diff --git a/crsn/src/asm/data/reg.rs b/crsn/src/asm/data/reg.rs
index d9333b8..25d2d11 100644
--- a/crsn/src/asm/data/reg.rs
+++ b/crsn/src/asm/data/reg.rs
@@ -1,6 +1,6 @@
 use std::fmt::{self, Display, Formatter};
 
-use crate::asm::error::Error;
+use crate::asm::error::CrsnError;
 
 /// Register name
 #[derive(Debug, Clone, Copy, Eq, PartialEq)]
@@ -34,6 +34,6 @@ pub fn parse_reg(name: &str) -> anyhow::Result<Register> {
         let val: u8 = rn.parse()?;
         Ok(Register::Gen(val))
     } else {
-        Err(Error::Parse(format!("Bad reg name: {}", name).into()))?
+        Err(CrsnError::Parse(format!("Bad reg name: {}", name).into()))?
     }
 }
diff --git a/crsn/src/asm/error.rs b/crsn/src/asm/error.rs
index dc89b9e..8ee60d4 100644
--- a/crsn/src/asm/error.rs
+++ b/crsn/src/asm/error.rs
@@ -8,7 +8,7 @@ use crate::asm::instr::Cond;
 
 /// csn_asm unified error type
 #[derive(Error, Debug)]
-pub enum Error {
+pub enum CrsnError {
     #[error("S-expression syntax error: {0:?}")]
     PreParse(#[from] Box<sexp::Error>),
     #[error("Parse error: {0:?}")]
diff --git a/crsn/src/asm/instr/cond.rs b/crsn/src/asm/instr/cond.rs
index d3bc847..4da4ddf 100644
--- a/crsn/src/asm/instr/cond.rs
+++ b/crsn/src/asm/instr/cond.rs
@@ -1,7 +1,7 @@
 use std::fmt::{self, Display, Formatter};
 use std::ops::Not;
 
-use crate::asm::error::Error;
+use crate::asm::error::CrsnError;
 
 /// Condition flag
 #[derive(Clone, Copy, Debug, Eq, PartialEq, Hash)]
@@ -45,7 +45,7 @@ pub enum Cond {
     NotCarry,
 }
 
-pub fn parse_cond(text: &str) -> Result<Cond, Error> {
+pub fn parse_cond(text: &str) -> Result<Cond, CrsnError> {
     Ok(match text.trim_end_matches('?') {
         "eq" | "=" | "==" => Cond::Equal,
         "ne" | "<>" | "!=" | "≠" => Cond::NotEqual,
@@ -66,7 +66,7 @@ pub fn parse_cond(text: &str) -> Result<Cond, Error> {
         "ov" | "^" => Cond::Overflow,
         "nov" | "!ov" | "!^" => Cond::NotOverflow,
         _ => {
-            return Err(Error::Parse(format!("Unknown cond: {}", text).into()));
+            return Err(CrsnError::Parse(format!("Unknown cond: {}", text).into()));
         }
     })
 }
diff --git a/crsn/src/asm/instr/flatten.rs b/crsn/src/asm/instr/flatten.rs
index 6cc2baa..da6436f 100644
--- a/crsn/src/asm/instr/flatten.rs
+++ b/crsn/src/asm/instr/flatten.rs
@@ -3,17 +3,17 @@ use std::sync::atomic::AtomicU32;
 
 use crate::asm::data::{Rd, RdData};
 use crate::asm::data::literal::{Label, Value};
-use crate::asm::error::{AsmError, Error};
+use crate::asm::error::{AsmError, CrsnError};
 use crate::asm::instr::{Cond, Instr, Op, Routine};
 use crate::builtin::defs::BuiltinOp;
 
 /// A trait for something that can turn into multiple instructions
 pub trait Flatten {
-    fn flatten(self, label_num: &AtomicU32) -> Result<Vec<Op>, Error>;
+    fn flatten(self, label_num: &AtomicU32) -> Result<Vec<Op>, CrsnError>;
 }
 
 impl Flatten for Instr {
-    fn flatten(self, label_num: &AtomicU32) -> Result<Vec<Op>, Error> {
+    fn flatten(self, label_num: &AtomicU32) -> Result<Vec<Op>, CrsnError> {
         let mut ops = vec![self.op];
 
         if let Some(branches) = self.branches {
@@ -22,7 +22,7 @@ impl Flatten for Instr {
             let end_lbl = Label::unique(label_num);
             for (cnt, (cond, branch)) in branches.into_iter().enumerate() {
                 if labels.contains_key(&cond) {
-                    return Err(Error::Asm(AsmError::ConditionalAlreadyUsed(cond)));
+                    return Err(CrsnError::Asm(AsmError::ConditionalAlreadyUsed(cond)));
                 }
 
                 let next_lbl = if cnt == branch_count - 1 {
@@ -49,7 +49,7 @@ impl Flatten for Instr {
 }
 
 impl Flatten for Routine {
-    fn flatten(self, label_num: &AtomicU32) -> Result<Vec<Op>, Error> {
+    fn flatten(self, label_num: &AtomicU32) -> Result<Vec<Op>, CrsnError> {
         let mut ops = vec![
             BuiltinOp::Routine(self.name.clone()).into(),
         ];
@@ -65,7 +65,7 @@ impl Flatten for Routine {
 }
 
