add bg_get_tile(), bg_get_tile_addr(), and bg_get_color()

2 files changed, 103 insertions, 18 deletions

diff --git a/gb.h b/gb.h
index e027ee7..358c1f2 100644
--- a/gb.h
+++ b/gb.h
@@ -124,7 +124,8 @@ struct gb_t_ {
     uint32_t frame;
 
     struct {
-      uint8_t x, y, tile;
+      int16_t x, y;
+      uint8_t tile;
       bool priority,
            y_flip,
            x_flip,
diff --git a/ops.yaml b/ops.yaml
index 26c1a87..a88e9e4 100644
--- a/ops.yaml
+++ b/ops.yaml
@@ -8088,6 +8088,7 @@ templates:
 
     #define UNUSED(a) ((void) (a))
     #define MIN(a, b) (((a) < (b)) ? (a) : (b))
+    #define BETWEEN(v, a, b) (((v) >= (a)) && ((v) <= (b)))
 
     // cpu flags
     #define FLAG(ctx, f) (cpu_rb(ctx, RB_F) & (F_##f))
@@ -10211,10 +10212,10 @@ templates:
         return;
       }
 
-      // decode oam
+      // decode and cache oam
       for (uint8_t i = 0; i < 40; i++) {
-        ctx->gpu.oam_cache[i].y = (ctx->mmu.oam[4 * i + 0] + 16) & 0xFF;
-        ctx->gpu.oam_cache[i].x = (ctx->mmu.oam[4 * i + 1] + 8) & 0xFF;
+        ctx->gpu.oam_cache[i].y = ctx->mmu.oam[4 * i + 0] - 10;
+        ctx->gpu.oam_cache[i].x = ctx->mmu.oam[4 * i + 1] - 8;
         ctx->gpu.oam_cache[i].tile = ctx->mmu.oam[4 * i + 2];
 
         const uint8_t flags = ctx->mmu.oam[4 * i + 3];
@@ -10228,6 +10229,87 @@ templates:
       ctx->gpu.oam_dirty = false;
     }
 
+    static uint8_t
+    bg_get_tile(
+      gb_t * const ctx,
+      const uint8_t ty,
+      const uint8_t tx
+    ) {
+      // calculate tile offset
+      return mmu_rb(ctx, (
+        // get bg tile map base
+        // (lcdc & (1 << 3))
+        ((ctx->gpu.lcdc & (1 << 3)) ? 0x9C00 : 0x9800) +
+
+        // calculate tile offset
+        ty * 32 + tx
+      ));
+    }
+
+    static uint16_t
+    bg_get_tile_addr(
+      gb_t * const ctx,
+      const uint8_t ty,
+      const uint8_t tx
+    ) {
+      // get tile
+      const uint8_t tile = bg_get_tile(ctx, ty, tx);
+
+      if (ctx->gpu.lcdc & (1 << 4)) {
+        // base: 0x8000, tile is unsigned
+        return 0x8000 + 16 * tile;
+      } else {
+        // base: 0x9000, tile is signed
+        return 0x9000 + 16 * ((int8_t) tile);
+      }
+    }
+
+    static uint8_t
+    bg_get_color(
+      gb_t * const ctx,
+      const uint8_t sy,
+      const uint8_t sx
+    ) {
+      // TODO: draw background
+      const uint8_t y = (ctx->gpu.scy + sy) & 0xFF,
+                    ty = y >> 3,
+                    py = y - (ty << 3);
+
+      const uint8_t x = (ctx->gpu.scx + sx) & 0xFF,
+                    tx = x >> 3,
+                    px = x - (tx << 3);
+
+      // get tile data address
+      const uint16_t addr = bg_get_tile_addr(ctx, ty, tx);
+
+      // get tile data
+      const uint16_t data = mmu_rw(ctx, addr + (2 * py));
+
+      // get bg palette entry
+      const uint8_t p_ofs = (
+        ((data >> (8 + (7 - px)) & 0x01) << 1) |
+        ((data >> (7 - px)) & 0x01)
+      );
+
+      // get pixel color
+      const uint8_t pc = (ctx->gpu.bgp >> (2 * p_ofs)) & 0x03;
+
+      // convert from 0-3 to RGB
+      switch (pc) {
+      case 0:
+        return 0xFF;
+      case 1:
+        return 0xC0;
+      case 2:
+        return 0x40;
+      case 3:
+        return 0x00;
+      }
+
+      // never reached
+      return 0xA0;
+    }
+
     static void
     gpu_draw_line(
       gb_t * const ctx
@@ -10242,12 +10324,11 @@ templates:
             ctx->gpu.oam_cache[i].priority &&
 
             // sprite is on screen
-            ctx->gpu.oam_cache[i].y < 144 &&
-            ctx->gpu.oam_cache[i].x < 160 &&
+            BETWEEN(ctx->gpu.oam_cache[i].y, 0, 144) &&
+            BETWEEN(ctx->gpu.oam_cache[i].x, 0, 160) &&
 
             // sprite is on current line
-            ctx->gpu.line >= ctx->gpu.oam_cache[i].y &&
-            ctx->gpu.line <= ctx->gpu.oam_cache[i].y
+            BETWEEN(ctx->gpu.line, ctx->gpu.oam_cache[i].y, ctx->gpu.oam_cache[i].y + 8)
           ) {
             const uint8_t max_j = MIN(8, 160 - ctx->gpu.oam_cache[i].x);
 
@@ -10262,12 +10343,16 @@ templates:
           }
         }
 
-        // TODO: draw background
-        for (uint8_t i = 0; i < 160; i++) {
-          // interim: set line to red
-          ctx->gpu.fb[3 * (ctx->gpu.line * 160 + i) + 0] = 0xff;
-          ctx->gpu.fb[3 * (ctx->gpu.line * 160 + i) + 1] = 0x00;
-          ctx->gpu.fb[3 * (ctx->gpu.line * 160 + i) + 2] = 0x00;
+        if (ctx->gpu.lcdc & 1) {
+          for (uint8_t i = 0; i < 160; i++) {
+            // get pixel color
+            const uint8_t c = bg_get_color(ctx, ctx->gpu.line, i);
+
+            // populate framebuffer
+            ctx->gpu.fb[3 * (ctx->gpu.line * 160 + i) + 0] = c;
+            ctx->gpu.fb[3 * (ctx->gpu.line * 160 + i) + 1] = c;
+            ctx->gpu.fb[3 * (ctx->gpu.line * 160 + i) + 2] = c;
+          }
         }
 
         // draw foreground sprites
@@ -10277,12 +10362,11 @@ templates:
             !ctx->gpu.oam_cache[i].priority &&
 
             // sprite is on screen
-            ctx->gpu.oam_cache[i].y < 144 &&
-            ctx->gpu.oam_cache[i].x < 160 &&
+            BETWEEN(ctx->gpu.oam_cache[i].y, 0, 144) &&
+            BETWEEN(ctx->gpu.oam_cache[i].x, 0, 160) &&
 
             // sprite is on current line
-            ctx->gpu.line >= ctx->gpu.oam_cache[i].y &&
-            ctx->gpu.line <= ctx->gpu.oam_cache[i].y
+            BETWEEN(ctx->gpu.line, ctx->gpu.oam_cache[i].y, ctx->gpu.oam_cache[i].y + 8)
           ) {
             const uint8_t max_j = MIN(8, 160 - ctx->gpu.oam_cache[i].x);