
+      0x00100100,
+      0x00000000 },
+  };
+  CpuFastSet(tiles[0], CHAR_BASE_ADR(0), 16 | COPY16);
+  CpuFastSet(tiles[1], CHAR_BASE_ADR(1), 16 | COPY16);
+
+  // (2) Create 4 separate palettes, each with empty index 0 and a different colour at index 1.
+  BG_PALETTE[0 * 16 + 1] = RGB5(31, 0, 0);     // Red
+  BG_PALETTE[1 * 16 + 1] = RGB5(0, 31, 0);     // Green
+  BG_PALETTE[2 * 16 + 1] = RGB5(0, 0, 31);     // Blue
+  BG_PALETTE[3 * 16 + 1] = RGB5(16, 16, 16);   // Grey
+
+  // (3) Create a map, 4 blocks of tiles, each using a different palette.
+  u16* screen_block = SCREEN_BASE_BLOCK(SCR_BASE_BLOCK);
+  for (int block_idx = 0; block_idx < 4; block_idx++) {
+    for (int tile_idx = 0; tile_idx < 1024; tile_idx++) {
+      screen_block[block_idx * 1024 + tile_idx] = CHAR_PALETTE(block_idx) | 0;
+    }
   }
 }
 
@@ -13,34 +59,36 @@ int main() {
   irqEnable(IRQ_VBLANK);
   REG_IME = 1;
 
-  SetMode(MODE_3 | BG2_ENABLE);
+  init_gfx();
+
+  u16 scroll_x = 0;
+  u16 scroll_y = 0;
 
-  u16 colour = 0;
   while(1) {
     VBlankIntrWait();
 
     scanKeys();
-    u16 keys_down = keysDown();
     u16 keys_up = keysUp();
 
-    if (keys_down & KEY_A) {
-      colour |= 0x001f;
+    if (keys_up & KEY_START) {
+      break;
     }
-    if (keys_up & KEY_A) {
-      colour &= ~0x001f;
+
+    if (keys_up & KEY_UP) {
+      scroll_y -= 32;
     }
-    if (keys_down & KEY_B) {
-      colour |= (0x1f << 10);
+    if (keys_up & KEY_DOWN) {
+      scroll_y += 32;
     }
-    if (keys_up & KEY_B) {
-      colour &= ~(0x001f << 10);
+    if (keys_up & KEY_LEFT) {
+      scroll_x -= 32;
     }
-
-    fill_screen(colour);
-
-    if (keys_down & KEY_START) {
-      break;
+    if (keys_up & KEY_RIGHT) {
+      scroll_x += 32;
     }
+
+    REG_BG0HOFS = scroll_x;
+    REG_BG0VOFS = scroll_y;
   }
 
   Stop();