 /// Convert jumps to relative skips
-fn numbered_labels_to_skips(ops: Vec<Op>) -> Result<Vec<Op>, Error> {
+fn numbered_labels_to_skips(ops: Vec<Op>) -> Result<Vec<Op>, CrsnError> {
     let mut label_positions = HashMap::<Label, usize>::new();
     for (n, op) in ops.iter().enumerate() {
         if let Op::BuiltIn(BuiltinOp::Label(name @ Label::Numbered(_))) = op {
@@ -85,7 +85,7 @@ fn numbered_labels_to_skips(ops: Vec<Op>) -> Result<Vec<Op>, Error> {
                     let skip = *dest as isize - n as isize + skipped;
                     cleaned.push(Op::BuiltIn(BuiltinOp::Skip(Rd::new(RdData::Immediate(skip as Value)))));
                 } else {
-                    return Err(Error::Asm(AsmError::LabelNotDefined(target)));
+                    return Err(CrsnError::Asm(AsmError::LabelNotDefined(target)));
                 }
             }
             Op::BuiltIn(BuiltinOp::JumpIf(cond, target @ Label::Numbered(_))) => {
@@ -93,7 +93,7 @@ fn numbered_labels_to_skips(ops: Vec<Op>) -> Result<Vec<Op>, Error> {
                     let skip = *dest as isize - n as isize + skipped;
                     cleaned.push(Op::BuiltIn(BuiltinOp::SkipIf(cond, Rd::new(RdData::Immediate(skip as Value)))));
                 } else {
-                    return Err(Error::Asm(AsmError::LabelNotDefined(target)));
+                    return Err(CrsnError::Asm(AsmError::LabelNotDefined(target)));
                 }
             }
             other => {
diff --git a/crsn/src/asm/mod.rs b/crsn/src/asm/mod.rs
index f41c510..faab7d8 100644
--- a/crsn/src/asm/mod.rs
+++ b/crsn/src/asm/mod.rs
@@ -10,7 +10,7 @@ pub mod parse;
 pub mod patches;
 
 /// Parse a program from string and assemble a low level instruction sequence from it.
-pub fn assemble(source: &str, parsers: Arc<Vec<Box<dyn CrsnExtension>>>) -> Result<Arc<Program>, error::Error> {
+pub fn assemble(source: &str, parsers: Arc<Vec<Box<dyn CrsnExtension>>>) -> Result<Arc<Program>, error::CrsnError> {
     let ops = parse::parse(source, &parsers)?;
 
     trace!("--- Compiled program ---");
diff --git a/crsn/src/asm/parse/mod.rs b/crsn/src/asm/parse/mod.rs
index 27b7497..fe08f4e 100644
--- a/crsn/src/asm/parse/mod.rs
+++ b/crsn/src/asm/parse/mod.rs
@@ -3,7 +3,7 @@ use std::sync::atomic::AtomicU32;
 pub use parse_instr::parse_instructions;
 use parse_routines::parse_routines;
 
-use crate::asm::error::Error;
+use crate::asm::error::CrsnError;
 use crate::asm::instr::{Flatten, Op, Routine};
 use crate::asm::parse::sexp_expect::expect_list;
 use crate::module::CrsnExtension;
@@ -17,7 +17,7 @@ mod parse_op;
 
 pub mod arg_parser;
 
-pub fn parse(source: &str, parsers: &[Box<dyn CrsnExtension>]) -> Result<Vec<Op>, Error> {
+pub fn parse(source: &str, parsers: &[Box<dyn CrsnExtension>]) -> Result<Vec<Op>, CrsnError> {
     let root = sexp::parse(source)?;
 
     let subs: Vec<Routine> = parse_routines(expect_list(Some(root), true)?, parsers)?;
diff --git a/crsn/src/asm/parse/parse_cond.rs b/crsn/src/asm/parse/parse_cond.rs
index 8532cb5..177aa80 100644
--- a/crsn/src/asm/parse/parse_cond.rs
+++ b/crsn/src/asm/parse/parse_cond.rs
@@ -1,18 +1,18 @@
 use sexp::Sexp;
 
-use crate::asm::error::Error;
+use crate::asm::error::CrsnError;
 use crate::asm::instr::{Cond, cond, Instr};
 use crate::asm::parse::parse_instr::parse_instructions;
 use crate::asm::parse::sexp_expect::{expect_list, expect_string_atom};
 use crate::asm::patches::TryRemove;
 use crate::module::CrsnExtension;
 
-pub fn parse_cond_branch(tok: Sexp, parsers: &[Box<dyn CrsnExtension>]) -> Result<(Cond, Vec<Instr>), Error> {
+pub fn parse_cond_branch(tok: Sexp, parsers: &[Box<dyn CrsnExtension>]) -> Result<(Cond, Vec<Instr>), CrsnError> {
     let mut list = expect_list(Some(tok), false)?;
     let kw = expect_string_atom(list.try_remove(0))?;
 
     if !kw.ends_with('?') {
-        return Err(Error::Parse(format!("Condition must end with '?': {}", kw).into()));
+        return Err(CrsnError::Parse(format!("Condition must end with '?': {}", kw).into()));
     }
 
     Ok((cond::parse_cond(&kw)?, parse_instructions(list, parsers)?))
diff --git a/crsn/src/asm/parse/parse_data.rs b/crsn/src/asm/parse/parse_data.rs
index 0569f51..ce2f5dc 100644
--- a/crsn/src/asm/parse/parse_data.rs
+++ b/crsn/src/asm/parse/parse_data.rs
@@ -4,21 +4,21 @@ use sexp::{Atom, Sexp};
 
 use crate::asm::data::{DataDisp, Rd, RdData, reg, Wr, WrData};
 use crate::asm::data::literal::Label;
-use crate::asm::error::Error;
+use crate::asm::error::CrsnError;
 use crate::asm::parse::sexp_expect::expect_string_atom;
 
 /// Parse a label
-pub fn parse_label(name: Option<Sexp>) -> Result<Label, Error> {
+pub fn parse_label(name: Option<Sexp>) -> Result<Label, CrsnError> {
     let name = expect_string_atom(name)?;
     Ok(Label::Named(name.trim_start_matches(':').into()))
 }
 
 /// Parse data disposition (address/value, without the read/write restriction)
-pub fn parse_data_disp(tok: Option<Sexp>) -> Result<DataDisp, Error> {
+pub fn parse_data_disp(tok: Option<Sexp>) -> Result<DataDisp, CrsnError> {
     let tok = if let Some(tok) = tok {
         tok
     } else {
-        return Err(Error::Parse("Expected data disposition token".into()));
+        return Err(CrsnError::Parse("Expected data disposition token".into()));
     };
 
     // TODO implement masks
@@ -41,7 +41,7 @@ pub fn parse_data_disp(tok: Option<Sexp>) -> Result<DataDisp, Error> {
             }
         }
         _ => {
-            Err(Error::Parse(format!("bad data disp: {:?}", tok).into()))
+            Err(CrsnError::Parse(format!("bad data disp: {:?}", tok).into()))
         }
     }
 }
diff --git a/crsn/src/asm/parse/parse_instr.rs b/crsn/src/asm/parse/parse_instr.rs
index ccaa022..0025de7 100644
--- a/crsn/src/asm/parse/parse_instr.rs
+++ b/crsn/src/asm/parse/parse_instr.rs
@@ -1,6 +1,6 @@
 use sexp::Sexp;
 
