diff --git a/crates/fayalite/src/lib.rs b/crates/fayalite/src/lib.rs
index eedb1bb..fba7ada 100644
--- a/crates/fayalite/src/lib.rs
+++ b/crates/fayalite/src/lib.rs
@@ -46,6 +46,7 @@ pub mod module;
 pub mod prelude;
 pub mod reg;
 pub mod reset;
+pub mod sim;
 pub mod source_location;
 pub mod testing;
 pub mod ty;
diff --git a/crates/fayalite/src/sim.rs b/crates/fayalite/src/sim.rs
new file mode 100644
index 0000000..3aa69a6
--- /dev/null
+++ b/crates/fayalite/src/sim.rs
@@ -0,0 +1,206 @@
+//! Fayalite Simulation
+
+use crate::{
+    bundle::{Bundle, BundleType},
+    enum_::Enum,
+    expr::Expr,
+    int::Bool,
+    intern::{Intern, Interned},
+    module::{
+        Block, Module, ModuleBody, NormalModuleBody, Stmt, StmtConnect, StmtFormal, StmtIf,
+        StmtMatch,
+    },
+    sim::interpreter::{Insn, Insns, InsnsBuilding},
+    source_location::SourceLocation,
+};
+use hashbrown::HashMap;
+use std::{cell::RefCell, rc::Rc};
+
+mod interpreter;
+
+#[derive(Debug)]
+struct CompilingModule {
+    compiled_module: Option<CompiledModule>,
+}
+
+#[derive(Debug, PartialEq, Eq, Hash, Clone, Copy)]
+enum CondStack {
+    Always,
+    IfTrue {
+        parent: Interned<CondStack>,
+        cond: Expr<Bool>,
+        source_location: SourceLocation,
+    },
+    IfFalse {
+        parent: Interned<CondStack>,
+        cond: Expr<Bool>,
+        source_location: SourceLocation,
+    },
+    MatchArm {
+        parent: Interned<CondStack>,
+        enum_expr: Expr<Enum>,
+        variant_index: usize,
+        source_location: SourceLocation,
+    },
+}
+
+#[derive(Debug)]
+pub struct Compiler {
+    insns: Insns<InsnsBuilding>,
+    modules: HashMap<Interned<Module<Bundle>>, Rc<RefCell<CompilingModule>>>,
+    module_queue: Vec<Interned<Module<Bundle>>>,
+}
+
+impl Compiler {
+    pub fn new() -> Self {
+        Self {
+            insns: Insns::new(),
+            modules: HashMap::new(),
+            module_queue: Vec::new(),
+        }
+    }
+    pub fn add_module(&mut self, module: Interned<Module<Bundle>>) {
+        self.modules.entry(module).or_insert_with(|| {
+            self.module_queue.push(module);
+            Rc::new(RefCell::new(CompilingModule {
+                compiled_module: None,
+            }))
+        });
+    }
+    fn compile_block(
+        &mut self,
+        module: Interned<Module<Bundle>>,
+        block: Block,
+        cond_stack: Interned<CondStack>,
+    ) {
+        let Block { memories, stmts } = block;
+        for memory in memories {
+            todo!("implement memory");
+        }
+        for stmt in stmts {
+            match stmt {
+                Stmt::Connect(StmtConnect {
+                    lhs,
+                    rhs,
+                    source_location,
+                }) => todo!(),
+                Stmt::Formal(StmtFormal {
+                    kind,
+                    clk,
+                    pred,
+                    en,
+                    text,
+                    source_location,
+                }) => todo!("implement simulating formal statements"),
+                Stmt::If(StmtIf {
+                    cond,
+                    source_location,
+                    blocks: [then_block, else_block],
+                }) => {
+                    self.compile_block(
+                        module,
+                        then_block,
+                        CondStack::IfTrue {
+                            parent: cond_stack,
+                            cond,
+                            source_location,
+                        }
+                        .intern_sized(),
+                    );
+                    self.compile_block(
+                        module,
+                        else_block,
+                        CondStack::IfFalse {
+                            parent: cond_stack,
+                            cond,
+                            source_location,
+                        }
+                        .intern_sized(),
+                    );
+                }
+                Stmt::Match(StmtMatch {
+                    expr,
+                    source_location,
+                    blocks,
+                }) => {
+                    for (variant_index, block) in blocks.into_iter().enumerate() {
+                        self.compile_block(
