diff --git a/snes-core/src/cpu/bus.rs b/snes-core/src/cpu/bus.rs
index 1870a0c..018d09d 100644
--- a/snes-core/src/cpu/bus.rs
+++ b/snes-core/src/cpu/bus.rs
@@ -49,7 +49,7 @@ impl Bus {
                 0x0000..=0x1FFF => MemoryMap::WRAM,
                 0x2100..=0x21FF => MemoryMap::PPU,
                 0x4016..=0x4017 => MemoryMap::Joypad,
-                0x4200..=0x420F => MemoryMap::CPU,
+                0x4200..=0x42FF => MemoryMap::CPU,
                 _ => MemoryMap::Cartridge,
             },
             _ => MemoryMap::Cartridge,
diff --git a/snes-core/src/cpu/internal_registers.rs b/snes-core/src/cpu/internal_registers.rs
index c7c745c..a92463b 100644
--- a/snes-core/src/cpu/internal_registers.rs
+++ b/snes-core/src/cpu/internal_registers.rs
@@ -16,12 +16,23 @@ impl InternalRegisters {
         }
     }
 
+    fn get_index(address: u16) -> usize {
+        (address - INTERNAL_REGISTERS_ADDRESS) as usize
+    }
+
     fn _read(&self, address: u16) -> u8 {
-        self.registers[(address - INTERNAL_REGISTERS_ADDRESS) as usize]
+        let index = InternalRegisters::get_index(address);
+        if index >= self.registers.len() {
+            return 0xFF;
+        }
+        self.registers[index]
     }
 
     fn _write(&mut self, address: u16, value: u8) {
-        self.registers[(address - INTERNAL_REGISTERS_ADDRESS) as usize] = value
+        let index = InternalRegisters::get_index(address);
+        if index < self.registers.len() {
+            self.registers[index] = value
+        }
     }
 
     pub fn read_external(&self, address: u16, ppu_registers: &PPURegisters) -> u8 {
@@ -73,7 +84,7 @@ mod ppu_general_test {
         ppu.registers.h_count = 339;
         ppu.registers.v_count = 224;
         ppu.dot_cycle();
-        assert_eq!(registers.read_vblank_nmi(&mut ppu.registers), 0x80);
+        assert_eq!(registers.read_vblank_nmi_mut(&mut ppu.registers), 0x80);
         // vblank bit is reset after read
         ppu.dot_cycle();
         assert_eq!(registers.read_vblank_nmi(&mut ppu.registers), 0x00);
diff --git a/snes-core/src/ppu/registers.rs b/snes-core/src/ppu/registers.rs
index 2f9b471..5bfef49 100644
--- a/snes-core/src/ppu/registers.rs
+++ b/snes-core/src/ppu/registers.rs
@@ -378,7 +378,7 @@ impl PPURegisters {
     }
 
     pub fn is_vblanking(&self) -> bool {
-        if self.h_count >= 1 && self.h_count <= 224 {
+        if self.v_count >= 1 && self.v_count <= 224 {
             return false
         }
         return true
@@ -577,17 +577,17 @@ mod ppu_registers_test {
     #[test]
     fn test_is_vblanking() {
         let mut registers = PPURegisters::new();
-        registers.h_count = 339;
+        registers.v_count = 339;
         assert_eq!(registers.is_vblanking(), true);
-        registers.h_count = 0;
+        registers.v_count = 0;
         assert_eq!(registers.is_vblanking(), true);
-        registers.h_count = 225;
+        registers.v_count = 225;
         assert_eq!(registers.is_vblanking(), true);
-        registers.h_count = 224;
+        registers.v_count = 224;
         assert_eq!(registers.is_vblanking(), false);
-        registers.h_count = 2;
+        registers.v_count = 2;
         assert_eq!(registers.is_vblanking(), false);
-        registers.h_count = 50;
+        registers.v_count = 50;
         assert_eq!(registers.is_vblanking(), false);
     }
 }
diff --git a/snes-frontend/src/ppu.rs b/snes-frontend/src/ppu.rs
index a5c4f1f..93692c3 100644
--- a/snes-frontend/src/ppu.rs
+++ b/snes-frontend/src/ppu.rs
@@ -155,6 +155,7 @@ pub fn registers_window(ppu_registers: &PPURegisters, show_registers: &mut bool,
             ui.text(format!("V: {}", ppu_registers.v_count));
             ui.text(format!("H: {}", ppu_registers.h_count));
             ui.text(format!("VBlanking: {}", ppu_registers.is_vblanking()));
+            ui.text(format!("NMI Flag:  {}", ppu_registers.vblank_nmi));
             ui.separator();
             ui.text("Registers:");
             for (index, register_value) in ppu_registers.registers().iter().enumerate() {