-use crate::asm::error::Error;
+use crate::asm::error::CrsnError;
 use crate::asm::instr::Instr;
 use crate::asm::parse::arg_parser::ArgParser;
 use crate::asm::parse::parse_cond::parse_cond_branch;
@@ -10,7 +10,7 @@ use crate::module::CrsnExtension;
 
 use super::parse_op::parse_op;
 
-pub fn parse_instructions(instrs: Vec<Sexp>, parsers: &[Box<dyn CrsnExtension>]) -> Result<Vec<Instr>, Error> {
+pub fn parse_instructions(instrs: Vec<Sexp>, parsers: &[Box<dyn CrsnExtension>]) -> Result<Vec<Instr>, CrsnError> {
     let mut parsed = vec![];
     for expr in instrs {
         let tokens = expect_list(Some(expr), false)?;
diff --git a/crsn/src/asm/parse/parse_op.rs b/crsn/src/asm/parse/parse_op.rs
index 2a24057..3b0c07d 100644
--- a/crsn/src/asm/parse/parse_op.rs
+++ b/crsn/src/asm/parse/parse_op.rs
@@ -1,10 +1,10 @@
-use crate::asm::error::Error;
+use crate::asm::error::CrsnError;
 use crate::asm::instr::Op;
 use crate::asm::parse::arg_parser::ArgParser;
 use crate::builtin::parse::BuiltinOps;
 use crate::module::{CrsnExtension, ParseOpRes};
 
-pub fn parse_op(keyword: &str, mut arg_tokens: ArgParser, parsers: &[Box<dyn CrsnExtension>]) -> Result<Op, Error> {
+pub fn parse_op(keyword: &str, mut arg_tokens: ArgParser, parsers: &[Box<dyn CrsnExtension>]) -> Result<Op, CrsnError> {
     // Include built-in instructions
     let builtins = [BuiltinOps::new()];
 
@@ -23,5 +23,5 @@ pub fn parse_op(keyword: &str, mut arg_tokens: ArgParser, parsers: &[Box<dyn Crs
         }
     }
 
-    return Err(Error::Parse(format!("Unknown instruction: {}", keyword).into()));
+    return Err(CrsnError::Parse(format!("Unknown instruction: {}", keyword).into()));
 }
diff --git a/crsn/src/asm/parse/parse_routines.rs b/crsn/src/asm/parse/parse_routines.rs
index b7cd761..081f169 100644
--- a/crsn/src/asm/parse/parse_routines.rs
+++ b/crsn/src/asm/parse/parse_routines.rs
@@ -1,14 +1,14 @@
 use sexp::Sexp;
 
 use crate::asm::data::literal::RoutineName;
-use crate::asm::error::Error;
+use crate::asm::error::CrsnError;
 use crate::asm::instr::Routine;
 use crate::asm::parse::parse_instr::parse_instructions;
 use crate::asm::parse::sexp_expect::{expect_list, expect_string_atom};
 use crate::asm::patches::TryRemove;
 use crate::module::CrsnExtension;
 
-pub fn parse_routines(routines: Vec<Sexp>, parsers: &[Box<dyn CrsnExtension>]) -> Result<Vec<Routine>, Error> {
+pub fn parse_routines(routines: Vec<Sexp>, parsers: &[Box<dyn CrsnExtension>]) -> Result<Vec<Routine>, CrsnError> {
     let mut parsed = vec![];
     for rt in routines {
         let mut def = expect_list(Some(rt), false)?;
diff --git a/crsn/src/asm/parse/sexp_expect.rs b/crsn/src/asm/parse/sexp_expect.rs
index 55c84e4..c0d8969 100644
--- a/crsn/src/asm/parse/sexp_expect.rs
+++ b/crsn/src/asm/parse/sexp_expect.rs
@@ -1,16 +1,16 @@
 use sexp::{Atom, Sexp};
 
-use crate::asm::error::Error;
+use crate::asm::error::CrsnError;
 
-pub fn expect_list(expr: Option<Sexp>, allow_empty: bool) -> Result<Vec<Sexp>, Error> {
+pub fn expect_list(expr: Option<Sexp>, allow_empty: bool) -> Result<Vec<Sexp>, CrsnError> {
     if let Some(expr) = expr {
         match &expr {
             Sexp::Atom(_) => {
-                return Err(Error::ParseIn("Expected a list".into(), expr));
+                return Err(CrsnError::ParseIn("Expected a list".into(), expr));
             }
             Sexp::List(list) => {
                 if !allow_empty && list.is_empty() {
-                    return Err(Error::ParseIn("Routine: Empty list".into(), expr));
+                    return Err(CrsnError::ParseIn("Routine: Empty list".into(), expr));
                 }
 
                 if let Sexp::List(list) = expr {
@@ -22,10 +22,10 @@ pub fn expect_list(expr: Option<Sexp>, allow_empty: bool) -> Result<Vec<Sexp>, E
         }
     }
 
-    Err(Error::Parse("Expected a list, got nothing".into()))
+    Err(CrsnError::Parse("Expected a list, got nothing".into()))
 }
 
-pub fn expect_atom(expr: Option<Sexp>) -> Result<Atom, Error> {
+pub fn expect_atom(expr: Option<Sexp>) -> Result<Atom, CrsnError> {
     if let Some(expr) = expr {
         match &expr {
             Sexp::Atom(_atom) => {
@@ -36,18 +36,18 @@ pub fn expect_atom(expr: Option<Sexp>) -> Result<Atom, Error> {
                 }
             }
             Sexp::List(_) => {
-                return Err(Error::ParseIn("Expected atom got list".into(), expr));
+                return Err(CrsnError::ParseIn("Expected atom got list".into(), expr));
             }
         }
     }
 
-    Err(Error::Parse("Expected atom, got nothing".into()))
+    Err(CrsnError::Parse("Expected atom, got nothing".into()))
 }
 
