diff --git a/crsn/src/asm/parse/parse_data.rs b/crsn/src/asm/parse/parse_data.rs
index 6e6a27a..b4f41f1 100644
--- a/crsn/src/asm/parse/parse_data.rs
+++ b/crsn/src/asm/parse/parse_data.rs
@@ -94,30 +94,48 @@ pub fn parse_data_disp(tok: Sexp, pcx: &ParserContext) -> Result<DataDisp, CrsnE
             }
 
             if let Some(reference) = s.strip_prefix('@') {
+                /* extension constants (pre-defined handles) */
+                for p in pcx.parsers {
+                    if let Some(val) = p.get_constant_value(&s) {
+                        return Ok(DataDisp::ImmObject(val));
+                    }
+                }
+
+                /* register aliases */
                 let pstate = pcx.state.borrow();
                 if let Some(reg) = pstate.reg_aliases.get(reference) {
-                    Ok(DataDisp::RegObject(*reg))
-                } else {
-                    let reg = reg::parse_reg(reference, &pos)?;
+                    return Ok(DataDisp::RegObject(*reg))
+                }
 
-                    if pstate.reg_aliases.values().find(|v| **v == reg).is_some() {
-                        Err(CrsnError::Parse(format!("Sym exists for register {}, direct access denied. Unsym it if needed.", reg).into(), pos))
-                    } else {
-                        Ok(DataDisp::RegObject(reg))
-                    }
+                let reg = reg::parse_reg(reference, &pos)?;
+
+                if pstate.reg_aliases.values().find(|v| **v == reg).is_some() {
+                    Err(CrsnError::Parse(format!("Sym exists for register {}, direct access denied. Unsym it if needed.", reg).into(), pos))
+                } else {
+                    Ok(DataDisp::RegObject(reg))
                 }
             } else if s.starts_with(|c: char| c.is_ascii_digit() || c == '-') {
                 Ok(DataDisp::Immediate(unsafe { std::mem::transmute(parse_i64(&s, &pos)?) }))
             } else {
+                /* extension constants */
+                for p in pcx.parsers {
+                    if let Some(val) = p.get_constant_value(&s) {
+                        return Ok(DataDisp::Immediate(val));
+                    }
+                }
+
+                /* program constants */
                 let pstate = pcx.state.borrow();
                 if let Some(val) = pstate.constants.get(&s) {
                     return Ok(DataDisp::Immediate(*val));
                 }
 
+                /* register aliases */
                 if let Some(val) = pstate.reg_aliases.get(&s) {
                     return Ok(DataDisp::Register(*val));
                 }
 
+                /* register */
                 let reg = reg::parse_reg(&s, &pos)?;
 
                 let pstate = pcx.state.borrow();
diff --git a/crsn/src/module/mod.rs b/crsn/src/module/mod.rs
index 847f4a4..dbb1e12 100644
--- a/crsn/src/module/mod.rs
+++ b/crsn/src/module/mod.rs
@@ -12,7 +12,7 @@ use crate::asm::instr::Flatten;
 use crate::asm::instr::op::OpKind;
 use crate::asm::parse::arg_parser::TokenParser;
 use crate::runtime::fault::Fault;
-use crate::runtime::run_thread::state::{RunState};
+use crate::runtime::run_thread::state::RunState;
 use crate::runtime::run_thread::ThreadInfo;
 
 mod eval_res;
@@ -63,6 +63,13 @@ pub trait CrsnExtension: Debug + Send + Sync + 'static {
         Ok(ParseRes::Unknown(tokens))
     }
 
+    /// Get value of an extension-provided constant.
+    /// This constant may be an object handle, or a constant value used as argument in some other instruction.
+    fn get_constant_value<'a>(&self, name: &str) -> Option<Value>
+    {
+        None
+    }
+
     /// Drop an object referenced by a handle
     fn drop_obj(&self, ti: &ThreadInfo, state: &mut RunState, handle: Value)
                 -> Result<Option<()>, Fault>