diff --git a/lib/src/processor/memory/mmio/gpu.rs b/lib/src/processor/memory/mmio/gpu.rs
index b9eb38b..9327f90 100644
--- a/lib/src/processor/memory/mmio/gpu.rs
+++ b/lib/src/processor/memory/mmio/gpu.rs
@@ -117,10 +117,11 @@ where
             match self.stat.mode {
                 DrawMode::HBlank => {
                     // mode 0: hblank
-                    if self.mode_clock >= 204 {
-                        self.mode_clock = 0;
+                    const HBLANK_MODE_CLOCK: usize = 204;
+                    if self.mode_clock >= HBLANK_MODE_CLOCK {
+                        self.mode_clock -= HBLANK_MODE_CLOCK;
                         self.scanline += 1;
-                        if self.scanline == 143 {
+                        if self.scanline == 144 {
                             self.enter_vblank(output);
                             interrupts.vblank = true;
                         } else {
@@ -130,8 +131,9 @@ where
                 }
                 DrawMode::VBlank => {
                     // mode 1: vblank
-                    if self.mode_clock >= 456 {
-                        self.mode_clock = 0;
+                    const VBLANK_MODE_CLOCK: usize = 456;
+                    if self.mode_clock >= VBLANK_MODE_CLOCK {
+                        self.mode_clock -= VBLANK_MODE_CLOCK;
                         self.scanline += 1;
                         if self.scanline == 153 {
                             self.exit_vblank(output);
@@ -139,17 +141,19 @@ where
                     }
                 }
                 DrawMode::Mode2 => {
+                    const MODE2_MODE_CLOCK: usize = 80;
                     // search oam for sprites on this line
                     // we dont really have to emulate this
-                    if self.mode_clock >= 80 {
-                        self.mode_clock = 0;
+                    if self.mode_clock >= MODE2_MODE_CLOCK {
+                        self.mode_clock -= MODE2_MODE_CLOCK;
                         self.stat.mode = DrawMode::Mode3;
                     }
                 }
                 DrawMode::Mode3 => {
+                    const MODE3_MODE_CLOCK: usize = 172;
                     // generate scanline
-                    if self.mode_clock >= 172 {
-                        self.mode_clock = 0;
+                    if self.mode_clock >= MODE3_MODE_CLOCK {
+                        self.mode_clock -= MODE3_MODE_CLOCK;
                         self.enter_hblank(output);
                     }
                 }