2026-05-14 19:54:20

Author: mohanson

src/gpu.rs

@@ -295,9 +295,6 @@ pub struct Gpu {
     // Extra dots to add to the mode3 threshold due to sprites on the current scanline.
     // Computed when mode2 fires; represents floor(T_penalty/4)*4 extra dots before mode0.
     sprite_penalty: u32,
-    // Set when LCDC bit 7 transitions 0->1. Line 0 after LCD enable starts in Mode 0
-    // (not Mode 2) and jumps directly to Mode 3 at dot 80.
-    lcdon_first_line: bool,
 }
 
 impl Gpu {
@@ -329,7 +326,6 @@ impl Gpu {
             prio: [(true, 0); SCREEN_W],
             dots: 0,
             sprite_penalty: 0,
-            lcdon_first_line: false,
         }
     }
 
@@ -868,7 +864,6 @@ impl Memory for Gpu {
                 } else if !was_on {
                     // LCD just enabled: line 0 starts in Mode 0, not Mode 2.
                     // Evaluate LYC=LY coincidence for the initial display state.
-                    self.lcdon_first_line = true;
                     self.stat.lyeq(self.ly == self.lc);
                     self.sigq_update();
                 }
@@ -993,8 +988,9 @@ impl Ticker for Gpu {
             }
             // Mode 2: OAM search (dots 0-79).
             if self.dots < 80 {
-                if self.lcdon_first_line {
-                    // Line 0 after LCD enable: stay in Mode 0, skip Mode 2 entirely.
+                // Line 0 after LCD enable: ly=0 and mode=0 (cleared at LCD-off) uniquely identify this case. Stay in
+                // Mode 0, skip Mode 2 entirely.
+                if self.stat.mode() == 0 && self.ly == 0 {
                     continue;
                 }
                 if self.stat.mode() == 2 {
@@ -1010,7 +1006,6 @@ impl Ticker for Gpu {
             }
             // Mode 3: Pixel transfer (dots 80-455, but may end early due to sprite penalty).
             if self.dots <= (80 + 172 + ((self.sx as u32 % 8 + 3) / 4) * 4) - 4 + self.sprite_penalty {
-                self.lcdon_first_line = false;
                 self.stat.data = (self.stat.data & !0x03) | 3;
                 // Mode 3 has no STAT interrupt source; the signal may fall to LOW here
                 // unless LYC=LY keeps it high.  Update to track any falling edge.