diff --git a/rustboyadvance-core/src/gba.rs b/rustboyadvance-core/src/gba.rs
index b1c31a7..0e30058 100644
--- a/rustboyadvance-core/src/gba.rs
+++ b/rustboyadvance-core/src/gba.rs
@@ -145,7 +145,7 @@ impl GameBoyAdvance {
     pub fn frame(&mut self) {
         self.key_poll();
 
-        let mut remaining_cycles = 280896 - self.overshoot_cycles;
+        let mut remaining_cycles = CYCLES_FULL_REFRESH - self.overshoot_cycles;
 
         while remaining_cycles > 0 {
             let cycles = self.step();
diff --git a/rustboyadvance-core/src/gpu/mod.rs b/rustboyadvance-core/src/gpu/mod.rs
index 453306a..3c343b9 100644
--- a/rustboyadvance-core/src/gpu/mod.rs
+++ b/rustboyadvance-core/src/gpu/mod.rs
@@ -48,6 +48,8 @@ pub mod consts {
     pub(super) const CYCLES_VDRAW: usize = 197120;
     pub(super) const CYCLES_VBLANK: usize = 83776;
 
+    pub const CYCLES_FULL_REFRESH: usize = 280896;
+
     pub const TILE_SIZE: u32 = 0x20;
 }
 pub use self::consts::*;
@@ -507,6 +509,7 @@ impl Gpu {
                 } else {
                     self.update_vcount(0, irqs);
                     self.dispstat.set_vblank_flag(false);
+                    self.dispstat.set_hblank_flag(false);
                     self.render_scanline();
                     self.cycles_left_for_current_state = CYCLES_HDRAW;
                     self.state = HDraw;
@@ -541,3 +544,99 @@ impl Gpu {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    struct NopDmaNotifer;
+    impl DmaNotifer for NopDmaNotifer {
+        fn notify(&mut self, _timing: u16) {}
+    }
+
+    #[derive(Default)]
+    struct TestVideoInterface {
+        frame_counter: usize,
+    }
+
+    impl VideoInterface for TestVideoInterface {
+        fn render(&mut self, _buffer: &[u32]) {
+            self.frame_counter += 1;
+        }
+    }
+
+    #[test]
+    fn test_gpu_state_machine() {
+        let mut gpu = Gpu::new();
+        let video = Rc::new(RefCell::new(TestVideoInterface::default()));
+        let video_clone: VideoDeviceRcRefCell = video.clone();
+        let mut irqs = IrqBitmask(0);
+        let mut dma_notifier = NopDmaNotifer;
+        let mut cycles_to_next_event = CYCLES_FULL_REFRESH;
+
+        gpu.dispstat.set_vcount_setting(0);
+        gpu.dispstat.set_vcount_irq_enable(true);
+
+        let mut total_cycles = 0;
+
+        macro_rules! update {
+            ($cycles:expr) => {
+                gpu.update(
+                    $cycles,
+                    &mut irqs,
+                    &mut cycles_to_next_event,
+                    &mut dma_notifier,
+                    &video_clone,
+                );
+                total_cycles += $cycles;
+            };
+        }
+
+        for line in 0..160 {
+            println!("line = {}", line);
+            assert_eq!(video.borrow().frame_counter, 0);
+            assert_eq!(gpu.vcount, line);
+            assert_eq!(gpu.state, GpuState::HDraw);
+            assert_eq!(gpu.dispstat.get_hblank_flag(), false);
+            assert_eq!(gpu.dispstat.get_vblank_flag(), false);
+
+            update!(CYCLES_HDRAW);
+
+            assert_eq!(gpu.state, GpuState::HBlank);
+            assert_eq!(gpu.dispstat.get_hblank_flag(), true);
+            assert_eq!(gpu.dispstat.get_vblank_flag(), false);
+
+            update!(CYCLES_HBLANK);
+
+            assert_eq!(irqs.LCD_VCounterMatch(), false);
+        }
+
+        assert_eq!(video.borrow().frame_counter, 1);
+
+        for line in 0..68 {
+            println!("line = {}", 160 + line);
+            assert_eq!(gpu.dispstat.get_hblank_flag(), false);
+            assert_eq!(gpu.dispstat.get_vblank_flag(), true);
+            assert_eq!(gpu.state, GpuState::VBlankHDraw);
+
+            update!(CYCLES_HDRAW);
+
+            assert_eq!(gpu.dispstat.get_hblank_flag(), true);
+            assert_eq!(gpu.dispstat.get_vblank_flag(), true);
+            assert_eq!(gpu.state, GpuState::VBlankHBlank);
+            assert_eq!(irqs.LCD_VCounterMatch(), false);
+
+            update!(CYCLES_HBLANK);
+        }
+
+        assert_eq!(video.borrow().frame_counter, 1);
+        assert_eq!(total_cycles, CYCLES_FULL_REFRESH);
+
+        assert_eq!(irqs.LCD_VCounterMatch(), true);
+        assert_eq!(gpu.cycles_left_for_current_state, CYCLES_HDRAW);
+        assert_eq!(gpu.state, GpuState::HDraw);
+        assert_eq!(gpu.vcount, 0);
+        assert_eq!(gpu.dispstat.get_vcount_flag(), true);
+        assert_eq!(gpu.dispstat.get_hblank_flag(), false);
+    }
+}
diff --git a/rustboyadvance-core/src/gpu/regs.rs b/rustboyadvance-core/src/gpu/regs.rs
index dad86f6..26aca24 100644
--- a/rustboyadvance-core/src/gpu/regs.rs
+++ b/rustboyadvance-core/src/gpu/regs.rs
@@ -91,10 +91,10 @@ bitfield! {
     pub get_vblank_flag, set_vblank_flag: 0;
     pub get_hblank_flag, set_hblank_flag: 1;
     pub get_vcount_flag, set_vcount_flag: 2;
-    pub vblank_irq_enable, _ : 3;
-    pub hblank_irq_enable, _ : 4;
-    pub vcount_irq_enable, _ : 5;
-    pub vcount_setting, _ : 15, 8;
+    pub vblank_irq_enable, set_vblank_irq_enable : 3;
+    pub hblank_irq_enable, set_hblank_irq_enable : 4;
+    pub vcount_irq_enable, set_vcount_irq_enable : 5;
+    pub vcount_setting, set_vcount_setting : 15, 8;
 }
 
 bitfield! {