Background rendering completion; implement backdrop override

src/ppu.c

@@ -61,7 +61,12 @@ void nes_ppu_render_line(nes_PPU* ppu, nes_PPU_Memory* mem) {
     uint8_t* ptr = ppu->line_data;
 
     if (!(mem->mask & ppu_Mask_Back)) {
-        memset(ptr, 0, nes_ppu_render_w);
+        // Emulate the happy part of the backdrop override quirk
+        int pal_idx = ((mem->addr & 0x3F00U) == 0x3F00U) ?
+                      (mem->addr & 0x1FU) : 0;
+        // Don't use the rendering palette (masked transparency)
+        memset(ptr, mem->pal_bank[0x300 + pal_idx],
+               nes_ppu_render_w);
     } else {
         int back_bank = nes_PPU_Nametable_Bank_Index +
                    ((mem->addr >> 10) & 3);
@@ -75,32 +80,33 @@ void nes_ppu_render_line(nes_PPU* ppu, nes_PPU_Memory* mem) {
         const uint8_t* nametable = mem->bank[back_bank] + (y_coarse * 32) + x_coarse;
         const uint8_t* attrs = mem->bank[back_bank] + 0x3C0U + ((y_coarse >> 2) << 3);
 
+        // Partial left column
         ptr += 8 - mem->x;
 
-        // TODO: Omit if left column is masked?
-        {
-        const uint8_t* pal = mem->palette +
-                (((attrs[x_coarse >> 2] >>
-                    (   (x_coarse & 2) +
-                        ((y_coarse & 2) << 1))) & 3) << 2);
-        const int ch = *nametable;
-        const int bank = (ch >> 6) + bank_off;
-        const int addr_off = ((ch & 0x3F) << 4) + y_fine;
-        const uint8_t* data = mem->bank[bank] + addr_off;
-        const uint8_t pl0 = data[0];
-        const uint8_t pl1 = data[8];
-        const int pat0 = (pl0 & 0x55) | ((pl1 << 1) & 0xAA);
-        const int pat1 = ((pl0 >> 1) & 0x55) | (pl1 & 0xAA);
-        switch (mem->x) {
-            case 0: ptr[-8] = pal[(pat1 >> 6) & 3];
-            case 1: ptr[-7] = pal[(pat0 >> 6) & 3];
-            case 2: ptr[-6] = pal[(pat1 >> 4) & 3];
-            case 3: ptr[-5] = pal[(pat0 >> 4) & 3];
-            case 4: ptr[-4] = pal[(pat1 >> 2) & 3];
-            case 5: ptr[-3] = pal[(pat0 >> 2) & 3];
-            case 6: ptr[-2] = pal[(pat1 >> 0) & 3];
-            case 7: ptr[-1] = pal[(pat0 >> 0) & 3];
-        }
+        // Omit if left column is masked
+        if (mem->mask & ppu_Mask_Left_Back) {
+            const uint8_t* pal = mem->palette +
+                    (((attrs[x_coarse >> 2] >>
+                        (   (x_coarse & 2) +
+                            ((y_coarse & 2) << 1))) & 3) << 2);
+            const int ch = *nametable;
+            const int bank = (ch >> 6) + bank_off;
+            const int addr_off = ((ch & 0x3F) << 4) + y_fine;
+            const uint8_t* data = mem->bank[bank] + addr_off;
+            const uint8_t pl0 = data[0];
+            const uint8_t pl1 = data[8];
+            const int pat0 = (pl0 & 0x55) | ((pl1 << 1) & 0xAA);
+            const int pat1 = ((pl0 >> 1) & 0x55) | (pl1 & 0xAA);
+            switch (mem->x) {
+                case 0: ptr[-8] = pal[(pat1 >> 6) & 3];
+                case 1: ptr[-7] = pal[(pat0 >> 6) & 3];
+                case 2: ptr[-6] = pal[(pat1 >> 4) & 3];
+                case 3: ptr[-5] = pal[(pat0 >> 4) & 3];
+                case 4: ptr[-4] = pal[(pat1 >> 2) & 3];
+                case 5: ptr[-3] = pal[(pat0 >> 2) & 3];
+                case 6: ptr[-2] = pal[(pat1 >> 0) & 3];
+                case 7: ptr[-1] = pal[(pat0 >> 0) & 3];
+            }
         }
 
         // TODO: Mapper ppu_bus
@@ -119,8 +125,6 @@ void nes_ppu_render_line(nes_PPU* ppu, nes_PPU_Memory* mem) {
             const uint8_t* data = mem->bank[bank] + addr_off;
             const uint8_t pl0 = data[0];
             const uint8_t pl1 = data[8];
-            //const int pat0 = ((pl0 & 0x55) << 1) | ((pl1 & 0x55) << 2);
-            //const int pat1 = ((pl0 & 0xAA) << 0) | ((pl1 & 0xAA) << 1);
             const int pat0 = (pl0 & 0x55) | ((pl1 << 1) & 0xAA);
             const int pat1 = ((pl0 >> 1) & 0x55) | (pl1 & 0xAA);
 
@@ -135,12 +139,14 @@ void nes_ppu_render_line(nes_PPU* ppu, nes_PPU_Memory* mem) {
             ptr += 8;
         }
 
+        // Left Nametable
         for (; x_coarse < 32; ++x_coarse) {
             render_bg();
             // TODO: Mapper ppu_bus
             ++nametable;
         }
 
+        // Right Nametable
         back_bank ^= 0b01;
         nametable = mem->bank[back_bank] + (y_coarse * 32);
         attrs = mem->bank[back_bank] + 0x3C0U + ((y_coarse >> 2) << 3);
@@ -150,6 +156,38 @@ void nes_ppu_render_line(nes_PPU* ppu, nes_PPU_Memory* mem) {
             // TODO: Mapper ppu_bus
             ++nametable;
         }
+
+        // Partial right column
+        {
+            const uint8_t* pal = mem->palette +
+                    (((attrs[x_coarse >> 2] >>
+                        (   (x_coarse & 2) +
+                            ((y_coarse & 2) << 1))) & 3) << 2);
+            const int ch = *nametable;
+            const int bank = (ch >> 6) + bank_off;
+            const int addr_off = ((ch & 0x3F) << 4) + y_fine;
+            const uint8_t* data = mem->bank[bank] + addr_off;
+            const uint8_t pl0 = data[0];
+            const uint8_t pl1 = data[8];
+            const int pat0 = (pl0 & 0x55) | ((pl1 << 1) & 0xAA);
+            const int pat1 = ((pl0 >> 1) & 0x55) | (pl1 & 0xAA);
+            switch (mem->x) {
+                case 8: ptr[7] = pal[(pat0 >> 0) & 3];
+                case 7: ptr[6] = pal[(pat1 >> 0) & 3];
+                case 6: ptr[5] = pal[(pat0 >> 2) & 3];
+                case 5: ptr[4] = pal[(pat1 >> 2) & 3];
+                case 4: ptr[3] = pal[(pat0 >> 4) & 3];
+                case 3: ptr[2] = pal[(pat1 >> 4) & 3];
+                case 2: ptr[1] = pal[(pat0 >> 6) & 3];
+                case 1: ptr[0] = pal[(pat1 >> 6) & 3];
+            }
+        }
+    }
+
+    // TODO: Mapper ppu_bus
+
+    if (!(mem->mask & ppu_Mask_Left_Back)) {
+        memset(ppu->line_data, mem->palette[0], 8);
     }
 
     // TODO: Draw Sprites