+                            module,
+                            block,
+                            CondStack::MatchArm {
+                                parent: cond_stack,
+                                enum_expr: expr,
+                                variant_index,
+                                source_location,
+                            }
+                            .intern_sized(),
+                        );
+                    }
+                }
+                Stmt::Declaration(stmt_declaration) => todo!(),
+            }
+        }
+    }
+    fn compile_module(&mut self, module: Interned<Module<Bundle>>) -> CompiledModule {
+        let step_entry_pc = self.insns.next_insn_pc();
+        match module.body() {
+            ModuleBody::Normal(NormalModuleBody { body }) => {
+                self.compile_block(module, body, CondStack::Always.intern_sized())
+            }
+            ModuleBody::Extern(_extern_module_body) => {
+                todo!("simulating extern module: {}", module.name_id());
+            }
+        }
+        self.insns.push(Insn::Return, module.source_location());
+        CompiledModule { step_entry_pc }
+    }
+    pub fn run(mut self) -> Compiled {
+        while let Some(module) = self.module_queue.pop() {
+            let compiling_module = self.modules[&module].clone();
+            let mut compiling_module = compiling_module.borrow_mut();
+            let CompilingModule { compiled_module } = &mut *compiling_module;
+            assert!(compiled_module.is_none());
+            *compiled_module = Some(self.compile_module(module));
+        }
+        Compiled {
+            insns: Insns::from(self.insns).intern_sized(),
+            modules: self
+                .modules
+                .into_iter()
+                .map(|(module, compiling_module)| {
+                    (
+                        module,
+                        Rc::into_inner(compiling_module)
+                            .expect("only reference is Compiler::modules")
+                            .into_inner()
+                            .compiled_module
+                            .expect("module is compiled by Compiler::run"),
+                    )
+                })
+                .collect(),
+        }
+    }
+}
+
+#[derive(Debug)]
+struct CompiledModule {
+    step_entry_pc: usize,
+}
+
+#[derive(Debug)]
+pub struct Compiled {
+    insns: Interned<Insns>,
+    modules: HashMap<Interned<Module<Bundle>>, CompiledModule>,
+}
+
+impl Compiled {
+    pub fn new<T: BundleType>(module: Module<T>) -> Self {
+        let mut compiler = Compiler::new();
+        compiler.add_module(module.canonical().intern());
+        compiler.run()
+    }
+}
+
+pub struct Simulation {
+    state: interpreter::State,
+}
diff --git a/crates/fayalite/src/sim/interpreter.rs b/crates/fayalite/src/sim/interpreter.rs
new file mode 100644
index 0000000..eed64e4
--- /dev/null
+++ b/crates/fayalite/src/sim/interpreter.rs
@@ -0,0 +1,464 @@
+use crate::{
+    intern::{Intern, Interned},
+    source_location::SourceLocation,
+};
+use num_bigint::BigInt;
+use std::{convert::Infallible, fmt, hash::Hash};
+
+pub(crate) type SmallUInt = u64;
+pub(crate) type SmallSInt = i64;
+
+macro_rules! impl_insns {
+    (
+        #[next_macro = $next_macro:ident, branch_macro = $branch_macro:ident]
+        $vis:vis fn $State:ident::$run:ident(&mut $self:ident, $insns:ident: &[$Insn:ident]) -> $run_ret_ty:ty {
+            #[pc]
+            let mut $pc:ident: usize = $pc_init:expr;
+            setup! {
+                $($setup:tt)*
+            }
+            $(let mut $local:ident = $local_init:expr;)*
+            main_loop!();
+            cleanup! {
+                $($cleanup:tt)*
+            }
+        }
+        $(
+            $(#[$insn_meta:meta])*
+            $insn_name:ident $({
+                $(
+                    $(#[$field_meta:meta])*
+                    $field_name:ident: $field_ty:ty,
+                )*
+            })? => $block:block
+        )+
+    ) => {
+        #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
+        $vis enum $Insn {
+            $(
+                $(#[$insn_meta])*
+                $insn_name $({
+                    $(
+                        $(#[$field_meta])*
+                        $field_name: $field_ty,
