diff --git a/compile_examples.sh b/compile_examples.sh
index cdc0960..5fec96e 100755
--- a/compile_examples.sh
+++ b/compile_examples.sh
@@ -4,8 +4,4 @@ set -e
 
 cargo build
 
-for file in examples/*.csn
-do
-  echo "--- $file ---"
-  target/debug/launcher -P "$file"
-done
+find examples -name '*.csn' -type f -exec target/debug/launcher -P {} \;
diff --git a/crsn/crsn-sexp/src/error.rs b/crsn/crsn-sexp/src/error.rs
index cb5dea9..8a3ada2 100644
--- a/crsn/crsn-sexp/src/error.rs
+++ b/crsn/crsn-sexp/src/error.rs
@@ -1,4 +1,5 @@
 use std::{cmp, fmt};
+use std::fmt::{Formatter, Debug};
 
 /// The representation of an s-expression parse error.
 pub struct Error {
@@ -9,7 +10,7 @@ pub struct Error {
 }
 
 /// Position in the input string
-#[derive(Debug, PartialEq, Clone, Default)]
+#[derive(PartialEq, Clone, Default)]
 pub struct SourcePosition {
     /// The line number on which the error occurred.
     pub line: u32,
@@ -25,6 +26,21 @@ impl fmt::Display for SourcePosition {
     }
 }
 
+impl Debug for SourcePosition {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        if f.alternate() {
+            f.debug_struct("SourcePosition")
+                .field("line", &self.line)
+                .field("column", &self.column)
+                .field("index", &self.index)
+                .finish()
+        } else {
+            // shorter version
+            write!(f, "Pos({}:{})", self.line, self.column)
+        }
+    }
+}
+
 /// Since errors are the uncommon case, they're boxed. This keeps the size of
 /// structs down, which helps performance in the common case.
 ///
diff --git a/crsn/src/asm/mod.rs b/crsn/src/asm/mod.rs
index 6798920..1974265 100644
--- a/crsn/src/asm/mod.rs
+++ b/crsn/src/asm/mod.rs
@@ -8,6 +8,8 @@ use crate::asm::parse::{ParserContext, ParserState};
 use crate::module::{CrsnExtension, CrsnUniq};
 use crate::runtime::program::Program;
 use crate::builtin::BuiltinOps;
+use crate::runtime::run_thread::{RunState, ThreadInfo, ThreadToken};
+use crate::runtime::frame::REG_COUNT;
 
 pub mod data;
 pub mod error;
@@ -18,11 +20,12 @@ pub mod patches;
 /// Parse a program from string and assemble a low level instruction sequence from it.
 pub fn assemble(source: &str, uniq : &CrsnUniq, mut parsers: Vec<Box<dyn CrsnExtension>>) -> Result<Arc<Program>, error::CrsnError> {
     parsers.insert(0, BuiltinOps::new());
-
     for p in &mut parsers {
         p.init(uniq);
     }
 
+    let parsers_arc = Arc::new(parsers);
+
     // remove first line if it looks like a shebang
     let source = if source.starts_with("#!") {
         if let Some(nl) = source.find('\n') {
@@ -34,18 +37,38 @@ pub fn assemble(source: &str, uniq : &CrsnUniq, mut parsers: Vec<Box<dyn CrsnExt
         source
     };
 
+    let ti = Arc::new(ThreadInfo {
+        id: ThreadToken(0),
+        uniq: Default::default(),
+        program: Program::new(vec![], parsers_arc.clone()).unwrap(),
+        cycle_time: Default::default(),
+        extensions: parsers_arc.clone(),
+    });
+
     let pcx = ParserContext {
-        parsers: &parsers,
+        parsers: &parsers_arc,
         state: RefCell::new(ParserState {
             reg_aliases: Default::default(),
             reg_alias_stack: vec![],
             global_reg_aliases: Default::default(),
             constants: Default::default(),
+
+            // This is a fake thread to pass to constant expressions when evaluating them.
+            // This allows to evaluate nearly all instructions at compile time.
+            const_eval: RunState {
+                thread_info: ti.clone(),
+                frame: Default::default(),
+                call_stack: vec![],
+                global_regs: [0; REG_COUNT],
+                ext_data: Default::default()
+            },
+            const_eval_ti: ti,
+            parsing_expr: false
         }),
     };
 
     let ops = parse::parse(source, &SourcePosition::default(), &pcx)?;
     let ops = labels_to_skips(ops)?;
 
