diff --git a/src/cpus/m68k/debugger.rs b/src/cpus/m68k/debugger.rs
index 58e94b6..3fbe792 100644
--- a/src/cpus/m68k/debugger.rs
+++ b/src/cpus/m68k/debugger.rs
@@ -1,7 +1,7 @@
 
 use crate::error::Error;
 use crate::system::System;
-use crate::memory::{Address, Addressable};
+use crate::devices::{Address, Addressable};
 
 use super::state::M68k;
 use super::decode::M68kDecoder;
diff --git a/src/cpus/m68k/decode.rs b/src/cpus/m68k/decode.rs
index 0bf5912..c6422cf 100644
--- a/src/cpus/m68k/decode.rs
+++ b/src/cpus/m68k/decode.rs
@@ -3,8 +3,7 @@ use std::fmt;
 
 use crate::error::Error;
 use crate::system::System;
-use crate::memory::{Address, Addressable};
-use crate::devices::AddressableDeviceRefMut;
+use crate::devices::{Address, Addressable, AddressableDeviceRefMut};
 
 use super::state::{M68kType, Exceptions};
 
diff --git a/src/cpus/m68k/execute.rs b/src/cpus/m68k/execute.rs
index 2895b02..988a32c 100644
--- a/src/cpus/m68k/execute.rs
+++ b/src/cpus/m68k/execute.rs
@@ -1,8 +1,7 @@
 
 use crate::system::System;
 use crate::error::{ErrorType, Error};
-use crate::memory::{Address, Addressable};
-use crate::devices::{Clock, Steppable, Interruptable};
+use crate::devices::{Clock, Address, Steppable, Interruptable, Addressable};
 
 use super::decode::{
     Instruction,
diff --git a/src/cpus/m68k/state.rs b/src/cpus/m68k/state.rs
index 9bc7388..e5ec3be 100644
--- a/src/cpus/m68k/state.rs
+++ b/src/cpus/m68k/state.rs
@@ -1,6 +1,6 @@
 
 use crate::system::System;
-use crate::memory::Address;
+use crate::devices::Address;
 use crate::timers::CpuTimer;
 
 use super::decode::M68kDecoder;
diff --git a/src/cpus/m68k/tests.rs b/src/cpus/m68k/tests.rs
index ea7489e..1151cf0 100644
--- a/src/cpus/m68k/tests.rs
+++ b/src/cpus/m68k/tests.rs
@@ -2,11 +2,11 @@
 #[cfg(test)]
 mod decode_tests {
     use crate::system::System;
-    use crate::devices::{Steppable, AddressableDeviceBox, wrap_addressable};
-    use crate::memory::{Address, Addressable, MemoryBlock};
+    use crate::memory::MemoryBlock;
+    use crate::devices::{Address, Addressable, Steppable, AddressableDeviceBox, wrap_addressable, MAX_READ};
 
     use crate::cpus::m68k::{M68k, M68kType};
-    use crate::cpus::m68k::decode::{Instruction, Target, Size, Sign, RegisterType, ShiftDirection};
+    use crate::cpus::m68k::decode::{Instruction, Target, Size, Sign, XRegister, ShiftDirection};
 
     const INIT_STACK: Address = 0x00002000;
     const INIT_ADDR: Address = 0x00000010;
@@ -136,7 +136,7 @@ mod decode_tests {
 
         let memory = get_decode_memory(&mut cpu, &system);
         let target = cpu.decoder.get_mode_as_target(&mut memory.borrow_mut(), 0b110, 0b010, Some(size)).unwrap();
-        assert_eq!(target, Target::IndirectARegXRegOffset(2, RegisterType::Data, 3, offset, size));
+        assert_eq!(target, Target::IndirectARegXRegOffset(2, XRegister::Data(3), offset, 0, size));
     }
 
     #[test]
@@ -194,7 +194,7 @@ mod decode_tests {
 
         let memory = get_decode_memory(&mut cpu, &system);
         let target = cpu.decoder.get_mode_as_target(&mut memory.borrow_mut(), 0b111, 0b011, Some(size)).unwrap();
-        assert_eq!(target, Target::IndirectPCXRegOffset(RegisterType::Data, 3, offset, size));
+        assert_eq!(target, Target::IndirectPCXRegOffset(XRegister::Data(3), offset, 0, size));
     }
 
     #[test]
@@ -331,8 +331,8 @@ mod decode_tests {
 #[cfg(test)]
 mod execute_tests {
     use crate::system::System;
-    use crate::devices::{Steppable, wrap_addressable};
-    use crate::memory::{Address, Addressable, MemoryBlock};
+    use crate::memory::MemoryBlock;
+    use crate::devices::{Address, Addressable, Steppable, wrap_addressable};
 
     use crate::cpus::m68k::{M68k, M68kType};
     use crate::cpus::m68k::decode::{Instruction, Target, Size, Sign, ShiftDirection};
@@ -419,22 +419,22 @@ mod execute_tests {
     fn instruction_andi_sr() {
         let (mut cpu, system) = init_test();
 