+                    )*
+                })?,
+            )+
+        }
+
+        impl $State {
+            $vis fn $run(&mut $self, $insns: &[$Insn]) -> $run_ret_ty {
+                let mut $pc: usize = $pc_init;
+                $($setup)*
+                let mut insn = $insns[$pc];
+                let retval = 'main_loop: loop {
+                    macro_rules! $next_macro {
+                        () => {
+                            $pc += 1;
+                            insn = $insns[$pc];
+                            continue 'main_loop;
+                        };
+                    }
+                    macro_rules! $branch_macro {
+                        ($next_pc:expr) => {
+                            $pc = $next_pc;
+                            insn = $insns[$pc];
+                            continue 'main_loop;
+                        };
+                    }
+                    let _: Infallible = match insn {
+                        $(
+                            $Insn::$insn_name $({
+                                $(
+                                    $field_name,
+                                )*
+                            })? => {
+                                $block
+                            }
+                        )+
+                    };
+                };
+                $($cleanup)*
+                retval
+            }
+        }
+    };
+}
+
+pub(crate) trait InsnsBuildingKind: Copy + Eq + fmt::Debug + Hash + Default {
+    type BoxedSlice<T: fmt::Debug + Clone + Eq + Hash + Send + Sync + 'static>: fmt::Debug
+        + Clone
+        + Eq
+        + std::ops::Deref<Target = [T]>;
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, Default)]
+pub(crate) struct InsnsBuildingDone;
+
+impl InsnsBuildingKind for InsnsBuildingDone {
+    type BoxedSlice<T: fmt::Debug + Clone + Eq + Hash + Send + Sync + 'static> = Interned<[T]>;
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, Default)]
+pub(crate) struct InsnsBuilding;
+
+impl InsnsBuildingKind for InsnsBuilding {
+    type BoxedSlice<T: fmt::Debug + Clone + Eq + Hash + Send + Sync + 'static> = Vec<T>;
+}
+
+#[derive(Clone, PartialEq, Eq, Hash)]
+pub(crate) struct Insns<BK: InsnsBuildingKind = InsnsBuildingDone> {
+    insns: BK::BoxedSlice<Insn>,
+    small_stack_size: usize,
+    big_stack_size: usize,
+    small_state_debug_names: BK::BoxedSlice<Interned<str>>,
+    big_state_debug_names: BK::BoxedSlice<Interned<str>>,
+    insn_source_locations: BK::BoxedSlice<SourceLocation>,
+}
+
+struct InsnsDebug<'a> {
+    insns: &'a [Insn],
+    insn_source_locations: &'a [SourceLocation],
+}
+
+impl<'a> fmt::Debug for InsnsDebug<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut debug_list = f.debug_list();
+        let mut last_source_location = None;
+        for (insn, source_location) in self.insns.iter().zip(self.insn_source_locations) {
+            if Some(source_location) != last_source_location {
+                debug_list.entry(&format_args!("// at: {source_location}\n{insn:?}"));
+            } else {
+                debug_list.entry(insn);
+            }
+            last_source_location = Some(source_location);
+        }
+        debug_list.finish()
+    }
+}
+
+impl<BK: InsnsBuildingKind> fmt::Debug for Insns<BK> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let Self {
+            insns,
+            small_stack_size,
+            big_stack_size,
+            small_state_debug_names,
+            big_state_debug_names,
+            insn_source_locations,
+        } = self;
+        f.debug_struct("Insns")
+            .field("small_stack_size", small_stack_size)
+            .field("big_stack_size", big_stack_size)
+            .field("small_state_debug_names", small_state_debug_names)
+            .field("big_state_debug_names", big_state_debug_names)
+            .field(
+                "insns",
+                &InsnsDebug {
+                    insns,
+                    insn_source_locations,
+                },
+            )
+            .finish_non_exhaustive()
+    }
+}
+
+impl Insns<InsnsBuilding> {
+    pub(crate) fn new() -> Self {
+        Self {
+            insns: Vec::new(),
+            small_stack_size: 0,
+            big_stack_size: 0,
+            small_state_debug_names: Vec::new(),
