diff --git a/rustboyadvance-core/src/cartridge/builder.rs b/rustboyadvance-core/src/cartridge/builder.rs
index 3e9f87f..c9e4d1b 100644
--- a/rustboyadvance-core/src/cartridge/builder.rs
+++ b/rustboyadvance-core/src/cartridge/builder.rs
@@ -7,6 +7,7 @@ use super::super::{GBAError, GBAResult};
 use super::backup::eeprom::*;
 use super::backup::flash::*;
 use super::backup::{BackupFile, BackupType};
+use super::gpio::Gpio;
 use super::header;
 use super::BackupMedia;
 use super::Cartridge;
@@ -126,9 +127,12 @@ impl GamepakBuilder {
 
         let backup = create_backup(self.save_type, self.save_path);
 
+        let gpio = Gpio::new();
+
         let size = bytes.len();
         Ok(Cartridge {
             header: header,
+            gpio: gpio,
             bytes: bytes.into_boxed_slice(),
             size: size,
             backup: backup,
diff --git a/rustboyadvance-core/src/cartridge/gpio.rs b/rustboyadvance-core/src/cartridge/gpio.rs
new file mode 100644
index 0000000..a7e1c9e
--- /dev/null
+++ b/rustboyadvance-core/src/cartridge/gpio.rs
@@ -0,0 +1,90 @@
+use super::rtc::Rtc;
+use super::{GPIO_PORT_CONTROL, GPIO_PORT_DATA, GPIO_PORT_DIRECTION};
+
+use bit::BitIndex;
+use serde::{Deserialize, Serialize};
+
+use std::cell::RefCell;
+
+#[derive(Serialize, Deserialize, Copy, Clone, Debug, PartialEq)]
+enum GpioDirection {
+    In = 0,
+    Out = 1,
+}
+
+#[derive(Serialize, Deserialize, Copy, Clone, Debug, PartialEq)]
+enum GpioPortControl {
+    WriteOnly = 0,
+    ReadWrite = 1,
+}
+
+trait GpioDevice: Sized {
+    fn write(&mut self);
+    fn read(&mut self);
+}
+
+enum GpioDeviceType {
+    Rtc,
+    SolarSensor,
+    Gyro,
+    None,
+}
+
+#[derive(Serialize, Deserialize, Clone, Debug)]
+pub struct Gpio {
+    rtc: Option<RefCell<Rtc>>,
+    direction: [GpioDirection; 4],
+    control: GpioPortControl,
+}
+
+impl Gpio {
+    pub fn new() -> Self {
+        Gpio {
+            rtc: None,
+            direction: [GpioDirection::In; 4],
+            control: GpioPortControl::WriteOnly,
+        }
+    }
+
+    pub fn is_readable(&self) -> bool {
+        self.control != GpioPortControl::WriteOnly
+    }
+
+    pub fn read(&self, addr: u32) -> u16 {
+        match addr {
+            GPIO_PORT_DATA => unimplemented!(),
+            GPIO_PORT_DIRECTION => {
+                let mut direction = 0u16;
+                for i in 0..4 {
+                    direction.set_bit(i, self.direction[i] == GpioDirection::Out);
+                }
+                direction
+            }
+            GPIO_PORT_CONTROL => self.control as u16,
+            _ => unreachable!(),
+        }
+    }
+
+    pub fn write(&mut self, addr: u32) -> u16 {
+        match addr {
+            GPIO_PORT_DATA => unimplemented!(),
+            GPIO_PORT_DIRECTION => {
+                let mut direction = 0u16;
+                for i in 0..4 {
+                    direction.set_bit(i, self.direction[i] == GpioDirection::Out);
+                }
+                direction
+            }
+            GPIO_PORT_CONTROL => self.control as u16,
+            _ => unreachable!(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test_gpio() {
+        unimplemented!();
+    }
+}
diff --git a/rustboyadvance-core/src/cartridge/mod.rs b/rustboyadvance-core/src/cartridge/mod.rs
index 5866a2f..4aa1000 100644
--- a/rustboyadvance-core/src/cartridge/mod.rs
+++ b/rustboyadvance-core/src/cartridge/mod.rs
@@ -13,10 +13,18 @@ use backup::flash::Flash;
 pub use backup::BackupType;
 use backup::{BackupFile, BackupMemoryInterface};
 
+mod gpio;
+mod rtc;
+use gpio::Gpio;
+
 mod builder;
 mod loader;
 pub use builder::GamepakBuilder;
 
+pub const GPIO_PORT_DATA: u32 = 0x0800_00C4;
+pub const GPIO_PORT_DIRECTION: u32 = 0x0800_00C6;
+pub const GPIO_PORT_CONTROL: u32 = 0x0800_00C8;
+
 #[derive(Serialize, Deserialize, Clone, Debug)]
 pub enum BackupMedia {
     Sram(BackupFile),
@@ -32,6 +40,7 @@ pub struct Cartridge {
     pub header: CartridgeHeader,
     bytes: Box<[u8]>,
     size: usize,
+    gpio: Gpio,
     symbols: Option<SymbolTable>, // TODO move it somewhere else
     pub(in crate) backup: BackupMedia,
 }
@@ -46,6 +55,13 @@ use super::sysbus::consts::*;
 
 pub const EEPROM_BASE_ADDR: u32 = 0x0DFF_FF00;
 
+fn is_gpio_access(addr: u32) -> bool {
+    match addr {
+        GPIO_PORT_DATA | GPIO_PORT_DIRECTION | GPIO_PORT_CONTROL => true,
+        _ => false,
+    }
+}
+
 impl Bus for Cartridge {
     fn read_8(&self, addr: Addr) -> u8 {
         let offset = (addr & 0x01ff_ffff) as usize;
@@ -66,6 +82,10 @@ impl Bus for Cartridge {
     }
 
     fn read_16(&self, addr: u32) -> u16 {
+        if is_gpio_access(addr) && self.gpio.is_readable() {
+            return self.gpio.read(addr);
+        }
+
         if addr & 0xff000000 == GAMEPAK_WS2_HI
             && (self.bytes.len() <= 16 * 1024 * 1024 || addr >= EEPROM_BASE_ADDR)
         {
@@ -88,6 +108,11 @@ impl Bus for Cartridge {
     }
 
     fn write_16(&mut self, addr: u32, value: u16) {
+        if is_gpio_access(addr) {
+            self.gpio.write(addr);
+            return;
+        }
+
         if addr & 0xff000000 == GAMEPAK_WS2_HI
             && (self.bytes.len() <= 16 * 1024 * 1024 || addr >= EEPROM_BASE_ADDR)
         {
diff --git a/rustboyadvance-core/src/cartridge/rtc.rs b/rustboyadvance-core/src/cartridge/rtc.rs
new file mode 100644
index 0000000..1c71cfd
--- /dev/null
+++ b/rustboyadvance-core/src/cartridge/rtc.rs
@@ -0,0 +1,27 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Serialize, Deserialize, Clone, Debug)]
+enum Port {
+    #[doc("Serial Clock")]
+    Sck = 0,
+    #[doc("Serial IO")]
+    Sio = 1,
+    #[doc("Chip Select")]
+    Cs = 2,
+}
+
+/// Model of the S3511 8pin RTC
+#[derive(Serialize, Deserialize, Clone, Debug)]
+pub struct Rtc {}
+
+impl Rtc {
+    pub fn new() -> Self {
+        Rtc {}
+    }
+
+    pub fn read_port(port: usize) -> u8 {
+        0
+    }
+
+    pub fn write_port(port: usize) {}
+}