diff --git a/src/emulator/main.rs b/src/emulator/main.rs
index 449b5ea..617b446 100644
--- a/src/emulator/main.rs
+++ b/src/emulator/main.rs
@@ -39,6 +39,16 @@ fn tick<M: MemoryMapper>(state: &mut EmulatorState<M>) {
         0x00 => {} // NOP
 
         /* Special */
+        0xd3 => {
+            // OUT
+            let port = state.next_byte();
+            state.write_io(port, state.a);
+        }
+        0xdb => {
+            // IN
+            let port = state.next_byte();
+            state.a = state.read_io(port);
+        }
         0xfb => state.ei = true,  // EI
         0xf3 => state.ei = false, // DI
         0x76 => {
diff --git a/src/emulator/mapper/mod.rs b/src/emulator/mapper/mod.rs
index 897e917..a887a3b 100644
--- a/src/emulator/mapper/mod.rs
+++ b/src/emulator/mapper/mod.rs
@@ -1,6 +1,19 @@
 pub trait MemoryMapper {
+    /// Read a byte at the specified address through the memory mapper
     fn read(&mut self, address: u16) -> u8;
+
+    /// Write a byte to the specified address through the memory mapper
     fn write(&mut self, address: u16, value: u8);
+
+    /// Receive a byte from a device on the I/O bus through the memory mapper
+    #[allow(unused_variables)]
+    fn read_io(&mut self, port: u8) -> u8 {
+        0
+    }
+
+    /// Send a byte to a device on the I/O bus through the memory mapper
+    #[allow(unused_variables)]
+    fn write_io(&mut self, port: u8, value: u8) {}
 }
 
 //#[cfg(test)]
diff --git a/src/emulator/structs.rs b/src/emulator/structs.rs
index 37570bf..aa82681 100644
--- a/src/emulator/structs.rs
+++ b/src/emulator/structs.rs
@@ -47,12 +47,20 @@ impl<M: MemoryMapper> EmulatorState<M> {
     }
 
     /// Read a byte at a specific address
+    ///
+    /// May return an indeterminate value if an invalid region is accessed.
+    /// May cause internal mapper state (eg. interrupts, emulated shift
+    /// registers) to be updated. Subsequent reads from the same address may
+    /// yield different values.
     #[inline]
     pub fn read_byte(&mut self, address: u16) -> u8 {
         self.mapper.read(address)
     }
 
     /// Write a byte at a specific address
+    ///
+    /// May do nothing if an invalid region is accessed. Subsequent reads
+    /// from the same address may yield different values.
     #[inline]
     pub fn write_byte(&mut self, address: u16, value: u8) {
         self.mapper.write(address, value);
@@ -67,6 +75,11 @@ impl<M: MemoryMapper> EmulatorState<M> {
     }
 
     /// Read a 16-bit word at a specific address
+    ///
+    /// May return an indeterminate value if an invalid region is accessed.
+    /// May cause internal mapper state (eg. interrupts, emulated shift
+    /// registers) to be updated. Subsequent reads from the same address may
+    /// yield different values.
     #[inline]
     pub fn read_word(&mut self, address: u16) -> u16 {
         u16::from_le_bytes([
@@ -76,6 +89,9 @@ impl<M: MemoryMapper> EmulatorState<M> {
     }
 
     /// Write a 16-bit word at a specific address
+    ///
+    /// May do nothing if an invalid region is accessed. Subsequent reads
+    /// from the same address may yield different values.
     #[inline]
     pub fn write_word(&mut self, address: u16, value: u16) {
         let [low, high] = u16::to_le_bytes(value);
@@ -90,6 +106,24 @@ impl<M: MemoryMapper> EmulatorState<M> {
         (self.pc, ..) = self.pc.overflowing_add(2);
         value
     }
+
+    /// Receive a byte from a device on the I/O bus
+    ///
+    /// May return an indeterminate value if an invalid device is accessed.
+    /// May (and usually will) cause internal mapper state (eg. interrupts)
+    /// to be updated.
+    #[inline]
+    pub fn read_io(&mut self, port: u8) -> u8 {
+        self.mapper.read_io(port)
+    }
+
+    /// Send a byte to a device on the I/O bus
+    ///
+    /// May do nothing if an invalid device is accessed.
+    #[inline]
+    pub fn write_io(&mut self, port: u8, value: u8) {
+        self.mapper.write_io(port, value);
+    }
 }
 
 impl<M: MemoryMapper + Default> Default for EmulatorState<M> {