diff --git a/nac3artiq/src/lib.rs b/nac3artiq/src/lib.rs
index 042a7ce7..a86947df 100644
--- a/nac3artiq/src/lib.rs
+++ b/nac3artiq/src/lib.rs
@@ -21,7 +21,7 @@ use parking_lot::RwLock;
 use nac3core::{
     codegen::{CodeGenTask, WithCall, WorkerRegistry},
     symbol_resolver::SymbolResolver,
-    toplevel::{composer::TopLevelComposer, TopLevelContext, TopLevelDef},
+    toplevel::{composer::TopLevelComposer, TopLevelContext, TopLevelDef, GenCall},
     typecheck::typedef::{FunSignature, FuncArg},
 };
 use nac3core::{
@@ -33,7 +33,7 @@ use crate::symbol_resolver::Resolver;
 
 mod symbol_resolver;
 
-#[derive(Clone, Copy)]
+#[derive(PartialEq, Clone, Copy)]
 enum Isa {
     RiscV,
     CortexA9,
@@ -171,18 +171,57 @@ impl Nac3 {
             _ => return Err(exceptions::PyValueError::new_err("invalid ISA")),
         };
         let primitive: PrimitiveStore = TopLevelComposer::make_primitives().0;
-        let (composer, builtins_def, builtins_ty) = TopLevelComposer::new(vec![(
-            "output_int".into(),
-            FunSignature {
-                args: vec![FuncArg {
-                    name: "x".into(),
-                    ty: primitive.int32,
-                    default_value: None,
-                }],
-                ret: primitive.none,
-                vars: HashMap::new(),
-            },
-        )]);
+        let mut builtins = vec![];
+        if isa == Isa::RiscV {
+            builtins.push((
+                "now_mu".into(),
+                FunSignature {
+                    args: vec![],
+                    ret: primitive.int64,
+                    vars: HashMap::new(),
+                },
+                Arc::new(GenCall::new(Box::new(
+                    |ctx, _, fun, args| {
+                        unimplemented!();
+                    }
+                )))
+            ));
+            builtins.push((
+                "at_mu".into(),
+                FunSignature {
+                    args: vec![FuncArg {
+                        name: "t".into(),
+                        ty: primitive.int64,
+                        default_value: None,
+                    }],
+                    ret: primitive.none,
+                    vars: HashMap::new(),
+                },
+                Arc::new(GenCall::new(Box::new(
+                    |ctx, _, fun, args| {
+                        unimplemented!();
+                    }
+                )))
+            ));
+            builtins.push((
+                "delay_mu".into(),
+                FunSignature {
+                    args: vec![FuncArg {
+                        name: "dt".into(),
+                        ty: primitive.int64,
+                        default_value: None,
+                    }],
+                    ret: primitive.none,
+                    vars: HashMap::new(),
+                },
+                Arc::new(GenCall::new(Box::new(
+                    |ctx, _, fun, args| {
+                        unimplemented!();
+                    }
+                )))
+            ));
+        }
+        let (composer, builtins_def, builtins_ty) = TopLevelComposer::new(builtins);
         Ok(Nac3 {
             isa,
             primitive,
diff --git a/nac3core/src/toplevel/composer.rs b/nac3core/src/toplevel/composer.rs
index eb085187..f7c08337 100644
--- a/nac3core/src/toplevel/composer.rs
+++ b/nac3core/src/toplevel/composer.rs
@@ -34,7 +34,7 @@ impl TopLevelComposer {
     /// return a composer and things to make a "primitive" symbol resolver, so that the symbol
     /// resolver can later figure out primitive type definitions when passed a primitive type name
     pub fn new(
-        builtins: Vec<(StrRef, FunSignature)>,
+        builtins: Vec<(StrRef, FunSignature, Arc<GenCall>)>,
     ) -> (Self, HashMap<StrRef, DefinitionId>, HashMap<StrRef, Type>) {
         let mut primitives = Self::make_primitives();
 
@@ -311,7 +311,7 @@ impl TopLevelComposer {
             }
         }
 
-        for (name, sig) in builtins {
+        for (name, sig, codegen_callback) in builtins {
             let fun_sig = unifier.add_ty(TypeEnum::TFunc(RefCell::new(sig)));
             built_in_ty.insert(name, fun_sig);
             built_in_id.insert(name, DefinitionId(definition_ast_list.len()));
@@ -320,11 +320,11 @@ impl TopLevelComposer {
                     name: name.into(),
                     simple_name: name,
                     signature: fun_sig,
-                    instance_to_stmt: HashMap::new(),
-                    instance_to_symbol: [("".into(), name.into())].iter().cloned().collect(),
+                    instance_to_stmt: Default::default(),
+                    instance_to_symbol: Default::default(),
                     var_id: Default::default(),
                     resolver: None,
-                    codegen_callback: None,
+                    codegen_callback: Some(codegen_callback),
                 })),
                 None,
             ));
diff --git a/nac3standalone/demo/classes.py b/nac3standalone/demo/classes.py
index b2c534c4..ba489b48 100644
--- a/nac3standalone/demo/classes.py
+++ b/nac3standalone/demo/classes.py
@@ -1,3 +1,8 @@
+@extern
+def output_int(x: int32):
+    ...
+
+
 class A:
     a: int32
     b: B
diff --git a/nac3standalone/demo/mandelbrot.py b/nac3standalone/demo/mandelbrot.py
index a2e00e89..bb133b7d 100644
--- a/nac3standalone/demo/mandelbrot.py
+++ b/nac3standalone/demo/mandelbrot.py
@@ -1,3 +1,8 @@
+@extern
+def output_asciiart(x: int32):
+    ...
+
+
 def run() -> int32:
     minX = -2.0
     maxX = 1.0
diff --git a/nac3standalone/src/main.rs b/nac3standalone/src/main.rs
index ed86b1ab..fa51958d 100644
--- a/nac3standalone/src/main.rs
+++ b/nac3standalone/src/main.rs
@@ -9,7 +9,7 @@ use nac3core::{
     codegen::{CodeGenTask, WithCall, WorkerRegistry},
     symbol_resolver::SymbolResolver,
     toplevel::{composer::TopLevelComposer, TopLevelDef},
-    typecheck::typedef::{FunSignature, FuncArg},
+    typecheck::typedef::FunSignature,
 };
 
 mod basic_symbol_resolver;
@@ -32,26 +32,7 @@ fn main() {
    };
 
     let primitive: PrimitiveStore = TopLevelComposer::make_primitives().0;
-    let (mut composer, builtins_def, builtins_ty) = TopLevelComposer::new(vec![
-        ("output_int".into(), FunSignature {
-            args: vec![FuncArg {
-                name: "x".into(),
-                ty: primitive.int32,
-                default_value: None,
-            }],
-            ret: primitive.none,
-            vars: HashMap::new(),
-        }),
-        ("output_asciiart".into(), FunSignature {
-            args: vec![FuncArg {
-                name: "x".into(),
-                ty: primitive.int32,
-                default_value: None,
-            }],
-            ret: primitive.none,
-            vars: HashMap::new(),
-        })
-    ]);
+    let (mut composer, builtins_def, builtins_ty) = TopLevelComposer::new(vec![]);
 
     let internal_resolver: Arc<ResolverInternal> = ResolverInternal {
         id_to_type: builtins_ty.into(),