fix: interrupt timing issue

Allows blargg's test ROM to pass.

CHANGELOG.md

@@ -20,7 +20,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
-*
+* Issue related to interrupt timing, reduce interrupt to 20 cycles instead of 24
 
 ## [0.9.18] - 2024-01-02
 

frontends/sdl/src/test.rs

@@ -70,6 +70,22 @@ mod tests {
         assert!(image_result);
     }
 
+    #[test]
+    fn test_blargg_interrupt_time() {
+        let result: [u8; FRAME_BUFFER_SIZE] = run_image_test(
+            "../../res/roms/test/blargg/interrupt_time/interrupt_time.gb",
+            Some(20000000),
+            TestOptions {
+                mode: Some(GameBoyMode::Cgb),
+                ..TestOptions::default()
+            },
+        )
+        .unwrap();
+        let image_result =
+            compare_images(&result, "res/test/blargg/interrupt_time/interrupt_time.png");
+        assert!(image_result);
+    }
+
     #[test]
     fn test_blargg_dmg_sound() {
         let result: [u8; FRAME_BUFFER_SIZE] = run_image_test(

src/cpu.rs

@@ -172,7 +172,7 @@ impl Cpu {
                     self.halted = false;
                 }
 
-                return 24;
+                return 20;
             } else if (self.mmu.ie & 0x02 == 0x02) && self.mmu.ppu().int_stat() {
                 debugln!("Going to run LCD STAT interrupt handler (0x48)");
 
@@ -190,7 +190,7 @@ impl Cpu {
                     self.halted = false;
                 }
 
-                return 24;
+                return 20;
             } else if (self.mmu.ie & 0x04 == 0x04) && self.mmu.timer().int_tima() {
                 debugln!("Going to run Timer interrupt handler (0x50)");
 
@@ -208,7 +208,7 @@ impl Cpu {
                     self.halted = false;
                 }
 
-                return 24;
+                return 20;
             } else if (self.mmu.ie & 0x08 == 0x08) && self.mmu.serial().int_serial() {
                 debugln!("Going to run Serial interrupt handler (0x58)");
 
@@ -226,7 +226,7 @@ impl Cpu {
                     self.halted = false;
                 }
 
-                return 24;
+                return 20;
             } else if (self.mmu.ie & 0x10 == 0x10) && self.mmu.pad().int_pad() {
                 debugln!("Going to run JoyPad interrupt handler (0x60)");
 
@@ -244,7 +244,7 @@ impl Cpu {
                     self.halted = false;
                 }
 
-                return 24;
+                return 20;
             }
         }