-pub fn expect_string_atom(expr: Option<Sexp>) -> Result<String, Error> {
+pub fn expect_string_atom(expr: Option<Sexp>) -> Result<String, CrsnError> {
     match expect_atom(expr) {
         Ok(Atom::S(s)) => Ok(s),
-        Ok(atom) => Err(Error::ParseIn("Expected string atom".into(), Sexp::Atom(atom))),
+        Ok(atom) => Err(CrsnError::ParseIn("Expected string atom".into(), Sexp::Atom(atom))),
         Err(e) => Err(e),
     }
 }
diff --git a/crsn/src/builtin/parse.rs b/crsn/src/builtin/parse.rs
index 7ae516d..ff34ec9 100644
--- a/crsn/src/builtin/parse.rs
+++ b/crsn/src/builtin/parse.rs
@@ -1,7 +1,7 @@
 use sexp::{Atom, Sexp};
 
 use crate::asm::data::literal::{Label, RoutineName};
-use crate::asm::error::Error;
+use crate::asm::error::CrsnError;
 use crate::asm::instr::cond::parse_cond;
 use crate::asm::instr::Op;
 use crate::asm::parse::arg_parser::ArgParser;
@@ -28,7 +28,7 @@ impl CrsnExtension for BuiltinOps {
         "builtin"
     }
 
-    fn parse_op(&self, keyword: &str, mut args: ArgParser) -> Result<ParseOpRes<Op>, Error> {
+    fn parse_op(&self, keyword: &str, mut args: ArgParser) -> Result<ParseOpRes<Op>, CrsnError> {
         Ok(ParseOpRes::Parsed(Op::BuiltIn(match keyword {
             "nop" => {
                 BuiltinOp::Nop
diff --git a/crsn/src/module/mod.rs b/crsn/src/module/mod.rs
index 5d023b8..7dca685 100644
--- a/crsn/src/module/mod.rs
+++ b/crsn/src/module/mod.rs
@@ -4,7 +4,7 @@ pub use eval_res::EvalRes;
 
 use crate::asm::data::literal::Value;
 use crate::asm::data::Mask;
-use crate::asm::error::Error;
+use crate::asm::error::CrsnError;
 use crate::asm::instr::Op;
 use crate::asm::parse::arg_parser::ArgParser;
 use crate::runtime::fault::Fault;
@@ -23,7 +23,7 @@ pub enum ParseOpRes<T> {
 
 impl ParseOpRes<Op> {
     /// Helper to construct an extension op
-    pub fn parsed(op : impl OpTrait) -> Self {
+    pub fn ext(op : impl OpTrait) -> Self {
         Self::Parsed(Op::Ext(Box::new(op)))
     }
 }
@@ -41,7 +41,7 @@ pub trait CrsnExtension: Debug + Send + Sync + 'static {
     /// the argument list and either return Ok or Err.
     ///
     /// If the instruction keyword is not recognized, return Unknown with the unchanged argument list.
-    fn parse_op(&self, keyword: &str, arg_tokens: ArgParser) -> Result<ParseOpRes<Op>, Error>;
+    fn parse_op(&self, keyword: &str, arg_tokens: ArgParser) -> Result<ParseOpRes<Op>, CrsnError>;
 
     /// Drop an object referenced by a handle
     fn drop_obj(&self,
diff --git a/crsn_arith/src/lib.rs b/crsn_arith/src/lib.rs
index e2f4f86..427a488 100644
--- a/crsn_arith/src/lib.rs
+++ b/crsn_arith/src/lib.rs
@@ -1,6 +1,27 @@
-pub use parse::ArithOps;
+use crsn::module::{CrsnExtension, ParseOpRes};
+use crsn::asm::parse::arg_parser::ArgParser;
+use crsn::asm::error::CrsnError;
+use crsn::asm::instr::Op;
 
 mod defs;
 mod parse;
 mod exec;
 
+#[derive(Debug, Clone)]
+pub struct ArithOps;
+
+impl ArithOps {
+    pub fn new() -> Box<dyn CrsnExtension> {
+        Box::new(Self)
+    }
+}
+
+impl CrsnExtension for ArithOps {
+    fn name(&self) -> &'static str {
+        "arith"
+    }
+
+    fn parse_op(&self, keyword: &str, args: ArgParser) -> Result<ParseOpRes<Op>, CrsnError> {
+        parse::parse(keyword, args)
+    }
+}
diff --git a/crsn_arith/src/parse.rs b/crsn_arith/src/parse.rs
index 316d216..c5f1284 100644
--- a/crsn_arith/src/parse.rs
+++ b/crsn_arith/src/parse.rs
@@ -1,455 +1,437 @@
 use crsn::asm::data::{Rd, Wr};
-use crsn::asm::error::Error;
+use crsn::asm::error::CrsnError;
 use crsn::asm::instr::Op;
 use crsn::asm::parse::arg_parser::ArgParser;
-use crsn::module::{CrsnExtension, ParseOpRes};
+use crsn::module::{ParseOpRes};
 
 use crate::defs::ArithOp;
 
-#[derive(Debug, Clone)]
-pub struct ArithOps {
-    _internal: ()
-}
-
-impl ArithOps {
-    pub fn new() -> Box<dyn CrsnExtension> {
-        Box::new(Self {
-            _internal: ()
-        })
-    }
-}
+pub(crate) fn parse(keyword: &str, mut args: ArgParser) -> Result<ParseOpRes<Op>, CrsnError> {
+    Ok(ParseOpRes::ext(match keyword {
+        "cmp" => {
+            ArithOp::Compare {
+                a: args.next_rd()?,
+                b: args.next_rd()?,
+            }
+        }
 
-impl CrsnExtension for ArithOps {
-    fn name(&self) -> &'static str {
-        "arith"
-    }
+        "tst" => {
+            let arg = args.next_rd()?;
+            ArithOp::Test { a: arg }
+        }
 
-    fn parse_op(&self, keyword: &str, mut args: ArgParser) -> Result<ParseOpRes<Op>, Error> {
-        Ok(ParseOpRes::parsed(match keyword {
-            "cmp" => {
-                ArithOp::Compare {
-                    a: args.next_rd()?,
-                    b: args.next_rd()?,
-                }
+        "inc" => {
+            let dst = args.next_wr()?;
+            ArithOp::Add {
+                dst,
+                a: dst.as_rd(),
+                b: Rd::immediate(1),
             }
+        }
 