-    Ok(Program::new(ops, Arc::new(parsers))?)
+    Ok(Program::new(ops, parsers_arc)?)
 }
diff --git a/crsn/src/asm/parse/arg_parser.rs b/crsn/src/asm/parse/arg_parser.rs
index 0177ad1..eef11aa 100644
--- a/crsn/src/asm/parse/arg_parser.rs
+++ b/crsn/src/asm/parse/arg_parser.rs
@@ -8,9 +8,10 @@ use crate::asm::parse::sexp_expect::expect_string_atom;
 use crate::asm::data::literal::Value;
 use crate::builtin::defs::BitMask;
 use crate::asm::patches::ErrWithPos;
+use std::fmt::{Debug, Formatter};
+use std::fmt;
 
 /// Utility for argument parsing
-#[derive(Debug)]
 pub struct TokenParser<'a> {
     orig_len: usize,
     args: Vec<Sexp>,
@@ -18,6 +19,16 @@ pub struct TokenParser<'a> {
     pub pcx: &'a ParserContext<'a>,
 }
 
+impl<'a> Debug for TokenParser<'a> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        f.debug_struct("TokenParser")
+            .field("orig_len", &self.orig_len)
+            .field("args", &self.args)
+            .field("start_pos", &self.start_pos)
+            .finish()
+    }
+}
+
 impl<'a> IntoIterator for TokenParser<'a> {
     type Item = Sexp;
     type IntoIter = std::vec::IntoIter<Sexp>;
@@ -42,6 +53,13 @@ impl<'a> TokenParser<'a> {
         }
     }
 
+    /// Prepend a token
+    pub fn prepend(&mut self, what: Sexp) {
+        // the list is reversed - actually, append it
+        self.args.push(what);
+        self.orig_len += 1;
+    }
+
     /// Get error if not empty.
     /// The argument is substituted into the phrase "Instruction needs ...!" - i.e. "one Wr argument and a list or string"
     pub fn ensure_empty(&self, what_arguments : &str) -> Result<(), CrsnError> {
diff --git a/crsn/src/asm/parse/mod.rs b/crsn/src/asm/parse/mod.rs
index 0107c02..a52125c 100644
--- a/crsn/src/asm/parse/mod.rs
+++ b/crsn/src/asm/parse/mod.rs
@@ -11,6 +11,8 @@ use crate::asm::error::CrsnError;
 use crate::asm::instr::Op;
 use crate::asm::parse::sexp_expect::expect_list;
 use crate::module::CrsnExtension;
+use crate::runtime::run_thread::{ThreadInfo, RunState};
+use std::sync::Arc;
 
 pub mod parse_cond;
 pub mod parse_instr;
@@ -28,7 +30,7 @@ pub struct ParserContext<'a> {
     pub state: RefCell<ParserState>,
 }
 
-#[derive(Default, Debug)]
+#[derive(Debug)]
 pub struct ParserState {
     /// Register aliases valid globally
     pub global_reg_aliases: HashMap<RegisterAlias, Register>,
@@ -41,6 +43,14 @@ pub struct ParserState {
 
     /// Global constants
     pub constants: HashMap<ConstantName, Value>,
+
+    /// Dummy run state used for const eval
+    pub const_eval: RunState,
+    // ThreadInfo is needed separately from RunState. TODO check if this can be refactored
+    pub const_eval_ti: Arc<ThreadInfo>,
+
+    /// True if we are in an expression parser context
+    pub parsing_expr : bool,
 }
 
 pub fn parse(source: &str, pos: &SourcePosition, parsers: &ParserContext) -> Result<Vec<Op>, CrsnError> {
diff --git a/crsn/src/asm/parse/parse_data.rs b/crsn/src/asm/parse/parse_data.rs
index 27e5152..c8251f6 100644
--- a/crsn/src/asm/parse/parse_data.rs
+++ b/crsn/src/asm/parse/parse_data.rs
@@ -6,9 +6,12 @@ use sexp::{Atom, Sexp, SourcePosition};
 use crate::asm::data::{DataDisp, Rd, RdData, reg, Wr, WrData, RdWr};
 use crate::asm::data::literal::{ConstantName, Label, RegisterAlias, Value};
 use crate::asm::error::CrsnError;
-use crate::asm::parse::ParserContext;
+use crate::asm::parse::{ParserContext, parse_instructions};
 use crate::asm::parse::sexp_expect::expect_string_atom;
 use crate::asm::patches::ErrWithPos;
+use std::sync::atomic::AtomicU32;
+use crate::module::OpTrait;
+use crate::asm::instr::Cond;
 
