diff --git a/crates/cpu/src/util.rs b/crates/cpu/src/util.rs
index e006bb1..0b53274 100644
--- a/crates/cpu/src/util.rs
+++ b/crates/cpu/src/util.rs
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: LGPL-3.0-or-later
 // See Notices.txt for copyright information
 
+pub mod array_vec;
 pub mod tree_reduce;
 
 pub(crate) const fn range_u32_len(range: &std::ops::Range<u32>) -> usize {
diff --git a/crates/cpu/src/util/array_vec.rs b/crates/cpu/src/util/array_vec.rs
new file mode 100644
index 0000000..761f53f
--- /dev/null
+++ b/crates/cpu/src/util/array_vec.rs
@@ -0,0 +1,265 @@
+// SPDX-License-Identifier: LGPL-3.0-or-later
+// See Notices.txt for copyright information
+
+use fayalite::{
+    expr::ops::{ExprCastTo, ExprIndex, ExprPartialEq, ExprPartialOrd},
+    int::SizeType,
+    intern::{Intern, Interned},
+    prelude::*,
+    ty::{MatchVariantWithoutScope, StaticType, TypeProperties},
+};
+use std::{marker::PhantomData, ops::Index};
+
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
+pub struct Length<Max: Size> {
+    ty: UInt,
+    _phantom: PhantomData<Max>,
+}
+
+impl<Max: Size> Length<Max> {
+    pub fn new(max: Max::SizeType) -> Self {
+        Self {
+            ty: UInt::range_inclusive(0..=Max::as_usize(max)),
+            _phantom: PhantomData,
+        }
+    }
+    pub fn ty(self) -> UInt {
+        self.ty
+    }
+    pub fn zero(self) -> Expr<Self> {
+        Self::from_uint_unchecked(self.ty.zero())
+    }
+    pub fn from_uint_unchecked(v: impl ToExpr<Type = UInt>) -> Expr<Self> {
+        Expr::from_canonical(Expr::canonical(v.to_expr()))
+    }
+    pub fn cast_from_uint_unchecked<SrcWidth: Size>(
+        self,
+        v: impl ToExpr<Type = UIntType<SrcWidth>>,
+    ) -> Expr<Self> {
+        Self::from_uint_unchecked(v.to_expr().cast_to(self.ty))
+    }
+    pub fn as_uint(this: impl ToExpr<Type = Self>) -> Expr<UInt> {
+        let this = this.to_expr();
+        this.cast_to(Expr::ty(this).ty)
+    }
+}
+
+impl<Max: Size, DestWidth: Size> ExprCastTo<UIntType<DestWidth>> for Length<Max> {
+    fn cast_to(src: Expr<Self>, to_type: UIntType<DestWidth>) -> Expr<UIntType<DestWidth>> {
+        Expr::<UInt>::from_canonical(Expr::canonical(src)).cast_to(to_type)
+    }
+}
+
+#[allow(non_upper_case_globals)]
+pub const Length: __LengthWithoutGenerics = __LengthWithoutGenerics {};
+
+#[non_exhaustive]
+pub struct __LengthWithoutGenerics {}
+
+impl<M: SizeType> Index<M> for __LengthWithoutGenerics {
+    type Output = Length<M::Size>;
+
+    fn index(&self, max: M) -> &Self::Output {
+        Interned::into_inner(Length::new(max).intern_sized())
+    }
+}
+
+impl<Max: Size> Type for Length<Max> {
+    type BaseType = UInt;
+    type MaskType = Bool;
+    type MatchVariant = Expr<Self>;
+    type MatchActiveScope = ();
+    type MatchVariantAndInactiveScope = MatchVariantWithoutScope<Self::MatchVariant>;
+    type MatchVariantsIter = std::iter::Once<Self::MatchVariantAndInactiveScope>;
+    fn match_variants(
+        this: Expr<Self>,
+        source_location: SourceLocation,