-        cpu.state.sr = 0x87AA;
+        cpu.state.sr = 0xA7AA;
         cpu.decoder.instruction = Instruction::ANDtoSR(0xF8FF);
 
         cpu.execute_current(&system).unwrap();
-        assert_eq!(cpu.state.sr, 0x80AA);
+        assert_eq!(cpu.state.sr, 0xA0AA);
     }
 
     #[test]
     fn instruction_ori_sr() {
         let (mut cpu, system) = init_test();
 
-        cpu.state.sr = 0x8755;
+        cpu.state.sr = 0xA755;
         cpu.decoder.instruction = Instruction::ORtoSR(0x00AA);
 
         cpu.execute_current(&system).unwrap();
-        assert_eq!(cpu.state.sr, 0x87FF);
+        assert_eq!(cpu.state.sr, 0xA7FF);
     }
 
     #[test]
diff --git a/src/devices.rs b/src/devices.rs
index 355c2eb..68d9c7e 100644
--- a/src/devices.rs
+++ b/src/devices.rs
@@ -4,10 +4,12 @@ use std::cell::{RefCell, RefMut};
 
 use crate::error::Error;
 use crate::system::System;
-use crate::memory::{Addressable};
 
 
+pub const MAX_READ: usize = 4;
+
 pub type Clock = u64;
+pub type Address = u64;
 
 /// A device that can change state over time.  The `step()` method will be called
 /// by the containing `System` when the system clock advances.  If an error occurs
@@ -25,6 +27,40 @@ pub trait Interruptable {
     fn interrupt_state_change(&mut self, state: bool, priority: u8, number: u8) -> Result<(), Error>;
 }
 
+/// A device that can be addressed to read data from or write data to the device.
+pub trait Addressable {
+    fn len(&self) -> usize;
+    fn read(&mut self, addr: Address, count: usize) -> Result<[u8; MAX_READ], Error>;
+    fn write(&mut self, addr: Address, data: &[u8]) -> Result<(), Error>;
+
+    fn read_u8(&mut self, addr: Address) -> Result<u8, Error> {
+        Ok(self.read(addr, 1)?[0])
+    }
+
+    fn read_beu16(&mut self, addr: Address) -> Result<u16, Error> {
+        Ok(read_beu16(&self.read(addr, 2)?))
+    }
+
+    fn read_beu32(&mut self, addr: Address) -> Result<u32, Error> {
+        Ok(read_beu32(&self.read(addr, 4)?))
+    }
+
+    fn write_u8(&mut self, addr: Address, value: u8) -> Result<(), Error> {
+        let data = [value];
+        self.write(addr, &data)
+    }
+
+    fn write_beu16(&mut self, addr: Address, value: u16) -> Result<(), Error> {
+        let data = write_beu16(value);
+        self.write(addr, &data)
+    }
+
+    fn write_beu32(&mut self, addr: Address, value: u32) -> Result<(), Error> {
+        let data = write_beu32(value);
+        self.write(addr, &data)
+    }
+}
+
 
 pub trait AddressableDevice: Addressable + Steppable { }
 pub trait InterruptableDevice: Interruptable + Steppable { }
@@ -52,3 +88,36 @@ pub enum Device {
 }
 
 
+
+#[inline(always)]
+pub fn read_beu16(data: &[u8]) -> u16 {
+    (data[0] as u16) << 8 |
+    (data[1] as u16)
+}
+
+#[inline(always)]
+pub fn read_beu32(data: &[u8]) -> u32 {
+    (data[0] as u32) << 24 |
+    (data[1] as u32) << 16 |
+    (data[2] as u32) << 8 |
+    (data[3] as u32)
+}
+
+#[inline(always)]
+pub fn write_beu16(value: u16) -> [u8; 2] {
+    [
+        (value >> 8) as u8,
+        value as u8,
+    ]
+}
+
+#[inline(always)]
+pub fn write_beu32(value: u32) -> [u8; 4] {
+    [
+        (value >> 24) as u8,
+        (value >> 16) as u8,
+        (value >> 8) as u8,
+        value as u8,
+    ]
+}
+
diff --git a/src/host/mod.rs b/src/host/mod.rs
new file mode 100644
index 0000000..b94a5da
--- /dev/null
+++ b/src/host/mod.rs
@@ -0,0 +1,3 @@
+ 
+pub mod ttys;
+
diff --git a/src/ttys.rs b/src/host/ttys.rs
similarity index 100%
rename from src/ttys.rs
rename to src/host/ttys.rs
diff --git a/src/main.rs b/src/main.rs
index 8f7036f..8d45390 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -4,11 +4,12 @@ mod error;
 mod memory;
 mod timers;
 mod devices;
-mod ttys;
 mod interrupts;
+mod system;
+
+mod host;
 mod cpus;
 mod peripherals;
-mod system;
 