 fn is_valid_identifier(name: &str) -> bool {
     // ascii symbols "!\"#$_&'()*+,-./:;<=>?@[\\]^_`{|}~"
@@ -87,7 +90,51 @@ pub fn parse_data_disp(tok: Sexp, pcx: &ParserContext) -> Result<DataDisp, CrsnE
         Sexp::Atom(Atom::QS(_s), pos) => {
             Err(CrsnError::Parse("Quoted string not expected here".into(), pos))
         }
-        Sexp::List(_list, pos) => {
+        Sexp::List(list, pos) => {
+            if let Some(Sexp::Atom(Atom::S(s), s_pos)) = list.first() {
+                let parsing_expr = pcx.state.borrow().parsing_expr;
+
+                if !s.starts_with('=') && !parsing_expr {
+                    return Err(CrsnError::Parse(format!("Invalid syntax, did you mean \"={}\"?", s).into(), s_pos.clone()));
+                }
+
+                // start expr parsing mode
+                let orig_parsing_expr = pcx.state.borrow().parsing_expr;
+                pcx.state.borrow_mut().parsing_expr = true;
+
+                let lmut = AtomicU32::new(0);
+
+                let mut expr_ops = parse_instructions(
+                    // TODO avoid this madness
+                    vec![Sexp::List(list, pos.clone())].into_iter(),
+                    &pos, pcx)?
+                    .flatten(&lmut)?;
+
+                if expr_ops.len() != 1 {
+                    return Err(CrsnError::Parse(format!("Expected one top level operation in an expression, got: {:?}", expr_ops).into(), pos.clone()));
+                }
+
+                let op = expr_ops.remove(0);
+
+                let mut state_mut = pcx.state.borrow_mut();
+                state_mut.const_eval.clear_all();
+
+                let ticl = state_mut.const_eval.thread_info.clone();
+                op.execute(&ticl, &mut state_mut.const_eval)
+                    .map_err(|f| CrsnError::ParseOther(Box::new(f), pos.clone()))?;
+
+                if state_mut.const_eval.test_cond(Cond::Invalid) {
+                    return Err(CrsnError::Parse("Expression evaluation failed - invalid flag set!".into(), pos.clone()));
+                }
+
+                if !orig_parsing_expr {
+                    // exit expr parsing mode
+                    state_mut.parsing_expr = false;
+                }
+
+                return Ok(DataDisp::Immediate(state_mut.const_eval.frame.gen[0]));
+            }
+
             Err(CrsnError::Parse("List not expected here".into(), pos))
         }
         Sexp::Atom(Atom::S(s), pos) => {
diff --git a/crsn/src/asm/parse/parse_instr.rs b/crsn/src/asm/parse/parse_instr.rs
index ac276a8..6fccd7e 100644
--- a/crsn/src/asm/parse/parse_instr.rs
+++ b/crsn/src/asm/parse/parse_instr.rs
@@ -1,4 +1,4 @@
-use sexp::{Sexp, SourcePosition, Atom};
+use sexp::{Sexp, SourcePosition, Atom, atom_s};
 
 use crate::asm::error::CrsnError;
 use crate::asm::instr::{Flatten, InstrWithBranches};
@@ -18,9 +18,20 @@ pub fn parse_instructions(items: impl Iterator<Item=Sexp>, pos: &SourcePosition,
         let (tokens, listpos) = expect_list(expr, false)?;
 
         let mut toki = tokens.into_iter();
-        let (name, namepos) = expect_string_atom(toki.next_or_err(listpos.clone(), "Expected instruction name token")?)?;
+        let (mut name, namepos) = expect_string_atom(toki.next_or_err(listpos.clone(), "Expected instruction name token")?)?;
+
+        let parsing_expr = pcx.state.borrow().parsing_expr;
+
+        if parsing_expr {
+            if let Some(n) = name.strip_prefix('=') {
+                name = n.to_string();
+            }
+        }
 
         if name == "proc" {
+            if parsing_expr {
+                return Err(CrsnError::Parse("Illegal syntax in const expression".into(), pos.clone()));
+            }
             parsed.push(parse_routine(toki, pos, pcx)?);
             continue;
         }
@@ -62,8 +73,13 @@ pub fn parse_instructions(items: impl Iterator<Item=Sexp>, pos: &SourcePosition,
             })
             .collect::<Vec<_>>();
 
-        let arg_tokens = TokenParser::new(toki.take(token_count - branch_tokens.len()).collect(), &listpos, pcx);
-        // debug!("branch_tokens: {:#?}", branch_tokens);
+        if parsing_expr && !branch_tokens.is_empty() {
+            return Err(CrsnError::Parse(format!("Conditional branches are not allowed in const expression: {:?}", branch_tokens).into(), pos.clone()));
+        }
+
+        let mut arg_parser = TokenParser::new(toki.take(token_count - branch_tokens.len()).collect(), &listpos, pcx);
+
+        trace!("Parse op: {}\nargs {:?}\nbranches {:?}", name, arg_parser, branch_tokens);
 
         let branches = {
             let mut branches = vec![];
@@ -77,7 +93,12 @@ pub fn parse_instructions(items: impl Iterator<Item=Sexp>, pos: &SourcePosition,
             }
         };
 
