diff --git a/crsn/src/asm/instr/mod.rs b/crsn/src/asm/instr/mod.rs
index d936545..1c69c87 100644
--- a/crsn/src/asm/instr/mod.rs
+++ b/crsn/src/asm/instr/mod.rs
@@ -11,7 +11,7 @@ pub mod cond;
 mod flatten;
 
 /// A higher-level instruction
-#[derive(Debug, Clone)]
+#[derive(Debug)]
 pub struct Instr {
     pub op: OpWrapper,
     pub branches: Option<Vec<(Cond, Vec<Instr>)>>,
diff --git a/crsn/src/asm/instr/op.rs b/crsn/src/asm/instr/op.rs
index cd3ffd4..7fc5a1a 100644
--- a/crsn/src/asm/instr/op.rs
+++ b/crsn/src/asm/instr/op.rs
@@ -1,8 +1,5 @@
 use std::fmt::Debug;
 
-use dyn_clonable::*;
-use sexp::Sexp;
-
 use crate::asm::data::{
     literal::DebugMsg, literal::Label,
     literal::RoutineName,
@@ -11,13 +8,13 @@ use crate::asm::data::{
 };
 use crate::asm::error::Error;
 use crate::asm::instr::Cond;
+use crate::asm::parse::arg_parser::ArgParser;
 use crate::runtime::fault::Fault;
 use crate::runtime::frame::{CallStack, StackFrame};
 use crate::runtime::program::Program;
-use crate::asm::parse::arg_parser::ArgParser;
 
 /// A higher level simple opration
-#[derive(Clone, Debug)]
+#[derive(Debug)]
 pub enum OpWrapper {
     /// Mark a jump target.
     Label(Label),
@@ -30,7 +27,7 @@ pub enum OpWrapper {
 }
 
 /// A low level instruction
-#[derive(Clone, Debug)]
+#[derive(Debug)]
 pub enum Op {
     /// Do nothing
     Nop,
@@ -72,18 +69,16 @@ impl From<Op> for OpWrapper {
     }
 }
 
-#[clonable]
-pub trait OpTrait: Clone + Debug + Send + 'static {
+pub trait OpTrait: Debug + Send + Sync + 'static {
     fn execute(&self, program: &Program, call_stack: &mut CallStack, frame: &mut StackFrame) -> Result<(), Fault>;
 }
 
 pub enum ParseOpResult {
-    Parsed(Box<dyn OpTrait>),
+    Parsed(Box<dyn OpTrait + Send>),
     Unknown(ArgParser),
 }
 
-#[clonable]
-pub trait AsmModule: Clone + Debug + Send + 'static {
+pub trait AsmModule: Debug + Send + 'static {
     /// Get name of the module
     fn name(&self) -> &'static str;
 
