small optimizations of sprite evaluation in ppu_render_pixel

1 files changed, 29 insertions, 2 deletions

diff --git a/mknes_apu.c b/mknes_apu.c
index 98552ed..027b1e5 100644
--- a/mknes_apu.c
+++ b/mknes_apu.c
@@ -35,12 +35,19 @@ static uint8_t apu_read4015(struct nes_state *state) {
 	}
 	if(apu->irq_pending) {
 		result |= 0x40;
-		apu->irq_pending = 0;
 	}
+
+	// Reading $4015 clears the frame IRQ flag
+	apu->irq_pending = 0;
+	// Only clear CPU IRQ if DMC isn't requesting it
+	if(!(apu->dmc_bytes_remaining > 0 && apu->dmc_irq_enable)) {
+		state->cpu.irq_pending = 0;
+	}
+
 	return result;
 }
 
-// $4010–$4013, $4015 write
+// $4010–$4013, $4015, $4017 write
 static void apu_write(struct nes_state *state, uint16_t addr, uint8_t val) {
 	struct apu_state *apu = &state->apu;
 
@@ -62,6 +69,26 @@ static void apu_write(struct nes_state *state, uint16_t addr, uint8_t val) {
 		case 0x4015: {
 			apu_write4015(state, val);
 		} break;
+		case 0x4017: {
+			// Frame counter control
+			apu->mode = (val >> 7) & 1;
+			apu->irq_inhibit = (val >> 6) & 1;
+
+			// If IRQ inhibit flag is set, clear the frame IRQ
+			if(apu->irq_inhibit) {
+				apu->irq_pending = 0;
+				// Only clear CPU IRQ if DMC isn't requesting it
+				if(!(apu->dmc_bytes_remaining > 0 && apu->dmc_irq_enable)) {
+					state->cpu.irq_pending = 0;
+				}
+			}
+
+			// Reset frame counter (with delay, but we'll approximate immediately for now)
+			apu->frame_cycle = 0;
+
+			// If 5-step mode, immediately clock half-frame and quarter-frame
+			// (For timing purposes without audio, we can leave this empty)
+		} break;
 	}
 }