-            "tst" => {
-                let arg = args.next_rd()?;
-                ArithOp::Test { a: arg }
+        "dec" => {
+            let dst = args.next_wr()?;
+            ArithOp::Sub {
+                dst,
+                a: dst.as_rd(),
+                b: Rd::immediate(1),
             }
+        }
 
-            "inc" => {
-                let dst = args.next_wr()?;
-                ArithOp::Add {
-                    dst,
-                    a: dst.as_rd(),
-                    b: Rd::immediate(1),
+        "add" => {
+            match args.len() {
+                3 => {
+                    ArithOp::Add {
+                        dst: args.next_wr()?,
+                        a: args.next_rd()?,
+                        b: args.next_rd()?,
+                    }
                 }
-            }
-
-            "dec" => {
-                let dst = args.next_wr()?;
-                ArithOp::Sub {
-                    dst,
-                    a: dst.as_rd(),
-                    b: Rd::immediate(1),
+                2 => {
+                    let dst = args.next_wr()?;
+                    ArithOp::Add {
+                        dst,
+                        a: dst.as_rd(),
+                        b: args.next_rd()?,
+                    }
+                }
+                _ => {
+                    return Err(CrsnError::Parse("Add requires 2 or 3 arguments".into()));
                 }
             }
+        }
 
-            "add" => {
-                match args.len() {
-                    3 => {
-                        ArithOp::Add {
-                            dst: args.next_wr()?,
-                            a: args.next_rd()?,
-                            b: args.next_rd()?,
-                        }
-                    }
-                    2 => {
-                        let dst = args.next_wr()?;
-                        ArithOp::Add {
-                            dst,
-                            a: dst.as_rd(),
-                            b: args.next_rd()?,
-                        }
-                    }
-                    _ => {
-                        return Err(Error::Parse("Add requires 2 or 3 arguments".into()));
+        "sub" => {
+            match args.len() {
+                3 => {
+                    ArithOp::Sub {
+                        dst: args.next_wr()?,
+                        a: args.next_rd()?,
+                        b: args.next_rd()?,
+                    }
+                }
+                2 => {
+                    let dst = args.next_wr()?;
+                    ArithOp::Sub {
+                        dst,
+                        a: dst.as_rd(),
+                        b: args.next_rd()?,
                     }
                 }
+                _ => {
+                    return Err(CrsnError::Parse("Sub requires 2 or 3 arguments".into()));
+                }
             }
+        }
 
-            "sub" => {
-                match args.len() {
-                    3 => {
-                        ArithOp::Sub {
-                            dst: args.next_wr()?,
-                            a: args.next_rd()?,
-                            b: args.next_rd()?,
-                        }
-                    }
-                    2 => {
-                        let dst = args.next_wr()?;
-                        ArithOp::Sub {
-                            dst,
-                            a: dst.as_rd(),
-                            b: args.next_rd()?,
-                        }
-                    }
-                    _ => {
-                        return Err(Error::Parse("Sub requires 2 or 3 arguments".into()));
+        "mul" => {
+            match args.len() {
+                3 => {
+                    ArithOp::Mul {
+                        dst: args.next_wr()?,
+                        a: args.next_rd()?,
+                        b: args.next_rd()?,
+                    }
+                }
+                2 => {
+                    let dst = args.next_wr()?;
+                    ArithOp::Mul {
+                        dst,
+                        a: dst.as_rd(),
+                        b: args.next_rd()?,
                     }
                 }
+                _ => {
+                    return Err(CrsnError::Parse("Mul requires 2 or 3 arguments".into()));
+                }
             }
+        }
 
-            "mul" => {
-                match args.len() {
-                    3 => {
-                        ArithOp::Mul {
-                            dst: args.next_wr()?,
-                            a: args.next_rd()?,
-                            b: args.next_rd()?,
-                        }
-                    }
-                    2 => {
-                        let dst = args.next_wr()?;
-                        ArithOp::Mul {
-                            dst,
-                            a: dst.as_rd(),
-                            b: args.next_rd()?,
-                        }
-                    }
-                    _ => {
-                        return Err(Error::Parse("Mul requires 2 or 3 arguments".into()));
+        "divr" => {
+            match args.len() {
+                3 => {
+                    let dst = args.next_wr()?;
+                    let rem = args.next_wr()?;
+                    let div = args.next_rd()?;
+                    ArithOp::Div {
+                        dst,
+                        rem,
+                        a: dst.as_rd(),
+                        div,
                     }
                 }
+                4 => {
+                    ArithOp::Div {
+                        dst: args.next_wr()?,
+                        rem: args.next_wr()?,
+                        a: args.next_rd()?,
+                        div: args.next_rd()?,
+                    }
+                }
+                _ => {
+                    return Err(CrsnError::Parse("DivR requires 3 or 4 arguments".into()));
+                }
             }
+        }
 
-            "divr" => {
-                match args.len() {
-                    3 => {
-                        let dst = args.next_wr()?;
-                        let rem = args.next_wr()?;
-                        let div = args.next_rd()?;
-                        ArithOp::Div {
-                            dst,
-                            rem,
-                            a: dst.as_rd(),
-                            div,
-                        }
-                    }
-                    4 => {
-                        ArithOp::Div {
-                            dst: args.next_wr()?,
-                            rem: args.next_wr()?,
-                            a: args.next_rd()?,
-                            div: args.next_rd()?,
-                        }
-                    }
-                    _ => {
-                        return Err(Error::Parse("DivR requires 3 or 4 arguments".into()));
+        "div" => {
+            match args.len() {
+                3 => {
+                    ArithOp::Div {
+                        dst: args.next_wr()?,
+                        rem: Wr::discard(),
+                        a: args.next_rd()?,
+                        div: args.next_rd()?,
+                    }
+                }
+                2 => {
+                    let dst = args.next_wr()?;
+                    let div = args.next_rd()?;
+                    ArithOp::Div {
+                        dst,
+                        rem: Wr::discard(),
+                        a: dst.as_rd(),
+                        div,
                     }
                 }
+                _ => {
+                    return Err(CrsnError::Parse("Div requires 2 or 3 arguments".into()));
+                }
             }
+        }
 