diff --git a/crsn/src/asm/mod.rs b/crsn/src/asm/mod.rs
index 859f0e5..00c9e96 100644
--- a/crsn/src/asm/mod.rs
+++ b/crsn/src/asm/mod.rs
@@ -12,326 +12,3 @@ pub fn assemble(source: &str, parsers: &[Box<dyn AsmModule>]) -> Result<Vec<Op>,
     let parsed = parse::parse(source, parsers)?;
     Ok(lower(parsed)?)
 }
-
-#[cfg(test)]
-mod tests {
-    use std::sync::atomic::AtomicU32;
-
-    use crate::asm::data::{DstDisp, Rd, Register, SrcDisp, Wr};
-    use crate::asm::data::literal::{Addr, Label};
-    use crate::asm::instr::{Flatten, OpWrapper, Instr, lower, Op};
-    use crate::asm::instr::Cond;
-    use crate::asm::parse::{parse, parse_instructions};
-
-    #[test]
-    fn test_parse_empty() {
-        let parsed = parse("
-            ()
-        ").unwrap();
-        assert_eq!(Vec::<OpWrapper>::new(), parsed);
-    }
-
-    #[test]
-    fn test_parse_empty_routine() {
-        let parsed = parse("
-            (
-                (hello)
-            )
-        ").unwrap();
-        assert_eq!(vec![
-            OpWrapper::Op(Op::Routine("hello".into())),
-            OpWrapper::Op(Op::Barrier(Some("Routine \"hello\" overrun".into())))
-        ], parsed);
-
-        let parsed = parse("
-            (
-                (hello)
-                (world)
-            )
-        ").unwrap();
-        assert_eq!(vec![
-            OpWrapper::Op(Op::Routine("hello".into())),
-            OpWrapper::Op(Op::Barrier(Some("Routine \"hello\" overrun".into()))),
-            OpWrapper::Op(Op::Routine("world".into())),
-            OpWrapper::Op(Op::Barrier(Some("Routine \"world\" overrun".into())))
-        ], parsed);
-    }
-
-    #[test]
-    fn test_parse_data_formats() {
-        let parsed = parse("
-            (
-                (move
-                    (mov r0 r1)
-                    (mov r15 7)
-                    (mov r15 0xabcd)
-                    (mov r7 0b11110000)
-                    (mov r7 arg1)
-                    (mov r255 arg255)
-                    (mov r7 res0)
-                    (mov r7 res255)
-                    (mov @r0 @r0) ; test in both Rd and Wr positions
-                    (mov @r0 @arg0)
-                    (mov @r0 @res0)
-                    (mov @123456 @0x123456)
-                    (mov @0b010101 @0b010101)
-                )
-            )
-        ").unwrap();
-        assert_eq!(vec![
-            OpWrapper::Op(Op::Routine("move".into())),
-            // (mov r0 r1)
-            OpWrapper::Op(Op::Mov(
-                Wr::new(DstDisp::Register(Register::Gen(0))),
-                Rd::new(SrcDisp::Register(Register::Gen(1))),
-            )),
-            // (mov r15 7)
-            OpWrapper::Op(Op::Mov(
-                Wr::new(DstDisp::Register(Register::Gen(15))),
-                Rd::new(SrcDisp::Immediate(7)),
-            )),
-            // (mov r15 0xabcd)
-            OpWrapper::Op(Op::Mov(
-                Wr::new(DstDisp::Register(Register::Gen(15))),
-                Rd::new(SrcDisp::Immediate(0xabcd)),
-            )),
-            // (mov r7 0b11110000)
-            OpWrapper::Op(Op::Mov(
-                Wr::new(DstDisp::Register(Register::Gen(7))),
-                Rd::new(SrcDisp::Immediate(0b11110000)),
-            )),
-            // (mov r7 arg1)
-            OpWrapper::Op(Op::Mov(
-                Wr::new(DstDisp::Register(Register::Gen(7))),
-                Rd::new(SrcDisp::Register(Register::Arg(1))),
-            )),
-            // (mov r255 arg255)
-            OpWrapper::Op(Op::Mov(
-                Wr::new(DstDisp::Register(Register::Gen(255))),
-                Rd::new(SrcDisp::Register(Register::Arg(255))),
-            )),
-            // (mov r7 res0)
-            OpWrapper::Op(Op::Mov(
-                Wr::new(DstDisp::Register(Register::Gen(7))),
-                Rd::new(SrcDisp::Register(Register::Res(0))),
-            )),
-            // (mov r7 res255)
-            OpWrapper::Op(Op::Mov(
-                Wr::new(DstDisp::Register(Register::Gen(7))),
-                Rd::new(SrcDisp::Register(Register::Res(255))),
-            )),
-            // (mov @r0 @r0)
-            OpWrapper::Op(Op::Mov(
-                Wr::new(DstDisp::RegisterPtr(Register::Gen(0))),
-                Rd::new(SrcDisp::RegisterPtr(Register::Gen(0))),
-            )),
-            // (mov @r0 @arg0)
-            OpWrapper::Op(Op::Mov(
-                Wr::new(DstDisp::RegisterPtr(Register::Gen(0))),
-                Rd::new(SrcDisp::RegisterPtr(Register::Arg(0))),
-            )),
