reduce screen update to 1/3

apps/litenes/include/fce.h

@@ -14,6 +14,7 @@ void fce_run();
 void fce_update_screen();
 
 extern byte canvas[257][520];
+extern int frame_cnt;
 
 extern char rom_mario_nes[];
 

apps/litenes/include/ppu.h

@@ -56,7 +56,6 @@ typedef struct {
 extern PPU_STATE ppu;
 extern byte ppu_latch;
 extern bool ppu_sprite_hit_occured;
-extern byte ppu_screen_background[264][248];
 
 word ppu_get_real_ram_address(word address);
 

apps/litenes/src/fce.c

@@ -4,9 +4,10 @@
 #include <ppu.h>
 #include <klib.h>
 
-//#define NOGUI
+#define NOGUI
 
 int key_state[256];
+int frame_cnt;
 static byte *buf;
 
 typedef struct {
@@ -138,18 +139,17 @@ static uint32_t row[1024];
 
 void fce_update_screen()
 {
-  static int frame = 0;
   int idx = ppu_ram_read(0x3F00);
 
   int w = _screen.width;
   int h = _screen.height;
 
-  frame ++;
+  frame_cnt ++;
 #ifdef NOGUI
-  if (frame % 1000 == 0) printf("Frame %d (%d FPS)\n", frame, frame * 1000 / _uptime());
+  if (frame_cnt % 1000 == 0) printf("Frame %d (%d FPS)\n", frame_cnt, frame_cnt * 1000 / _uptime());
   return;
 #endif
-  if (frame % 2 != 0) return;
+  if (frame_cnt % 3 != 0) return;
 
   int pad = (w - h) / 2;
   for (int y = 0; y < h; y ++) {

apps/litenes/src/ppu.c

@@ -3,6 +3,7 @@
 #include <fce.h>
 #include <memory.h>
 #include <klib.h>
+#include <stdint.h>
 
 static const word ppu_base_nametable_addresses[4] = { 0x2000, 0x2400, 0x2800, 0x2C00 };
 
@@ -14,11 +15,13 @@ byte ppu_addr_latch;
 PPU_STATE ppu;
 byte ppu_latch;
 bool ppu_sprite_hit_occured = false;
-byte ppu_screen_background[264][248];
+byte ppu_screen_background[264][264];
 
 
 // preprocess tables
 static byte XHL[8][256][256];
+static uint64_t XHL64[256][256];
+static uint64_t XHLmask[256][256];
 static word ppu_ram_map[0x4000];
 
 static inline void draw(int col, int row, int idx) {
@@ -140,11 +143,17 @@ inline void ppu_ram_write(word address, byte data)
 
 // Rendering
 static void table_init() {
-  for (int x = 0; x < 8; x ++)
+  for (int x = 0; x < 8; x ++) {
     for (int h = 0; h < 256; h ++)
       for (int l = 0; l < 256; l ++) {
-        XHL[x][h][l] = (((h >> (7 - x)) & 1) << 1) | ((l >> (7 - x)) & 1);
+        int col = (((h >> (7 - x)) & 1) << 1) | ((l >> (7 - x)) & 1);
+        XHL[x][h][l] = col;
+        XHL64[h][l] |= (uint64_t)col << (x * 8);
+        if (col == 0) {
+          XHLmask[h][l] |= (uint64_t)0xff << (x * 8);
+        }
       }
+  }
 
   for (int x = 0; x < 0x4000; x ++) {
     ppu_ram_map[x] = ppu_get_real_ram_address(x);
@@ -157,6 +166,9 @@ void ppu_draw_background_scanline(bool mirror)
     int taddr = ppu_base_nametable_address() + (tile_y << 5) + (mirror ? 0x400 : 0);
     int y_in_tile = ppu.scanline & 0x7;
     int scroll_base = - ppu.PPUSCROLL_X + (mirror ? 256 : 0);
+
+    int palette_cache[4], do_update = frame_cnt % 3 == 0;
+
     word attribute_address = (ppu_base_nametable_address() + (mirror ? 0x400 : 0) + 0x3C0 +  -1 + ((ppu.scanline >> 5) << 3));
 
     for (tile_x = ppu_shows_background_in_leftmost_8px() ? 0 : 1; tile_x < 32; tile_x++) {
@@ -183,15 +195,22 @@ void ppu_draw_background_scanline(bool mirror)
         palette_attribute &= 3;
         word palette_address = 0x3F00 + (palette_attribute << 2);
 
-        for (int x = 0; x < 8; x ++) {
+        palette_cache[1] = ppu_ram_read(palette_address + 1);
-            byte color = XHL[x][h][l];
+        palette_cache[2] = ppu_ram_read(palette_address + 2);
-//            byte color = (((h >> (7 - x)) & 1) << 1) | ((l >> (7 - x)) & 1);
+        palette_cache[3] = ppu_ram_read(palette_address + 3);
-            if (color != 0) {
+
-                int idx = ppu_ram_read(palette_address + color);
+        if (do_update) {
-                draw(scroll_base + x, ppu.scanline + 1, idx);
+            for (int x = 0; x < 8; x ++) {
-                ppu_screen_background[(tile_x << 3) + x][ppu.scanline] = color;
+                byte color = XHL[x][h][l];
+                // lookup-table is much faster on x86.
+                // byte color = (((h >> (7 - x)) & 1) << 1) | ((l >> (7 - x)) & 1);
+                if (color != 0) {
+                    draw(scroll_base + x, ppu.scanline + 1, palette_cache[color]);
+                }
             }
         }
+        uint64_t *ptr = (uint64_t*)&ppu_screen_background[ppu.scanline][(tile_x << 3)];
+        *ptr = (XHL64[h][l]) | (XHLmask[h][l] & (*ptr)) ;
 
         taddr ++;
         scroll_base += 8;
@@ -245,7 +264,7 @@ void ppu_draw_sprite_scanline()
                 }
 
                 // Checking sprite 0 hit
-                if (ppu_shows_background() && !ppu_sprite_hit_occured && n == 0 && ppu_screen_background[screen_x][sprite_y + y_in_tile] == color) {
+                if (ppu_shows_background() && !ppu_sprite_hit_occured && n == 0 && ppu_screen_background[sprite_y + y_in_tile][screen_x] == color) {
                     ppu_set_sprite_0_hit(true);
                     ppu_sprite_hit_occured = true;
                 }