diff --git a/src/processor/memory.rs b/src/processor/memory.rs
index 487d685..e76099b 100644
--- a/src/processor/memory.rs
+++ b/src/processor/memory.rs
@@ -1,9 +1,9 @@
-use self::mmio::{Apu, Joypad};
+use self::mmio::{Apu, Joypad, Serial};
 pub use self::rom::Rom;
-use crate::{processor::SplitRegister, verbose_println};
+use crate::{processor::SplitRegister, util::set_bit, verbose_println, Cpu};
 use gilrs::ConnectedGamepadsIterator;
 use minifb::Key;
-use std::io::{stdout, Write};
+// use std::io::{stdout, Write};
 
 mod mmio;
 pub(crate) mod rom;
@@ -27,12 +27,11 @@ pub struct Memory {
     pub(super) user_mode: bool,
     joypad: Joypad,
     apu: Apu,
+    serial: Serial,
 }
 
 impl Memory {
     pub fn init(bootrom: Vec<u8>, bootrom_enabled: bool, rom: Rom) -> Self {
-        let mut apu = Apu::default();
-        apu.init();
         Self {
             bootrom,
             bootrom_enabled,
@@ -48,7 +47,8 @@ impl Memory {
             io: [0xFF; 76],
             user_mode: false,
             joypad: Joypad::default(),
-            apu,
+            apu: Apu::init_default(),
+            serial: Serial::default(),
         }
     }
 
@@ -85,33 +85,16 @@ impl Memory {
                 // change this with MBC code...
                 self.rom.set(address, data);
             }
-            0x8000..0xA000 => {
-                self.vram[(address - 0x8000) as usize] = data;
-            }
-            0xA000..0xC000 => {
-                self.rom.set_ram(address, data);
-            }
-            0xC000..0xE000 => {
-                self.ram[(address - 0xC000) as usize] = data;
-            }
-            0xE000..0xFE00 => {
-                self.ram[(address - 0xE000) as usize] = data;
-            }
-            0xFE00..0xFEA0 => {
-                self.oam[(address - 0xFE00) as usize] = data;
-            }
+            0x8000..0xA000 => self.vram[(address - 0x8000) as usize] = data,
+            0xA000..0xC000 => self.rom.set_ram(address, data),
+            0xC000..0xE000 => self.ram[(address - 0xC000) as usize] = data,
+            0xE000..0xFE00 => self.ram[(address - 0xE000) as usize] = data,
+            0xFE00..0xFEA0 => self.oam[(address - 0xFE00) as usize] = data,
             0xFEA0..0xFF00 => {}
-            0xFF00..0xFF4C => {
-                self.set_io(address, data);
-                stdout().flush().unwrap();
-            }
-            0xFF50 => {
-                self.bootrom_enabled = false;
-            }
+            0xFF00..0xFF4C => self.set_io(address, data),
+            0xFF50 => self.bootrom_enabled = false,
             0xFF4C..0xFF50 | 0xFF51..0xFF80 => {}
-            0xFF80..0xFFFF => {
-                self.cpu_ram[(address - 0xFF80) as usize] = data;
-            }
+            0xFF80..0xFFFF => self.cpu_ram[(address - 0xFF80) as usize] = data,
             0xFFFF => {
                 verbose_println!("interrupts set to {:#b}", data);
                 verbose_println!("                / {:#X}", data);
@@ -124,6 +107,8 @@ impl Memory {
         // range: 0xFF00 - 0xFF4B inclusive
         match address {
             0xFF00 => self.joypad.as_register(),
+            0xFF01 => self.serial.get_queued(),
+            0xFF02 => self.serial.get_control(),
             0xFF10..0xFF40 => self.apu.get_register(address),
             _ => self.io[(address - 0xFF00) as usize],
         }
@@ -140,12 +125,8 @@ impl Memory {
                     // joypad
                     self.joypad.mmio_write(data);
                 }
-                0xFF02 => {
-                    if data == 0x81 {
-                        print!("{}", self.get(0xFF01) as char);
-                        stdout().flush().unwrap();
-                    }
-                }
+                0xFF01 => self.serial.update_queued(data),
+                0xFF02 => self.serial.update_control(data),
                 0xFF04 => self.io[addr_l] = 0,
                 0xFF07 => self.masked_io(addr_l, data, 0b111),
                 0xFF0F => self.masked_io(addr_l, data, 0b11111),
@@ -219,6 +200,15 @@ impl Memory {
     }
 }
 