-            // (mov @r0 @res0)
-            OpWrapper::Op(Op::Mov(
-                Wr::new(DstDisp::RegisterPtr(Register::Gen(0))),
-                Rd::new(SrcDisp::RegisterPtr(Register::Res(0))),
-            )),
-            // (mov @123456 @0x123456)
-            OpWrapper::Op(Op::Mov(
-                Wr::new(DstDisp::ImmediatePtr(Addr(123456))),
-                Rd::new(SrcDisp::ImmediatePtr(Addr(0x123456))),
-            )),
-            //  (mov @0b010101 @0b010101)
-            OpWrapper::Op(Op::Mov(
-                Wr::new(DstDisp::ImmediatePtr(Addr(0b010101))),
-                Rd::new(SrcDisp::ImmediatePtr(Addr(0b010101))),
-            )),
-            OpWrapper::Op(Op::Barrier(Some("Routine \"move\" overrun".into()))),
-        ], parsed);
-    }
-
-    fn parse_single_instr(src: &str) -> anyhow::Result<Instr> {
-        Ok(parse_instructions(vec![sexp::parse(src)?])?.remove(0))
-    }
-
-    fn parse_single_op(src: &str) -> anyhow::Result<Vec<OpWrapper>> {
-        let num = AtomicU32::new(0);
-        Ok(parse_single_instr(src)?.flatten(&num)?)
-    }
-
-    #[test]
-    fn test_parse_single() {
-        let parsed = parse_single_op("(mov r0 r1)").unwrap();
-        assert_eq!(vec![
-            OpWrapper::Op(Op::Mov(
-                Wr::new(DstDisp::Register(Register::Gen(0))),
-                Rd::new(SrcDisp::Register(Register::Gen(1))),
-            )),
-        ], parsed);
-    }
-
-    #[test]
-    fn test_branches_instr() {
-        let parsed = parse_single_instr("
-            (cmp r0 r1 (eq? (mov r0 r0) (mov r1 r2)) (>? (mov r0 r0) (mov r1 r1)))
-        ").unwrap();
-        assert_eq!(
-            Instr {
-                op: OpWrapper::Op(Op::Cmp(
-                    Rd::new(SrcDisp::Register(Register::Gen(0))),
-                    Rd::new(SrcDisp::Register(Register::Gen(1))),
-                )),
-                branches: Some(vec![
-                    (
-                        Cond::Equal,
-                        vec![
-                            Instr {
-                                op: OpWrapper::Op(Op::Mov(
-                                    Wr::new(DstDisp::Register(Register::Gen(0))),
-                                    Rd::new(SrcDisp::Register(Register::Gen(0))),
-                                )),
-                                branches: None,
-                            },
-                            Instr {
-                                op: OpWrapper::Op(Op::Mov(
-                                    Wr::new(DstDisp::Register(Register::Gen(1))),
-                                    Rd::new(SrcDisp::Register(Register::Gen(2))),
-                                )),
-                                branches: None,
-                            }
-                        ]
-                    ),
-                    (
-                        Cond::Greater,
-                        vec![
-                            Instr {
-                                op: OpWrapper::Op(Op::Mov(
-                                    Wr::new(DstDisp::Register(Register::Gen(0))),
-                                    Rd::new(SrcDisp::Register(Register::Gen(0))),
-                                )),
-                                branches: None,
-                            },
-                            Instr {
-                                op: OpWrapper::Op(Op::Mov(
-                                    Wr::new(DstDisp::Register(Register::Gen(1))),
-                                    Rd::new(SrcDisp::Register(Register::Gen(1))),
-                                )),
-                                branches: None,
-                            }
-                        ]
-                    )
-                ]),
-            }
-            , parsed);
-    }
-
-    #[test]
-    fn test_branches_op() {
-        let parsed = parse_single_op("
-            (cmp r0 r1
-                (eq?
-                    (mov r0 r0)
-                    (mov r1 r2))
-                (>?
-                    (mov r0 r0)
-                    (mov r1 r1)))
-        ").unwrap();
-        assert_eq!(
-            vec![
-                OpWrapper::Op(Op::Cmp(
-                    Rd::new(SrcDisp::Register(Register::Gen(0))),
-                    Rd::new(SrcDisp::Register(Register::Gen(1))),
-                )),
-                OpWrapper::JumpIf(Cond::NotEqual, Label::Numbered(1)),
-                OpWrapper::Op(Op::Mov(
-                    Wr::new(DstDisp::Register(Register::Gen(0))),