-            "div" => {
-                match args.len() {
-                    3 => {
-                        ArithOp::Div {
-                            dst: args.next_wr()?,
-                            rem: Wr::discard(),
-                            a: args.next_rd()?,
-                            div: args.next_rd()?,
-                        }
-                    }
-                    2 => {
-                        let dst = args.next_wr()?;
-                        let div = args.next_rd()?;
-                        ArithOp::Div {
-                            dst,
-                            rem: Wr::discard(),
-                            a: dst.as_rd(),
-                            div,
-                        }
-                    }
-                    _ => {
-                        return Err(Error::Parse("Div requires 2 or 3 arguments".into()));
+        "mod" => {
+            match args.len() {
+                3 => {
+                    ArithOp::Mod {
+                        dst: args.next_wr()?,
+                        a: args.next_rd()?,
+                        div: args.next_rd()?,
+                    }
+                }
+                2 => {
+                    let dst = args.next_wr()?;
+                    let div = args.next_rd()?;
+                    ArithOp::Mod {
+                        dst,
+                        a: dst.as_rd(),
+                        div,
                     }
                 }
+                _ => {
+                    return Err(CrsnError::Parse("Mod requires 2 or 3 arguments".into()));
+                }
             }
+        }
 
-            "mod" => {
-                match args.len() {
-                    3 => {
-                        ArithOp::Mod {
-                            dst: args.next_wr()?,
-                            a: args.next_rd()?,
-                            div: args.next_rd()?,
-                        }
-                    }
-                    2 => {
-                        let dst = args.next_wr()?;
-                        let div = args.next_rd()?;
-                        ArithOp::Mod {
-                            dst,
-                            a: dst.as_rd(),
-                            div,
-                        }
-                    }
-                    _ => {
-                        return Err(Error::Parse("Mod requires 2 or 3 arguments".into()));
+        "and" => {
+            match args.len() {
+                3 => {
+                    ArithOp::And {
+                        dst: args.next_wr()?,
+                        a: args.next_rd()?,
+                        b: args.next_rd()?,
+                    }
+                }
+                2 => {
+                    let dst = args.next_wr()?;
+                    ArithOp::And {
+                        dst,
+                        a: dst.as_rd(),
+                        b: args.next_rd()?,
                     }
                 }
+                _ => {
+                    return Err(CrsnError::Parse("And requires 2 or 3 arguments".into()));
+                }
             }
+        }
 
-            "and" => {
-                match args.len() {
-                    3 => {
-                        ArithOp::And {
-                            dst: args.next_wr()?,
-                            a: args.next_rd()?,
-                            b: args.next_rd()?,
-                        }
-                    }
-                    2 => {
-                        let dst = args.next_wr()?;
-                        ArithOp::And {
-                            dst,
-                            a: dst.as_rd(),
-                            b: args.next_rd()?,
-                        }
-                    }
-                    _ => {
-                        return Err(Error::Parse("And requires 2 or 3 arguments".into()));
+        "or" => {
+            match args.len() {
+                3 => {
+                    ArithOp::Or {
+                        dst: args.next_wr()?,
+                        a: args.next_rd()?,
+                        b: args.next_rd()?,
                     }
                 }
+                2 => {
+                    let dst = args.next_wr()?;
+                    ArithOp::Or {
+                        dst,
+                        a: dst.as_rd(),
+                        b: args.next_rd()?,
+                    }
+                }
+                _ => {
+                    return Err(CrsnError::Parse("Or requires 2 or 3 arguments".into()));
+                }
             }
+        }
 
-            "or" => {
-                match args.len() {
-                    3 => {
-                        ArithOp::Or {
-                            dst: args.next_wr()?,
-                            a: args.next_rd()?,
-                            b: args.next_rd()?,
-                        }
-                    }
-                    2 => {
-                        let dst = args.next_wr()?;
-                        ArithOp::Or {
-                            dst,
-                            a: dst.as_rd(),
-                            b: args.next_rd()?,
-                        }
-                    }
-                    _ => {
-                        return Err(Error::Parse("Or requires 2 or 3 arguments".into()));
+        "xor" => {
+            match args.len() {
+                3 => {
+                    ArithOp::Xor {
+                        dst: args.next_wr()?,
+                        a: args.next_rd()?,
+                        b: args.next_rd()?,
                     }
                 }
+                2 => {
+                    let dst = args.next_wr()?;
+                    ArithOp::Xor {
+                        dst,
+                        a: dst.as_rd(),
+                        b: args.next_rd()?,
+                    }
+                }
+                _ => {
+                    return Err(CrsnError::Parse("Xor requires 2 or 3 arguments".into()));
+                }
             }
+        }
 
-            "xor" => {
-                match args.len() {
-                    3 => {
-                        ArithOp::Xor {
-                            dst: args.next_wr()?,
-                            a: args.next_rd()?,
-                            b: args.next_rd()?,
-                        }
-                    }
-                    2 => {
-                        let dst = args.next_wr()?;
-                        ArithOp::Xor {
-                            dst,
-                            a: dst.as_rd(),
-                            b: args.next_rd()?,
-                        }
-                    }
-                    _ => {
-                        return Err(Error::Parse("Xor requires 2 or 3 arguments".into()));
+        "cpl" => {
+            match args.len() {
+                2 => {
+                    ArithOp::Cpl {
+                        dst: args.next_wr()?,
+                        a: args.next_rd()?,
+                    }
+                }
+                1 => {
+                    let dst = args.next_wr()?;
+                    ArithOp::Cpl {
+                        dst,
+                        a: dst.as_rd(),
                     }
                 }
+                _ => {
+                    return Err(CrsnError::Parse("Cpl requires 1 or 2 arguments".into()));
+                }
             }
+        }
 
-            "cpl" => {
-                match args.len() {
-                    2 => {
-                        ArithOp::Cpl {
-                            dst: args.next_wr()?,
-                            a: args.next_rd()?,
-                        }
+        "rol" => {
+            match args.len() {
+                3 => {
+                    ArithOp::Rol {
+                        dst: args.next_wr()?,
+                        a: args.next_rd()?,
+                        n: args.next_rd()?,
                     }
-                    1 => {
-                        let dst = args.next_wr()?;
-                        ArithOp::Cpl {
-                            dst,
-                            a: dst.as_rd(),
-                        }
+                }
+                2 => {
+                    let dst = args.next_wr()?;
+                    ArithOp::Rol {
+                        dst,
+                        a: dst.as_rd(),
+                        n: args.next_rd()?,
                     }
-                    _ => {
-                        return Err(Error::Parse("Cpl requires 1 or 2 arguments".into()));
+                }
+                1 => {
+                    let dst = args.next_wr()?;
+                    ArithOp::Rol {
+                        dst,
+                        a: dst.as_rd(),
+                        n: Rd::immediate(1),
                     }
                 }
+                _ => {
+                    return Err(CrsnError::Parse("Rol requires 1, 2 or 3 arguments".into()));
+                }
             }
+        }
 
