diff --git a/src/nes/bus.rs b/src/nes/bus.rs
index 09513af..2c90bd1 100644
--- a/src/nes/bus.rs
+++ b/src/nes/bus.rs
@@ -53,7 +53,7 @@ impl Memory for Bus {
     fn readb(&self, addr: Addr) -> Byte {
         if let Some(cartridge) = &self.cartridge {
             if CART_ADDR_RANGE[0] <= addr && addr <= CART_ADDR_RANGE[1] {
-                return cartridge.borrow().readb(addr)
+                return cartridge.borrow().readb(addr).unwrap()  // does not fail.
             } 
         }
         if RAM_ADDR_RANGE[0] <= addr && addr <= RAM_ADDR_RANGE[1] {
@@ -71,7 +71,7 @@ impl Memory for Bus {
     fn writeb(&mut self, addr: Addr, data: Byte) {
         if let Some(cartridge) = &mut self.cartridge {
             if CART_ADDR_RANGE[0] <= addr && addr <= CART_ADDR_RANGE[1] {
-                cartridge.borrow_mut().writeb(addr, data)
+                cartridge.borrow_mut().writeb(addr, data);
             } 
         }
         if RAM_ADDR_RANGE[0] <= addr && addr <= RAM_ADDR_RANGE[1] {
diff --git a/src/nes/cartridge.rs b/src/nes/cartridge.rs
index 71b4db2..ff19382 100644
--- a/src/nes/cartridge.rs
+++ b/src/nes/cartridge.rs
@@ -1,5 +1,4 @@
 use crate::nes::mappers::*;
-use crate::nes::Memory;
 use failure::Error;
 use std::io::prelude::*;
 use std::fs::File;
@@ -102,21 +101,38 @@ impl Cartridge {
             mapper: Box::new(Mapper0::new(1, 1))
         }
     }
-}
 
-impl Memory for Cartridge {
-    fn readb(&self, addr: Addr) -> Byte {
-        let mapped_addr = self.mapper.map_read_addr(addr);
-        self.prg_rom[mapped_addr as usize]
+    pub fn readb(&self, addr: Addr) -> Option<Byte> {
+        if let Some(mapped_addr) = self.mapper.map_read_addr(addr) {
+            return Some(self.prg_rom[mapped_addr as usize])
+        }
+        None
     }
 
-    fn writeb(&mut self, addr: Addr, data: Byte) {
-        let mapped_addr = self.mapper.map_write_addr(addr);
-        self.prg_rom[mapped_addr as usize] = data;
+    pub fn writeb(&mut self, addr: Addr, data: Byte) -> bool {
+        if let Some(mapped_addr) = self.mapper.map_write_addr(addr) {
+            self.prg_rom[mapped_addr as usize] = data;
+            return true
+        }
+        false
+    }
+
+    // Read from cartridge if the cartridge has readable VRAM/VROM
+    pub fn readb_ppu(&self, addr: Addr) -> Option<Byte> {
+        // TODO
+        None
+    }
+
+    // Let the cartridge handle the ppu write. Returns true if cartridge
+    // handled the write, false otherwise
+    pub fn writeb_ppu(&mut self, addr: Addr, data: Byte) -> bool {
+        // TODO
+        false
     }
 }
 
 
+
 #[cfg(test)]
 mod tests {
     use super::*;
diff --git a/src/nes/mappers.rs b/src/nes/mappers.rs
index 316101d..a61b390 100644
--- a/src/nes/mappers.rs
+++ b/src/nes/mappers.rs
@@ -1,8 +1,8 @@
 use crate::nes::types::*;
 
 pub trait Mapper {
-    fn map_read_addr(&self, addr: Addr) -> Addr;
-    fn map_write_addr(&self, addr: Addr) -> Addr;
+    fn map_read_addr(&self, addr: Addr) -> Option<Addr>;
+    fn map_write_addr(&self, addr: Addr) -> Option<Addr>;
 }
 
 // Mapper 0
@@ -29,24 +29,24 @@ impl Mapper0 {
 }
 
 impl Mapper for Mapper0 {
-    fn map_read_addr(&self, addr: Addr) -> Addr {
+    fn map_read_addr(&self, addr: Addr) -> Option<Addr> {
         if 0x8000 <= addr && addr <= 0xFFFF { 
             if self.prg_banks > 1 {
-                return addr & 0x7fff;
+                return Some(addr & 0x7fff);
             } else {
-                return addr & 0x3fff;
+                return Some(addr & 0x3fff);
             }
         }
-        0x0000
+        None
     }
-    fn map_write_addr(&self, addr: Addr) -> Addr {
+    fn map_write_addr(&self, addr: Addr) -> Option<Addr> {
         if 0x8000 <= addr && addr <= 0xFFFF { 
             if self.prg_banks > 1 {
-                return addr & 0x7fff;
+                return Some(addr & 0x7fff);
             } else {
-                return addr & 0x3fff;
+                return Some(addr & 0x3fff);
             }
         }
-        0x0000
+        None
     }
 }
diff --git a/src/nes/ppubus.rs b/src/nes/ppubus.rs
index 2da56a9..889d26e 100644
--- a/src/nes/ppubus.rs
+++ b/src/nes/ppubus.rs
@@ -35,6 +35,13 @@ impl PPUBus {
 
 impl PPUMemory for PPUBus {
     fn readb_ppu(&self, addr: Addr) -> Byte {
+        // give the cartridge a chance to handle it
+        if let Some(cartridge) = &self.cartridge {
+            if let Some(data) = cartridge.borrow().readb_ppu(addr) {
+                return data
+            }
+        }
+
         if PATTERN_ADDR_RANGE[0] <= addr && addr <= PATTERN_ADDR_RANGE[1] {
             let table = if addr < 0x1000 { 0 } else { 1 };
             return self.pattern_memory[table as usize][(addr % 0x1000) as usize]
@@ -60,6 +67,13 @@ impl PPUMemory for PPUBus {
         0x00
     }
     fn writeb_ppu(&mut self, addr: Addr, data: Byte) {
+        // give the cartridge a chance to handle it
+        if let Some(cartridge) = &self.cartridge {
+            if cartridge.borrow_mut().writeb_ppu(addr, data) {
+                return
+            }
+        }
+
         if PATTERN_ADDR_RANGE[0] <= addr && addr <= PATTERN_ADDR_RANGE[1] {
             let table = if addr < 0x1000 { 0 } else { 1 };
             self.pattern_memory[table as usize][(addr % 0x1000) as usize] = data;