-                    Rd::new(SrcDisp::Register(Register::Gen(0))),
-                )),
-                OpWrapper::Op(Op::Mov(
-                    Wr::new(DstDisp::Register(Register::Gen(1))),
-                    Rd::new(SrcDisp::Register(Register::Gen(2))),
-                )),
-                OpWrapper::Jump(Label::Numbered(0)),
-                OpWrapper::Label(Label::Numbered(1)),
-                OpWrapper::JumpIf(Cond::LowerOrEqual, Label::Numbered(0)),
-                OpWrapper::Op(Op::Mov(
-                    Wr::new(DstDisp::Register(Register::Gen(0))),
-                    Rd::new(SrcDisp::Register(Register::Gen(0))),
-                )),
-                OpWrapper::Op(Op::Mov(
-                    Wr::new(DstDisp::Register(Register::Gen(1))),
-                    Rd::new(SrcDisp::Register(Register::Gen(1))),
-                )),
-                OpWrapper::Label(Label::Numbered(0)),
-            ], parsed);
-    }
-
-
-    #[test]
-    fn test_jumps_to_skips() {
-        let parsed = parse("(
-            (foo
-                (:foo)
-                (:unused)
-                (:whatever)
-                (mov r0 r0)
-                (j :foo)
-                (j :foo)
-                (mov r0 r0)
-                (mov r0 r0)
-                (j :whatever)
-                (j.if eq :whatever)
-            )
-        )").unwrap();
-
-        assert_eq!(
-            vec![
-                Op::Routine("foo".into()).into(),
-                OpWrapper::Label(Label::Named("foo".to_string())),
-                OpWrapper::Label(Label::Named("unused".to_string())),
-                OpWrapper::Label(Label::Named("whatever".to_string())),
-                Op::Mov(
-                    Wr::new(DstDisp::Register(Register::Gen(0))),
-                    Rd::new(SrcDisp::Register(Register::Gen(0))),
-                ).into(),
-                OpWrapper::Jump(Label::Named("foo".to_string())),
-                OpWrapper::Jump(Label::Named("foo".to_string())),
-                Op::Mov(
-                    Wr::new(DstDisp::Register(Register::Gen(0))),
-                    Rd::new(SrcDisp::Register(Register::Gen(0))),
-                ).into(),
-                Op::Mov(
-                    Wr::new(DstDisp::Register(Register::Gen(0))),
-                    Rd::new(SrcDisp::Register(Register::Gen(0))),
-                ).into(),
-                OpWrapper::Jump(Label::Named("whatever".to_string())),
-                OpWrapper::JumpIf(Cond::Equal, Label::Named("whatever".to_string())),
-                Op::Barrier(Some("Routine \"foo\" overrun".into())).into(),
-            ], parsed);
-
-        // Labels are removed and jumps become skips
-
-        let cleaned = lower(parsed).unwrap();
-
-        assert_eq!(
-            vec![
-                Op::Routine("foo".into()),
-                Op::Mov(
-                    Wr::new(DstDisp::Register(Register::Gen(0))),
-                    Rd::new(SrcDisp::Register(Register::Gen(0))),
-                ),
-                Op::Skip(Rd::new(SrcDisp::Immediate(-1))),
-                Op::Skip(Rd::new(SrcDisp::Immediate(-2))),
-                Op::Mov(
-                    Wr::new(DstDisp::Register(Register::Gen(0))),
-                    Rd::new(SrcDisp::Register(Register::Gen(0))),
-                ),
-                Op::Mov(
-                    Wr::new(DstDisp::Register(Register::Gen(0))),
-                    Rd::new(SrcDisp::Register(Register::Gen(0))),
-                ),
-                Op::Skip(Rd::new(SrcDisp::Immediate(-5))),
-                Op::SkipIf(Cond::Equal, Rd::new(SrcDisp::Immediate(-6))),
-                Op::Barrier(Some("Routine \"foo\" overrun".into())),
-            ], cleaned);
-    }
-}
diff --git a/crsn/src/asm/parse/parse_op.rs b/crsn/src/asm/parse/parse_op.rs
index e6a4ab4..e13bd63 100644
--- a/crsn/src/asm/parse/parse_op.rs
+++ b/crsn/src/asm/parse/parse_op.rs
@@ -1,16 +1,13 @@
-use sexp::Sexp;
-
 use crate::asm::data::literal::{Label, RoutineName};
-use crate::asm::error::{AsmError, Error};
-use crate::asm::instr::{OpWrapper, Op};
+use crate::asm::error::Error;
+use crate::asm::instr::{Op, OpWrapper};
 use crate::asm::instr::cond::parse_cond;
 use crate::asm::instr::op::{AsmModule, ParseOpResult};
-use crate::asm::parse::parse_data::{parse_label, parse_rd, parse_wr};
-use crate::asm::parse::sexp_expect::expect_string_atom;
 use crate::asm::parse::arg_parser::ArgParser;
+use crate::asm::parse::parse_data::{parse_label, parse_rd};
+use crate::asm::parse::sexp_expect::expect_string_atom;
 