+            big_state_debug_names: Vec::new(),
+            insn_source_locations: Vec::new(),
+        }
+    }
+    pub(crate) fn last_insn_pc(&self) -> usize {
+        self.insns
+            .len()
+            .checked_sub(1)
+            .expect("no last instruction")
+    }
+    pub(crate) fn next_insn_pc(&self) -> usize {
+        self.insns.len()
+    }
+    pub(crate) fn push(&mut self, insn: Insn, source_location: SourceLocation) {
+        self.insns.push(insn);
+        self.insn_source_locations.push(source_location);
+    }
+}
+
+impl From<Insns<InsnsBuilding>> for Insns {
+    fn from(input: Insns<InsnsBuilding>) -> Self {
+        let Insns {
+            insns,
+            small_stack_size,
+            big_stack_size,
+            small_state_debug_names,
+            big_state_debug_names,
+            insn_source_locations,
+        } = input;
+        Insns {
+            insns: Intern::intern_owned(insns),
+            small_stack_size,
+            big_stack_size,
+            small_state_debug_names: Intern::intern_owned(small_state_debug_names),
+            big_state_debug_names: Intern::intern_owned(big_state_debug_names),
+            insn_source_locations: Intern::intern_owned(insn_source_locations),
+        }
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct State {
+    pub(crate) insns: Interned<Insns>,
+    pub(crate) pc: usize,
+    pub(crate) small_stack: Box<[SmallUInt]>,
+    pub(crate) small_stack_top: usize,
+    pub(crate) small_states: Box<[SmallUInt]>,
+    pub(crate) big_stack: Box<[BigInt]>,
+    pub(crate) big_stack_top: usize,
+    pub(crate) big_states: Box<[BigInt]>,
+}
+
+impl State {
+    pub(crate) fn setup_call(&mut self, entry_pc: usize) {
+        let Self {
+            insns: _,
+            pc,
+            small_stack: _,
+            small_stack_top,
+            small_states: _,
+            big_stack: _,
+            big_stack_top,
+            big_states: _,
+        } = self;
+        *pc = entry_pc;
+        *small_stack_top = 0;
+        *big_stack_top = 0;
+    }
+}
+
+impl_insns! {
+    #[next_macro = next, branch_macro = branch]
+    pub(crate) fn State::run(&mut self, insns: &[Insn]) -> () {
+        #[pc]
+        let mut pc: usize = self.pc;
+        setup! {
+            let small_states = &mut *self.small_states;
+            let small_stack = &mut *self.small_stack;
+            let mut small_stack_top = self.small_stack_top;
+            macro_rules! small_stack_push {
+                ($v:expr) => {
+                    {
+                        small_stack[small_stack_top] = $v;
+                        small_stack_top += 1;
+                    }
+                };
+            }
+            macro_rules! small_stack_pop {
+                () => {
+                    {
+                        small_stack_top -= 1;
+                        small_stack[small_stack_top]
+                    }
+                };
+            }
+            let big_states = &mut *self.big_states;
+            let big_stack = &mut *self.big_stack;
+            let mut big_stack_top = self.big_stack_top;
+            macro_rules! big_stack_push {
+                ($v:expr) => {
+                    {
+                        big_stack[big_stack_top] = $v;
+                        big_stack_top += 1;
+                    }
+                };
+            }
+            macro_rules! big_stack_pop {
+                () => {
+                    {
+                        big_stack_top -= 1;
+                        big_stack[big_stack_top]
+                    }
+                };
+            }
+        }
+        main_loop!();
+        cleanup! {
+            self.pc = pc;
+            self.small_stack_top = small_stack_top;
+        }
+    }
+    ReadSmall {
+        index: u32,
+    } => {
+        small_stack_push!(small_states[index as usize]);
+        next!();
+    }
+    SExtSmall {
+        /// number of bits in a [`SmallSInt`] that aren't used
+        unused_bit_count: u8,
+    } => {
+        let mut value = small_stack_pop!() as SmallSInt;
+        value <<= unused_bit_count;
+        value >>= unused_bit_count;
+        small_stack_push!(value as SmallUInt);
+        next!();
+    }
+    ZExtSmall {
+        /// number of bits in a [`SmallUInt`] that aren't used
