diff --git a/src/bytecode.rs b/src/bytecode.rs
index ee869b7..52a07ae 100644
--- a/src/bytecode.rs
+++ b/src/bytecode.rs
@@ -14,6 +14,43 @@
 //! Variables offsets are calculated as `local offsets`. Local offsets are calculated relative to
 //! the start of the space of the stack required by that function. The interpreter must keep track
 //! of the stack start of each function, to be able to calculate these offsets.
+//!
+//! # Function calls
+//! After the function returns, the interpreter resets its stack manually back
+//! to the length before the call. This means the interpreter has to do some bookkeeping, but it has
+//! to do that anyways.
+//!
+//! # ABI
+//! Function arguments are passed on the stack and can be loaded just like local variables. They belong
+//! to the stack frame of the new function and are cleaned up after returning, leaving the return value where
+//! the stack frame was.
+//!
+//! When a call happens, the current stack offset is pushed onto the stack as a `Value::Native` and
+//! the element before it is stored as the new offset.
+//! Then all parameters are pushed onto the stack, from last to first.
+//! Afterwards, execution of the code is started. A function always has to return, and compiler
+//! inserts `return null` at the end of every function implicitly.
+//!
+//! If a return happens, the VM loads the current value on the stack. It then goes to the start
+//! of the stack frame and saves the `Value::Native` that stores the old stack offset and loads that
+//! into its stack offset. It then removes the whole stack frame from the stack, and pushes the
+//! returned value.
+//!
+//! ```text
+//!  old stack offset ╮     ╭ Parameters ╮       ╭ local
+//!                   v     v            v       v
+//! ───────┬────────────┬───────────┬──────────┬─────────╮
+//! Num(6) │ Native(20) │   Num(5)  │  Num(6)  │  Num(5) │
+//! ───────┴────────────┴───────────┴──────────┴─────────╯
+//!        ╰────────────────────────────────────────────────── current stack frame
+//!  ^                  ^
+//!  ╰─ old local       ╰╮      
+//!                      │
+//!                      │
+//! Vm                   │
+//! Current stack offset ╯
+//!
+//! ```
 
 use crate::errors::Span;
 use crate::vm::Value;
@@ -24,7 +61,7 @@ use debug2::Formatter;
 #[derive(Debug)]
 pub struct FnBlock<'bc> {
     /// The bytecode of the function
-    pub code: Vec<'bc, Instr>,
+    pub code: Vec<'bc, Instr<'bc>>,
     /// The sizes of the stack required by the function after the instruction at the same index. This is only used
     /// during compilation to calculate local variable offsets.
     pub stack_sizes: Vec<'bc, usize>,
@@ -50,7 +87,7 @@ impl debug2::Debug for FnBlock<'_> {
 /// A bytecode instruction. For more details on the structure of the bytecode, read the module level docs [`bytecode`](`self`)
 #[derive(Debug, Clone, Copy)]
 #[cfg_attr(feature = "pretty", derive(debug2::Debug))]
-pub enum Instr {
+pub enum Instr<'bc> {
     /// An operation that does nothing.
     Nop,
 
@@ -86,6 +123,10 @@ pub enum Instr {
     /// Same as `JmpFalse`, but unconditional
     Jmp(isize),
 
+    Call(&'bc FnBlock<'bc>),
+
+    Return,
+
     /// Shrinks the stack by `usize` elements, should always be emitted before backwards jumps
     ShrinkStack(usize),
 }
diff --git a/src/vm.rs b/src/vm.rs
index 645dc04..93df565 100644
--- a/src/vm.rs
+++ b/src/vm.rs
@@ -28,12 +28,20 @@ pub fn execute<'bc>(
 #[derive(Debug, Clone, Copy)]
 #[cfg_attr(feature = "pretty", derive(debug2::Debug))]
 pub enum Value {
+    /// `null`
     Null,
+    /// A boolean value
     Bool(bool),
+    /// A floating point number
     Num(f64),
+    /// An interned string
     String(Symbol),
+    /// An array of values
     Array,
+    /// A map from string to value
     Object(Object),
+    /// A value that is stored by the vm for bookkeeping and should never be accessed for anythign else
+    Native(usize),
 }
 
 const _: () = _check_val_size();
@@ -170,6 +178,7 @@ impl<'bc> Vm<'bc, '_> {
                 }
             }
             Instr::Jmp(pos) => self.pc = (self.pc as isize + pos) as usize,
+            Instr::Call(_) => todo!(),
             Instr::ShrinkStack(size) => {
                 assert!(self.stack.len() >= size);
                 let new_len = self.stack.len() - size;
@@ -223,6 +232,7 @@ impl Display for Value {
             Value::String(str) => f.write_str(str.as_str()),
             Value::Array => todo!(),
             Value::Object(_) => todo!(),
+            Value::Native(_) => panic!("Called display on native value!"),
         }
     }
 }