 pub fn parse_op(keyword: &str, far: bool, mut arg_tokens: ArgParser, parsers: &[Box<dyn AsmModule>]) -> Result<OpWrapper, Error> {
-
     Ok(match keyword {
         "j" => {
             let dest = parse_label(arg_tokens.next())?;
diff --git a/crsn/src/runtime/fault.rs b/crsn/src/runtime/fault.rs
index 4498342..c76275f 100644
--- a/crsn/src/runtime/fault.rs
+++ b/crsn/src/runtime/fault.rs
@@ -60,6 +60,9 @@ pub enum Fault {
 
     #[error("Call stack underflow")]
     CallStackUnderflow,
+
+    #[error("Attempt to read undefined extension data store")]
+    ExtDataNotDefined,
 }
 
 #[derive(Error, Debug)]
diff --git a/crsn/src/runtime/program.rs b/crsn/src/runtime/program.rs
index e172aa3..1713e12 100644
--- a/crsn/src/runtime/program.rs
+++ b/crsn/src/runtime/program.rs
@@ -1,10 +1,12 @@
+
 use std::collections::HashMap;
 
 use crate::asm::data::literal::{Addr, Label, RoutineName};
 use crate::asm::instr::Op;
 use crate::runtime::fault::Fault;
+use std::sync::Arc;
 
-#[derive(Clone, Debug)]
+#[derive(Debug)]
 pub struct Program {
     ops: Vec<Op>,
     routines: HashMap<RoutineName, Addr>,
@@ -13,7 +15,7 @@ pub struct Program {
 }
 
 impl Program {
-    pub fn new(ops: Vec<Op>) -> Self {
+    pub fn new(ops: Vec<Op>) -> Arc<Self> {
         let mut p = Self {
             ops,
             routines: Default::default(),
@@ -21,7 +23,7 @@ impl Program {
             barriers: Default::default(),
         };
         p.scan();
-        p
+        Arc::new(p)
     }
 
     /// Find all the named things
diff --git a/crsn/src/runtime/run_thread.rs b/crsn/src/runtime/run_thread.rs
index 146b050..4ba46c4 100644
--- a/crsn/src/runtime/run_thread.rs
+++ b/crsn/src/runtime/run_thread.rs
@@ -1,3 +1,5 @@
+use std::any::{Any, TypeId};
+use std::collections::HashMap;
 use std::thread::JoinHandle;
 use std::time::Duration;
 
@@ -5,6 +7,8 @@ use crate::asm::data::literal::Addr;
 use crate::runtime::exec::EvalRes;
 use crate::runtime::frame::{CallStack, StackFrame};
 use crate::runtime::program::Program;
+use std::sync::Arc;
+
 
 const CYCLE_TIME: Duration = Duration::from_millis(0);
 //const CYCLE_TIME : Duration = Duration::from_millis(100);
@@ -20,11 +24,13 @@ pub struct RunThread {
     /// Call stack
     pub call_stack: CallStack,
     /// Program to run
-    pub program: Program,
+    pub program: Arc<Program>,
+    /// Extension data
+    ext_data: HashMap<TypeId, Box<dyn Any + Send>>,
 }
 
 impl RunThread {
-    pub fn new(id: ThreadToken, program: Program, pc: Addr, args: &[u64]) -> Self {
+    pub fn new(id: ThreadToken, program: Arc<Program>, pc: Addr, args: &[u64]) -> Self {
         let sf = StackFrame::new(pc, args);
 
         Self {
@@ -32,6 +38,7 @@ impl RunThread {
             frame: sf,
             call_stack: vec![],
             program,
+            ext_data: Default::default(),
         }
     }
 
@@ -58,5 +65,13 @@ impl RunThread {
             }
         }
     }
-}
 
+    pub fn ext_mut<T: Send + Default + 'static>(&mut self) -> &mut T {
+        if !self.ext_data.contains_key(&TypeId::of::<T>()) {
+            self.ext_data.insert(TypeId::of::<T>(), Box::new(T::default()));
+        }
+
+        self.ext_data.get_mut(&TypeId::of::<T>()).unwrap()
+            .downcast_mut().unwrap()
+    }
+}
diff --git a/crsn_arith/src/parse.rs b/crsn_arith/src/parse.rs
index b981fa1..70d80bd 100644
--- a/crsn_arith/src/parse.rs
+++ b/crsn_arith/src/parse.rs
@@ -1,8 +1,8 @@
 use crsn::asm::data::{Rd, Wr};
-use crsn::asm::error::{AsmError, Error};
-use crsn::asm::instr::op::{AsmModule, OpTrait, ParseOpResult};
-use crsn::asm::parse::parse_data::{parse_rd, parse_wr};
-use crsn::sexp::Sexp;
+use crsn::asm::error::{Error};
+use crsn::asm::instr::op::{AsmModule, ParseOpResult};
+
+
 
 use crate::defs::ArithOp;
 use crsn::asm::parse::arg_parser::ArgParser;