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src/dusksaturn/display/displaysaturn.c

@@ -25,12 +25,29 @@ errorret_t displaySaturnInit(void) {
     VDP2_TVMD_HORZ_NORMAL_A,
     VDP2_TVMD_VERT_224
   );
-  vdp2_tvmd_display_set();
+
+  /* Back-screen color shown where no VDP1/VDP2 plane draws a pixel.
+   * MSB=1 so VDP2 treats it as direct RGB in mixed-color mode. */
+  vdp2_scrn_back_color_set(
+    VDP2_VRAM_ADDR(3, 0x01FFFE),
+    RGB1555(1, 0, 3, 15)
+  );
+
+  /* Sprite priority 0 → invisible in VDP2.  Set slot 0 to priority 6 so
+   * VDP1 pixels are composited above the back-screen. */
+  vdp2_sprite_priority_set(0, 6);
 
   /* Disable all NBG/RBG scroll planes; game content drawn entirely via VDP1. */
   vdp2_scrn_display_set(VDP2_SCRN_DISP_NONE);
 
+  vdp2_tvmd_display_set();
+
   errorChain(renderSaturnInit());
+
+  /* Commit all VDP2 shadow registers to hardware before the first frame. */
+  vdp2_sync();
+  vdp2_sync_wait();
+
   errorOk();
 }
 
@@ -45,6 +62,12 @@ errorret_t displaySaturnSwap(void) {
   vdp1_sync();
   vdp2_sync();
   vdp2_sync_wait();
+  /* Wait for VDP1's VBlank-OUT handler to complete.  In auto mode VDP1 begins
+   * rendering at VBlank-IN; vdp2_sync_wait() returns shortly after that point,
+   * while VDP1 is still reading from VDP1 VRAM.  vdp1_sync_wait() blocks until
+   * VBlank-OUT, by which time VDP1 has finished and swapped framebuffers.  This
+   * prevents the next frame's DMA from overwriting VDP1 VRAM mid-render. */
+  vdp1_sync_wait();
   DISPLAY.whichBuffer ^= 1;
   errorOk();
 }

src/dusksaturn/display/render/rendersaturn.c

@@ -85,6 +85,16 @@ _Static_assert(sizeof(saturncmd_t) == sizeof(vdp1_cmdt_t),
 #define SATURNCMD_PMOD_RGB_DIRECT       (5u << 3) /* RGB1555 in COLR, no palette */
 #define SATURNCMD_PMOD_SPD              (0x0040u) /* do not skip index 0 */
 #define SATURNCMD_PMOD_ECD              (0x0080u) /* end-code disable */
+/* Bit 15 of PMOD: forces the MSB of every pixel written to the VDP1 framebuffer
+ * to 1.  In VDP2 mixed-color mode (SPCLMD=1, set by vdp1_env_default_set), a
+ * framebuffer pixel with MSB=1 is treated as a direct RGB555 color; MSB=0 is a
+ * CRAM palette index.  Always set this for direct-color drawing. */
+#define SATURNCMD_PMOD_MSB              (0x8000u)
+
+/* Last 16-word-aligned CRAM slot (word 2032 of 2048): reserved for the clear
+ * polygon color.  Placed well above any texture palette range (max 8 × 256
+ * = 2048 words, but we only have a handful of textures in practice). */
+#define SATURN_CLEAR_CRAM_ADDR          (0x07F0u)
 
 /* ---- Texture table ------------------------------------------------------- */
 
@@ -118,15 +128,23 @@ static float saturnViewTgtX = 0.0f, saturnViewTgtY = 0.0f, saturnViewTgtZ = 0.0f
 
 /* ---- Helpers ------------------------------------------------------------- */
 
-/* Convert color_t RGBA → VDP1/VDP2 RGB1555. */
+/* Convert color_t RGBA → VDP1/VDP2 RGB1555 (no MSB).
+ * Saturn RGB1555: bit[15]=MSB, bits[14:10]=R, bits[9:5]=G, bits[4:0]=B. */
 static uint16_t toRGB1555(color_t c) {
   return (uint16_t)(
-    ((uint16_t)(c.b >> 3) << 10) |
+    ((uint16_t)(c.r >> 3) << 10) |
     ((uint16_t)(c.g >> 3) <<  5) |
-    ((uint16_t)(c.r >> 3))
+    ((uint16_t)(c.b >> 3))
   );
 }
 
+/* Convert color_t → RGB1555 with bit[15]=1 (direct-color flag).
+ * For VDP1 polygon CMDCOLR: bit[15]=1 tells VDP1 to use the color as direct
+ * RGB555 rather than a CRAM palette index. */
+static uint16_t toRGB1555Direct(color_t c) {
+  return 0x8000u | toRGB1555(c);
+}
+
 /* Write palette into VDP2 CRAM at the texture's slot.
  * CRAM is mapped at 0x25F00000.  Each 256-entry slot = 512 bytes. */
 static void uploadPalette(saturntexentry_t *e) {
@@ -368,8 +386,8 @@ errorret_t renderSaturnFlush(ropbuffer_t *buf) {
     sc = &saturnCmdBuf[0];
     memoryZero(sc, sizeof(saturncmd_t));
     sc->ctrl = SATURNCMD_CTRL_SYSCLIP;
-    sc->xb   = (int16_t)(DUSK_DISPLAY_WIDTH  - 1);
+    sc->xc   = (int16_t)(DUSK_DISPLAY_WIDTH  - 1);
-    sc->yb   = (int16_t)(DUSK_DISPLAY_HEIGHT - 1);
+    sc->yc   = (int16_t)(DUSK_DISPLAY_HEIGHT - 1);
 
     /* [1] User clip – mirrors full screen; disabled per-command by default. */
     sc = &saturnCmdBuf[1];
@@ -394,14 +412,20 @@ errorret_t renderSaturnFlush(ropbuffer_t *buf) {
     switch(op) {
       case ROP_CLEAR: {
         const ropclear_t *c = (const ropclear_t *)hdr;
-        /* Fill the entire screen with a solid RGB polygon.
+        /* VDP1 polygon CM=0: CMDCOLR is a CRAM word address (16-aligned),
-         * RGB direct mode (PMOD = SATURNCMD_PMOD_RGB_DIRECT) places the
+         * not a direct color value.  Write the desired RGB1555 color into our
-         * RGB1555 color in COLR directly — no palette lookup required. */
+         * reserved CRAM slot so VDP1 reads it during rasterization.
+         * PMOD MSB_ENABLE then ORs bit[15]=1 into the framebuffer pixel so
+         * VDP2 (SPCLMD=1) treats it as direct RGB555 rather than a palette
+         * index. */
+        volatile uint16_t *cram = (volatile uint16_t *)0x25F00000;
+        cram[SATURN_CLEAR_CRAM_ADDR] = toRGB1555(c->color);
+
         saturncmd_t *poly = allocCmd();
         if(poly) {
           poly->ctrl = SATURNCMD_CTRL_POLYGON;
-          poly->pmod = SATURNCMD_PMOD_RGB_DIRECT;
+          poly->pmod = SATURNCMD_PMOD_MSB | SATURNCMD_PMOD_ECD | SATURNCMD_PMOD_SPD;
-          poly->colr = toRGB1555(c->color);
+          poly->colr = SATURN_CLEAR_CRAM_ADDR;
           poly->xa   = 0;
           poly->ya   = 0;
           poly->xb   = (int16_t)(DUSK_DISPLAY_WIDTH  - 1);