-            "rol" => {
-                match args.len() {
-                    3 => {
-                        ArithOp::Rol {
-                            dst: args.next_wr()?,
-                            a: args.next_rd()?,
-                            n: args.next_rd()?,
-                        }
-                    }
-                    2 => {
-                        let dst = args.next_wr()?;
-                        ArithOp::Rol {
-                            dst,
-                            a: dst.as_rd(),
-                            n: args.next_rd()?,
-                        }
-                    }
-                    1 => {
-                        let dst = args.next_wr()?;
-                        ArithOp::Rol {
-                            dst,
-                            a: dst.as_rd(),
-                            n: Rd::immediate(1),
-                        }
-                    }
-                    _ => {
-                        return Err(Error::Parse("Rol requires 1, 2 or 3 arguments".into()));
+        "ror" => {
+            match args.len() {
+                3 => {
+                    ArithOp::Ror {
+                        dst: args.next_wr()?,
+                        a: args.next_rd()?,
+                        n: args.next_rd()?,
                     }
                 }
-            }
-
-            "ror" => {
-                match args.len() {
-                    3 => {
-                        ArithOp::Ror {
-                            dst: args.next_wr()?,
-                            a: args.next_rd()?,
-                            n: args.next_rd()?,
-                        }
-                    }
-                    2 => {
-                        let dst = args.next_wr()?;
-                        ArithOp::Ror {
-                            dst,
-                            a: dst.as_rd(),
-                            n: args.next_rd()?,
-                        }
-                    }
-                    1 => {
-                        let dst = args.next_wr()?;
-                        ArithOp::Ror {
-                            dst,
-                            a: dst.as_rd(),
-                            n: Rd::immediate(1),
-                        }
-                    }
-                    _ => {
-                        return Err(Error::Parse("Ror requires 1, 2 or 3 arguments".into()));
+                2 => {
+                    let dst = args.next_wr()?;
+                    ArithOp::Ror {
+                        dst,
+                        a: dst.as_rd(),
+                        n: args.next_rd()?,
+                    }
+                }
+                1 => {
+                    let dst = args.next_wr()?;
+                    ArithOp::Ror {
+                        dst,
+                        a: dst.as_rd(),
+                        n: Rd::immediate(1),
                     }
                 }
+                _ => {
+                    return Err(CrsnError::Parse("Ror requires 1, 2 or 3 arguments".into()));
+                }
             }
+        }
 
-            "lsl" | "asl" => {
-                match args.len() {
-                    3 => {
-                        ArithOp::Lsl {
-                            dst: args.next_wr()?,
-                            a: args.next_rd()?,
-                            n: args.next_rd()?,
-                        }
-                    }
-                    2 => {
-                        let dst = args.next_wr()?;
-                        ArithOp::Lsl {
-                            dst,
-                            a: dst.as_rd(),
-                            n: args.next_rd()?,
-                        }
-                    }
-                    1 => {
-                        let dst = args.next_wr()?;
-                        ArithOp::Lsl {
-                            dst,
-                            a: dst.as_rd(),
-                            n: Rd::immediate(1),
-                        }
-                    }
-                    _ => {
-                        return Err(Error::Parse("Lsl requires 1, 2 or 3 arguments".into()));
+        "lsl" | "asl" => {
+            match args.len() {
+                3 => {
+                    ArithOp::Lsl {
+                        dst: args.next_wr()?,
+                        a: args.next_rd()?,
+                        n: args.next_rd()?,
+                    }
+                }
+                2 => {
+                    let dst = args.next_wr()?;
+                    ArithOp::Lsl {
+                        dst,
+                        a: dst.as_rd(),
+                        n: args.next_rd()?,
                     }
                 }
+                1 => {
+                    let dst = args.next_wr()?;
+                    ArithOp::Lsl {
+                        dst,
+                        a: dst.as_rd(),
+                        n: Rd::immediate(1),
+                    }
+                }
+                _ => {
+                    return Err(CrsnError::Parse("Lsl requires 1, 2 or 3 arguments".into()));
+                }
             }
+        }
 
-            "lsr" => {
-                match args.len() {
-                    3 => {
-                        ArithOp::Lsr {
-                            dst: args.next_wr()?,
-                            a: args.next_rd()?,
-                            n: args.next_rd()?,
-                        }
-                    }
-                    2 => {
-                        let dst = args.next_wr()?;
-                        ArithOp::Lsr {
-                            dst,
-                            a: dst.as_rd(),
-                            n: args.next_rd()?,
-                        }
-                    }
-                    1 => {
-                        let dst = args.next_wr()?;
-                        ArithOp::Lsr {
-                            dst,
-                            a: dst.as_rd(),
-                            n: Rd::immediate(1),
-                        }
-                    }
-                    _ => {
-                        return Err(Error::Parse("Lsr requires 1, 2 or 3 arguments".into()));
+        "lsr" => {
+            match args.len() {
+                3 => {
+                    ArithOp::Lsr {
+                        dst: args.next_wr()?,
+                        a: args.next_rd()?,
+                        n: args.next_rd()?,
+                    }
+                }
+                2 => {
+                    let dst = args.next_wr()?;
+                    ArithOp::Lsr {
+                        dst,
+                        a: dst.as_rd(),
+                        n: args.next_rd()?,
+                    }
+                }
+                1 => {
+                    let dst = args.next_wr()?;
+                    ArithOp::Lsr {
+                        dst,
+                        a: dst.as_rd(),
+                        n: Rd::immediate(1),
                     }
                 }
+                _ => {
+                    return Err(CrsnError::Parse("Lsr requires 1, 2 or 3 arguments".into()));
+                }
             }
+        }
 