+impl Cpu {
+    pub fn advance_mmio_clocks(&mut self, steps: usize) {
+        self.memory.apu.tick(steps);
+        if self.memory.serial.tick(steps) {
+            self.memory.set(0xFF0F, set_bit(self.memory.get(0xFF0F), 3));
+        }
+    }
+}
+
 pub fn masked_update(current: u8, data: u8, mask: u8) -> u8 {
     (current & (!mask)) | (data & mask)
 }
diff --git a/src/processor/memory/mmio/apu.rs b/src/processor/memory/mmio/apu.rs
index 869e040..45915eb 100644
--- a/src/processor/memory/mmio/apu.rs
+++ b/src/processor/memory/mmio/apu.rs
@@ -1,6 +1,6 @@
 use self::types::{Channels, DacSample, Mixer, VinEnable, Volume};
 use crate::{
-    processor::{memory::Address, timer::CLOCK_SPEED, Cpu},
+    processor::{memory::Address, timer::CLOCK_SPEED},
     util::{get_bit, set_or_clear_bit},
 };
 use async_ringbuf::{AsyncHeapProducer, AsyncHeapRb};
@@ -95,6 +95,12 @@ impl Default for Apu {
 const CYCLES_PER_FRAME: usize = 70224;
 
 impl Apu {
+    pub fn init_default() -> Self {
+        let mut r = Self::default();
+        r.init();
+        r
+    }
+
     pub fn init(&mut self) {
         let sample_rate = self.config.sample_rate.0;
         let rb_len = sample_rate as usize / 60;
@@ -150,7 +156,7 @@ impl Apu {
         }
     }
 
-    fn tick(&mut self, steps: usize) {
+    pub fn tick(&mut self, steps: usize) {
         self.buffer.append(
             &mut izip!(
                 self.channels.one.tick(steps).into_iter(),
@@ -303,9 +309,3 @@ impl Apu {
         }
     }
 }
-
-impl Cpu {
-    pub fn advance_apu_clock(&mut self, steps: usize) {
-        self.memory.apu.tick(steps);
-    }
-}
diff --git a/src/processor/memory/mmio/mod.rs b/src/processor/memory/mmio/mod.rs
index b00cd38..f14a15c 100644
--- a/src/processor/memory/mmio/mod.rs
+++ b/src/processor/memory/mmio/mod.rs
@@ -1,4 +1,6 @@
 mod apu;
 mod joypad;
+mod serial;
 pub use apu::Apu;
 pub use joypad::Joypad;
+pub use serial::Serial;
diff --git a/src/processor/memory/mmio/serial.rs b/src/processor/memory/mmio/serial.rs
new file mode 100644
index 0000000..606f2f2
--- /dev/null
+++ b/src/processor/memory/mmio/serial.rs
@@ -0,0 +1,99 @@
+use std::io::{stdout, Write};
+
+use crate::util::get_bit;
+
+enum ClockSource {
+    Internal,
+    External,
+}
+
+#[allow(dead_code)]
+enum ClockSpeed {
+    Normal,
+    Fast,
+}
+
+#[allow(dead_code)]
+struct SerialControl {
+    transfer_in_progress: bool,
+    clock_speed: ClockSpeed,
+    clock_source: ClockSource,
+}
+
+impl Default for SerialControl {
+    fn default() -> Self {
+        Self {
+            transfer_in_progress: false,
+            clock_speed: ClockSpeed::Normal,
+            clock_source: ClockSource::Internal,
+        }
+    }
+}
+
+pub struct Serial {
+    byte: u8,
+    bits_transferred: u8,
+    control: SerialControl,
+}
+
+impl Serial {
+    pub fn tick(&mut self, steps: usize) -> bool {
+        let mut will_interrupt = false;
+        for _ in 0..steps {
+            if self.control.transfer_in_progress {
+                let (remainder, finished) = self.bits_transferred.overflowing_sub(1);
+                self.bits_transferred = if finished {
+                    self.control.transfer_in_progress = false;
+                    will_interrupt = true;
+                    print!("{}", self.byte as char);
+                    stdout().flush().unwrap();
+                    7
+                } else {
+                    remainder
+                }
+            }
+        }
+        will_interrupt
+    }
+
+    pub fn update_queued(&mut self, data: u8) {
+        self.byte = data;
+    }
+
+    pub fn get_queued(&self) -> u8 {
+        self.byte
+    }
+
+    pub fn update_control(&mut self, data: u8) {
+        self.control.transfer_in_progress = get_bit(data, 7);
+        // CGB - add clock speed
+        self.control.clock_source = if get_bit(data, 0) {
+            ClockSource::Internal
+        } else {
+            ClockSource::External
+        };
+    }
+
+    pub fn get_control(&self) -> u8 {
+        0b01111110
+            | (if self.control.transfer_in_progress {
+                0b1
+            } else {
+                0b0
+            } << 7)
+            | match self.control.clock_source {
+                ClockSource::Internal => 0b1,
+                ClockSource::External => 0b0,
+            }
+    }
+}
+
+impl Default for Serial {
+    fn default() -> Self {
+        Self {
+            byte: 0,
+            bits_transferred: 7,
+            control: SerialControl::default(),
+        }
+    }
+}
diff --git a/src/processor/timer.rs b/src/processor/timer.rs
index fcce700..d0d510a 100644
--- a/src/processor/timer.rs
+++ b/src/processor/timer.rs
@@ -28,7 +28,7 @@ impl Cpu {
         let clock_cycles = (machine_cycles as usize) * 4;
 
         self.advance_gpu_clock(clock_cycles);
-        self.advance_apu_clock(clock_cycles);
+        self.advance_mmio_clocks(clock_cycles);
 
         self.timers.div_counter += clock_cycles;
         let mut div_diff = (self.timers.div_counter / 256) as u8;