+        unused_bit_count: u8,
+    } => {
+        let mut value = small_stack_pop!();
+        value <<= unused_bit_count;
+        value >>= unused_bit_count;
+        small_stack_push!(value);
+        next!();
+    }
+    AndSmall => {
+        let rhs = small_stack_pop!();
+        let lhs = small_stack_pop!();
+        let value = lhs & rhs;
+        small_stack_push!(value);
+        next!();
+    }
+    OrSmall => {
+        let rhs = small_stack_pop!();
+        let lhs = small_stack_pop!();
+        let value = lhs | rhs;
+        small_stack_push!(value);
+        next!();
+    }
+    XorSmall => {
+        let rhs = small_stack_pop!();
+        let lhs = small_stack_pop!();
+        let value = lhs ^ rhs;
+        small_stack_push!(value);
+        next!();
+    }
+    NotSmall => {
+        let value = small_stack_pop!();
+        small_stack_push!(!value);
+        next!();
+    }
+    CmpEqSmall => {
+        let rhs = small_stack_pop!();
+        let lhs = small_stack_pop!();
+        let value = (lhs == rhs) as SmallUInt;
+        small_stack_push!(value);
+        next!();
+    }
+    CmpNeSmall => {
+        let rhs = small_stack_pop!();
+        let lhs = small_stack_pop!();
+        let value = (lhs != rhs) as SmallUInt;
+        small_stack_push!(value);
+        next!();
+    }
+    CmpLTSmallUInt => {
+        let rhs = small_stack_pop!();
+        let lhs = small_stack_pop!();
+        let value = (lhs < rhs) as SmallUInt;
+        small_stack_push!(value);
+        next!();
+    }
+    CmpLESmallUInt => {
+        let rhs = small_stack_pop!();
+        let lhs = small_stack_pop!();
+        let value = (lhs <= rhs) as SmallUInt;
+        small_stack_push!(value);
+        next!();
+    }
+    CmpGTSmallUInt => {
+        let rhs = small_stack_pop!();
+        let lhs = small_stack_pop!();
+        let value = (lhs > rhs) as SmallUInt;
+        small_stack_push!(value);
+        next!();
+    }
+    CmpGESmallUInt => {
+        let rhs = small_stack_pop!();
+        let lhs = small_stack_pop!();
+        let value = (lhs >= rhs) as SmallUInt;
+        small_stack_push!(value);
+        next!();
+    }
+    CmpLTSmallSInt => {
+        let rhs = small_stack_pop!() as SmallSInt;
+        let lhs = small_stack_pop!() as SmallSInt;
+        let value = (lhs < rhs) as SmallUInt;
+        small_stack_push!(value);
+        next!();
+    }
+    CmpLESmallSInt => {
+        let rhs = small_stack_pop!() as SmallSInt;
+        let lhs = small_stack_pop!() as SmallSInt;
+        let value = (lhs <= rhs) as SmallUInt;
+        small_stack_push!(value);
+        next!();
+    }
+    CmpGTSmallSInt => {
+        let rhs = small_stack_pop!() as SmallSInt;
+        let lhs = small_stack_pop!() as SmallSInt;
+        let value = (lhs > rhs) as SmallUInt;
+        small_stack_push!(value);
+        next!();
+    }
+    CmpGESmallSInt => {
+        let rhs = small_stack_pop!() as SmallSInt;
+        let lhs = small_stack_pop!() as SmallSInt;
+        let value = (lhs >= rhs) as SmallUInt;
+        small_stack_push!(value);
+        next!();
+    }
+    AddSmall => {
+        let rhs = small_stack_pop!();
+        let lhs = small_stack_pop!();
+        let value = lhs.wrapping_add(rhs);
+        small_stack_push!(value);
+        next!();
+    }
+    SubSmall => {
+        let rhs = small_stack_pop!();
+        let lhs = small_stack_pop!();
+        let value = lhs.wrapping_sub(rhs);
+        small_stack_push!(value);
+        next!();
+    }
+    NegSmall => {
+        let value = small_stack_pop!();
+        let value = value.wrapping_neg();
+        small_stack_push!(value);
+        next!();
+    }
+    MulSmall => {
+        let rhs = small_stack_pop!();
+        let lhs = small_stack_pop!();
+        let value = lhs.wrapping_mul(rhs);
+        small_stack_push!(value);
+        next!();
+    }
+    ConstU32Small {
+        value: u32,
+    } => {
+        small_stack_push!(value as SmallUInt);
+        next!();
+    }
+    ConstS32Small {
+        value: i32,
+    } => {
+        small_stack_push!(value as SmallUInt);
+        next!();
+    }
+    WriteSmall {
+        index: u32,
+    } => {
+        small_states[index as usize] = small_stack_pop!();
+        next!();
+    }
+    Return => {
+        break;
+    }
+}