diff --git a/rust/src/chunk.rs b/rust/src/chunk.rs
index 44d1e9f..df29583 100644
--- a/rust/src/chunk.rs
+++ b/rust/src/chunk.rs
@@ -1,12 +1,13 @@
 #[repr(u8)]
 pub enum OpCode {
+    Constant,
     Return,
 }
 
 type Value = f32;
 
 pub struct Chunk {
-    code: Vec<OpCode>,
+    code: Vec<u8>,
     constants: Vec<Value>,
 }
 
@@ -19,13 +20,16 @@ impl Chunk {
     }
 
     // https://doc.rust-lang.org/nomicon/vec/vec-push-pop.html
-    pub fn write(&mut self, op_code: OpCode) {
-        self.code.push(op_code);
+    pub fn write(&mut self, byte: u8) {
+        self.code.push(byte);
     }
 
-    pub fn add_constant(&mut self, value: Value) -> usize{
+    pub fn add_constant(&mut self, value: Value) -> u8 {
         self.constants.push(value);
-        self.constants.len() - 1
+
+        assert!(self.constants.len() <= u8::MAX.into(), "Too many constants");
+
+        (self.constants.len() - 1) as u8
     }
 
     pub fn disassemble(&self, name: &str) {
@@ -41,7 +45,9 @@ impl Chunk {
         print!("{:04} ", offset);
 
         match self.code[offset] {
-            OpCode::Return => self.simple_instruction("OP_RETURN", offset),
+            0 => self.constant_instruction("OP_CONSTANT", offset),
+            1 => self.simple_instruction("OP_RETURN", offset),
+            _ => unreachable!(),
         }
     }
 
@@ -49,6 +55,13 @@ impl Chunk {
         println!("{}", name);
         offset + 1
     }
+
+    fn constant_instruction(&self, name: &str, offset: usize) -> usize {
+        let constant_index = self.code[offset+1];
+        let value = self.constants[constant_index as usize];
+        println!("{:<16} {:04} '{}'", name, constant_index, value);
+        offset + 2
+    }
 }
 
 impl Default for Chunk {
diff --git a/rust/src/main.rs b/rust/src/main.rs
index f1a76d2..fe15590 100644
--- a/rust/src/main.rs
+++ b/rust/src/main.rs
@@ -5,7 +5,11 @@ pub mod vec;
 
 fn main() {
     let mut chunk = Chunk::default();
-    chunk.write(OpCode::Return);
+    chunk.write(OpCode::Return as u8);
+
+    let constant = chunk.add_constant(1.2);
+    chunk.write(OpCode::Constant as u8);
+    chunk.write(constant);
 
     chunk.disassemble("test chunk");
 }