 use crate::system::System;
 use crate::memory::MemoryBlock;
diff --git a/src/memory.rs b/src/memory.rs
index b315ae4..28b39ca 100644
--- a/src/memory.rs
+++ b/src/memory.rs
@@ -3,46 +3,7 @@ use std::fs;
 
 use crate::error::Error;
 use crate::system::System;
-use crate::devices::{Clock, Steppable, AddressableDeviceBox};
-
-
-pub const MAX_READ: usize = 4;
-
-pub type Address = u64;
-
-/// A device that can be addressed to read data from or write data to the device.
-pub trait Addressable {
-    fn len(&self) -> usize;
-    fn read(&mut self, addr: Address, count: usize) -> Result<[u8; MAX_READ], Error>;
-    fn write(&mut self, addr: Address, data: &[u8]) -> Result<(), Error>;
-
-    fn read_u8(&mut self, addr: Address) -> Result<u8, Error> {
-        Ok(self.read(addr, 1)?[0])
-    }
-
-    fn read_beu16(&mut self, addr: Address) -> Result<u16, Error> {
-        Ok(read_beu16(&self.read(addr, 2)?))
-    }
-
-    fn read_beu32(&mut self, addr: Address) -> Result<u32, Error> {
-        Ok(read_beu32(&self.read(addr, 4)?))
-    }
-
-    fn write_u8(&mut self, addr: Address, value: u8) -> Result<(), Error> {
-        let data = [value];
-        self.write(addr, &data)
-    }
-
-    fn write_beu16(&mut self, addr: Address, value: u16) -> Result<(), Error> {
-        let data = write_beu16(value);
-        self.write(addr, &data)
-    }
-
-    fn write_beu32(&mut self, addr: Address, value: u32) -> Result<(), Error> {
-        let data = write_beu32(value);
-        self.write(addr, &data)
-    }
-}
+use crate::devices::{Clock, Address, Steppable, Addressable, AddressableDeviceBox, MAX_READ};
 
 
 pub struct MemoryBlock {
@@ -189,35 +150,3 @@ impl Addressable for Bus {
 }
 
 
-#[inline(always)]
-pub fn read_beu16(data: &[u8]) -> u16 {
-    (data[0] as u16) << 8 |
-    (data[1] as u16)
-}
-
-#[inline(always)]
-pub fn read_beu32(data: &[u8]) -> u32 {
-    (data[0] as u32) << 24 |
-    (data[1] as u32) << 16 |
-    (data[2] as u32) << 8 |
-    (data[3] as u32)
-}
-
-#[inline(always)]
-pub fn write_beu16(value: u16) -> [u8; 2] {
-    [
-        (value >> 8) as u8,
-        value as u8,
-    ]
-}
-
-#[inline(always)]
-pub fn write_beu32(value: u32) -> [u8; 4] {
-    [
-        (value >> 24) as u8,
-        (value >> 16) as u8,
-        (value >> 8) as u8,
-        value as u8,
-    ]
-}
-
diff --git a/src/peripherals/ata.rs b/src/peripherals/ata.rs
index 5fe47f5..f815909 100644
--- a/src/peripherals/ata.rs
+++ b/src/peripherals/ata.rs
@@ -3,8 +3,7 @@ use std::fs;
 
 use crate::error::Error;
 use crate::system::System;
-use crate::memory::{Address, Addressable, MAX_READ};
-use crate::devices::{Clock, Steppable};
+use crate::devices::{Clock, Address, Steppable, Addressable, MAX_READ};
 
 
 const ATA_REG_DATA_WORD: Address        = 0x20;
diff --git a/src/peripherals/mc68681.rs b/src/peripherals/mc68681.rs
index aa2ef84..bd33c69 100644
--- a/src/peripherals/mc68681.rs
+++ b/src/peripherals/mc68681.rs
@@ -1,9 +1,9 @@
 
 use crate::error::Error;
 use crate::system::System;
-use crate::devices::{Clock, Steppable};
-use crate::memory::{Address, Addressable, MAX_READ};
-use crate::ttys::{SimplePty, SharedSimplePty};
+use crate::devices::{Clock, Address, Steppable, Addressable, MAX_READ};
+
+use crate::host::ttys::{SimplePty, SharedSimplePty};
 
 
 const REG_MR1A_MR2A: Address = 0x01;
diff --git a/src/system.rs b/src/system.rs
index 333eed7..862aee3 100644
--- a/src/system.rs
+++ b/src/system.rs
@@ -2,9 +2,9 @@
 use std::cell::{RefCell, RefMut};
 
 use crate::error::Error;
+use crate::memory::Bus;
 use crate::interrupts::InterruptController;
-use crate::memory::{Address, Bus};
-use crate::devices::{Device, AddressableDeviceBox, InterruptableDeviceBox, Clock};
+use crate::devices::{Address, Device, AddressableDeviceBox, InterruptableDeviceBox, Clock};
 
 
 pub struct System {