-            "asr" => {
-                match args.len() {
-                    3 => {
-                        ArithOp::Asr {
-                            dst: args.next_wr()?,
-                            a: args.next_rd()?,
-                            n: args.next_rd()?,
-                        }
-                    }
-                    2 => {
-                        let dst = args.next_wr()?;
-                        ArithOp::Asr {
-                            dst,
-                            a: dst.as_rd(),
-                            n: args.next_rd()?,
-                        }
-                    }
-                    1 => {
-                        let dst = args.next_wr()?;
-                        ArithOp::Asr {
-                            dst,
-                            a: dst.as_rd(),
-                            n: Rd::immediate(1),
-                        }
-                    }
-                    _ => {
-                        return Err(Error::Parse("Asr requires 1, 2 or 3 arguments".into()));
+        "asr" => {
+            match args.len() {
+                3 => {
+                    ArithOp::Asr {
+                        dst: args.next_wr()?,
+                        a: args.next_rd()?,
+                        n: args.next_rd()?,
                     }
                 }
+                2 => {
+                    let dst = args.next_wr()?;
+                    ArithOp::Asr {
+                        dst,
+                        a: dst.as_rd(),
+                        n: args.next_rd()?,
+                    }
+                }
+                1 => {
+                    let dst = args.next_wr()?;
+                    ArithOp::Asr {
+                        dst,
+                        a: dst.as_rd(),
+                        n: Rd::immediate(1),
+                    }
+                }
+                _ => {
+                    return Err(CrsnError::Parse("Asr requires 1, 2 or 3 arguments".into()));
+                }
             }
+        }
 
-            _other => {
-                return Ok(ParseOpRes::Unknown(args));
-            }
-        }))
-    }
+        _other => {
+            return Ok(ParseOpRes::Unknown(args));
+        }
+    }))
 }
+
diff --git a/crsn_stacks/src/exec.rs b/crsn_stacks/src/exec.rs
index bd4f8c9..6cb71a4 100644
--- a/crsn_stacks/src/exec.rs
+++ b/crsn_stacks/src/exec.rs
@@ -67,7 +67,7 @@ impl OpTrait for StackOp {
     //
 }
 
-pub(crate) fn drop_stack(state: &mut RunState, handle: Value) -> Result<Option<()>, Fault> {
+pub(crate) fn drop_obj(state: &mut RunState, handle: Value) -> Result<Option<()>, Fault> {
     let stacks: &mut Stacks = state.ext_mut();
     Ok(stacks.store.remove(&handle).map(|_| ()))
 }
diff --git a/crsn_stacks/src/lib.rs b/crsn_stacks/src/lib.rs
index e182b9f..78b8c7f 100644
--- a/crsn_stacks/src/lib.rs
+++ b/crsn_stacks/src/lib.rs
@@ -1,6 +1,27 @@
-pub use parse::StackOps;
+use crsn::module::{CrsnExtension, ParseOpRes};
+use crsn::asm::parse::arg_parser::ArgParser;
+use crsn::asm::instr::Op;
+use crsn::asm::error::CrsnError;
 
 mod defs;
 mod parse;
 mod exec;
 
+#[derive(Debug, Clone)]
+pub struct StackOps;
+
+impl StackOps {
+    pub fn new() -> Box<dyn CrsnExtension> {
+        Box::new(Self)
+    }
+}
+
+impl CrsnExtension for StackOps {
+    fn name(&self) -> &'static str {
+        "stacks"
+    }
+
+    fn parse_op(&self, keyword: &str, args: ArgParser) -> Result<ParseOpRes<Op>, CrsnError> {
+        parse::parse(keyword, args)
+    }
+}
diff --git a/crsn_stacks/src/parse.rs b/crsn_stacks/src/parse.rs
index ee5bbbf..a149859 100644
--- a/crsn_stacks/src/parse.rs
+++ b/crsn_stacks/src/parse.rs
@@ -1,61 +1,37 @@
-use crsn::asm::data::literal::Value;
-use crsn::asm::error::Error;
+
+use crsn::asm::error::CrsnError;
 use crsn::asm::instr::Op;
 use crsn::asm::parse::arg_parser::ArgParser;
-use crsn::module::{CrsnExtension, ParseOpRes};
-use crsn::runtime::fault::Fault;
-use crsn::runtime::run_thread::{RunState, ThreadInfo};
+use crsn::module::{ParseOpRes};
 
-use crate::defs::StackOp;
 
-#[derive(Debug, Clone)]
-pub struct StackOps {
-    _internal: ()
-}
 
-impl StackOps {
-    pub fn new() -> Box<dyn CrsnExtension> {
-        Box::new(Self {
-            _internal: ()
-        })
-    }
-}
-
-impl CrsnExtension for StackOps {
-    fn name(&self) -> &'static str {
-        "stacks"
-    }
-
-    fn parse_op(&self, keyword: &str, mut args: ArgParser) -> Result<ParseOpRes<Op>, Error> {
-        Ok(ParseOpRes::parsed(match keyword {
-            "stack" => {
-                StackOp::NewStack {
-                    dst: args.next_wr()?,
-                }
-            }
+use crate::defs::StackOp;
 
-            "push" => {
-                StackOp::Push {
-                    obj: args.next_rdobj()?,
-                    src: args.next_rd()?,
-                }
+pub(crate) fn parse(keyword: &str, mut args: ArgParser) -> Result<ParseOpRes<Op>, CrsnError> {
+    Ok(ParseOpRes::ext(match keyword {
+        "stack" => {
+            StackOp::NewStack {
+                dst: args.next_wr()?,
             }
+        }
 
-            "pop" => {
-                StackOp::Pop {
-                    dst: args.next_wr()?,
-                    obj: args.next_rdobj()?,
-                }
+        "push" => {
+            StackOp::Push {
+                obj: args.next_rdobj()?,
+                src: args.next_rd()?,
             }
+        }
 
-            _other => {
-                return Ok(ParseOpRes::Unknown(args));
+        "pop" => {
+            StackOp::Pop {
+                dst: args.next_wr()?,
+                obj: args.next_rdobj()?,
             }
-        }))
-    }
+        }
 
-    fn drop_obj(&self, _ti: &ThreadInfo, state: &mut RunState, handle: Value) -> Result<Option<()>, Fault>
-    {
-        crate::exec::drop_stack(state, handle)
-    }
+        _other => {
+            return Ok(ParseOpRes::Unknown(args));
+        }
+    }))
 }