-        if let Some(op) = parse_op(name.as_str(), arg_tokens, &namepos)? {
+        if parsing_expr {
+            // hackity hack for const expr eval
+            arg_parser.prepend(atom_s("r0"));
+        }
+
+        if let Some(op) = parse_op(name.as_str(), arg_parser, &namepos)? {
             parsed.push(Box::new(InstrWithBranches {
                 op,
                 pos: namepos,
diff --git a/crsn/src/asm/parse/parse_op.rs b/crsn/src/asm/parse/parse_op.rs
index 9fb058d..ca057fd 100644
--- a/crsn/src/asm/parse/parse_op.rs
+++ b/crsn/src/asm/parse/parse_op.rs
@@ -16,6 +16,10 @@ pub fn parse_op<'a>(mut keyword: &str, mut arg_tokens: TokenParser<'a>, spos: &S
         keyword = &keyword[..pos];
     }
 
+    if cond.is_some() && arg_tokens.pcx.state.borrow().parsing_expr {
+        return Err(CrsnError::Parse("Conditional suffixes are not allowed in const expression".into(), spos.clone()));
+    }
+
     for p in arg_tokens.pcx.parsers {
         arg_tokens = match p.parse_op(spos, keyword, arg_tokens) {
             Ok(ParseRes::Parsed(kind)) => return Ok(Some(Op {
diff --git a/crsn/src/module/mod.rs b/crsn/src/module/mod.rs
index 1b160de..07442f9 100644
--- a/crsn/src/module/mod.rs
+++ b/crsn/src/module/mod.rs
@@ -52,7 +52,7 @@ pub trait OpTrait: Debug + Send + Sync + 'static {
 
 /// CRSN initializer object.
 /// Only one should be created for the lifespan of the parser and runtime.
-#[derive(Default)]
+#[derive(Default,Debug)]
 pub struct CrsnUniq {
     object_handle_counter : AtomicU64
 }
diff --git a/crsn/src/runtime/run_thread/info.rs b/crsn/src/runtime/run_thread/info.rs
index 7559559..7e2ed08 100644
--- a/crsn/src/runtime/run_thread/info.rs
+++ b/crsn/src/runtime/run_thread/info.rs
@@ -7,6 +7,7 @@ use crate::runtime::program::Program;
 use crate::runtime::run_thread::ThreadToken;
 use crate::module::{CrsnExtension, CrsnUniq};
 
+#[derive(Debug)]
 pub struct ThreadInfo {
     /// Thread ID
     pub id: ThreadToken,
diff --git a/crsn/src/runtime/run_thread/state.rs b/crsn/src/runtime/run_thread/state.rs
index 1315ea5..48133e5 100644
--- a/crsn/src/runtime/run_thread/state.rs
+++ b/crsn/src/runtime/run_thread/state.rs
@@ -25,6 +25,16 @@ pub struct RunState {
     pub ext_data: ExtensionDataStore,
 }
 
+impl RunState {
+    /// Clear everything. Caution: when done at runtime, this effectively reboots the thread
+    pub fn clear_all(&mut self) {
+        self.frame = Default::default();
+        self.call_stack = Default::default();
+        self.global_regs = Default::default();
+        self.ext_data = Default::default();
+    }
+}
+
 impl Debug for RunState {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         f.debug_struct("RunState")
diff --git a/examples/expr.csn b/examples/expr.csn
new file mode 100644
index 0000000..5368c93
--- /dev/null
+++ b/examples/expr.csn
@@ -0,0 +1,12 @@
+(
+    (def FOO 14)
+
+    (ld r0 (=add 14 1))
+    (cmp r0 15 (ne? (fault "1")))
+
+    (ld r0 (=add FOO 1))
+    (cmp r0 15 (ne? (fault "2")))
+
+    (ld r0 (=add (sub FOO 2) 3))
+    (cmp r0 15 (ne? (fault "3")))
+)
diff --git a/launcher/src/main.rs b/launcher/src/main.rs
index c8e8fa0..13f441a 100644
--- a/launcher/src/main.rs
+++ b/launcher/src/main.rs
@@ -151,6 +151,7 @@ fn main() -> anyhow::Result<()> {
     ])?;
 
     if config.assemble_only {
+        println!("--- {} ---", config.program_file);
         for (n, op) in parsed.ops.iter().enumerate() {
             if config.assemble_debug {
                 println!("{:04} : {:?}", n, op);