+    ) -> Self::MatchVariantsIter {
+        let _ = source_location;
+        std::iter::once(MatchVariantWithoutScope(this))
+    }
+
+    fn mask_type(&self) -> Self::MaskType {
+        Bool
+    }
+
+    fn canonical(&self) -> CanonicalType {
+        self.ty.canonical()
+    }
+
+    fn from_canonical(canonical_type: CanonicalType) -> Self {
+        let ty = <UInt>::from_canonical(canonical_type);
+        if let Some(known_max) = Max::KNOWN_VALUE {
+            assert_eq!(ty, UInt::range_inclusive(0..=known_max));
+        }
+        Self {
+            ty,
+            _phantom: PhantomData,
+        }
+    }
+
+    fn source_location() -> SourceLocation {
+        SourceLocation::caller()
+    }
+}
+
+impl<Max: KnownSize> StaticType for Length<Max> {
+    const TYPE: Self = Self {
+        ty: UInt {
+            width: Max::VALUE.next_power_of_two().ilog2() as usize,
+        },
+        _phantom: PhantomData,
+    };
+    const MASK_TYPE: Self::MaskType = Bool;
+    const TYPE_PROPERTIES: TypeProperties = {
+        let mut p = <UInt<1>>::TYPE_PROPERTIES;
+        p.bit_width = Self::TYPE.ty.width;
+        p
+    };
+    const MASK_TYPE_PROPERTIES: TypeProperties = Bool::TYPE_PROPERTIES;
+}
+
+impl<Max: Size> ExprPartialEq<Self> for Length<Max> {
+    fn cmp_eq(lhs: Expr<Self>, rhs: Expr<Self>) -> Expr<Bool> {
+        Self::as_uint(lhs).cmp_eq(Self::as_uint(rhs))
+    }
+    fn cmp_ne(lhs: Expr<Self>, rhs: Expr<Self>) -> Expr<Bool> {
+        Self::as_uint(lhs).cmp_ne(Self::as_uint(rhs))
+    }
+}
+
+impl<Max: Size> ExprPartialOrd<Self> for Length<Max> {
+    fn cmp_lt(lhs: Expr<Self>, rhs: Expr<Self>) -> Expr<Bool> {
+        Self::as_uint(lhs).cmp_lt(Self::as_uint(rhs))
+    }
+    fn cmp_le(lhs: Expr<Self>, rhs: Expr<Self>) -> Expr<Bool> {
+        Self::as_uint(lhs).cmp_le(Self::as_uint(rhs))
+    }
+    fn cmp_gt(lhs: Expr<Self>, rhs: Expr<Self>) -> Expr<Bool> {
+        Self::as_uint(lhs).cmp_gt(Self::as_uint(rhs))
+    }
+    fn cmp_ge(lhs: Expr<Self>, rhs: Expr<Self>) -> Expr<Bool> {
+        Self::as_uint(lhs).cmp_ge(Self::as_uint(rhs))
+    }
+}
+
+/// like [`std::vec::Vec`], except with a [`Expr`] for [`len()`][`Self::len()`] and a fixed capacity
+#[hdl]
+pub struct ArrayVec<T: Type, N: Size> {
+    elements: ArrayType<T, N>,
+    len: Length<N>,
+}
+
+impl<T: Type, N: Size> ArrayVec<T, N> {
+    #[hdl]
+    pub fn new(self) -> Expr<Self> {
+        #[hdl]
+        ArrayVec {
+            elements: self.elements.uninit(),
+            len: self.len.zero(),
+        }
+    }
+    pub fn element(self) -> T {
+        self.elements.element()
+    }
+    pub fn elements_ty(self) -> ArrayType<T, N> {
+        self.elements
+    }
+    pub fn elements_unchecked(this: impl ToExpr<Type = Self>) -> Expr<ArrayType<T, N>> {
+        this.to_expr().elements
+    }
+    #[hdl]
+    pub fn from_parts_unchecked(
+        elements: impl ToExpr<Type = ArrayType<T, N>>,
+        len: impl ToExpr<Type = Length<N>>,
+    ) -> Expr<Self> {
+        let elements = elements.to_expr();
+        let len = len.to_expr();
+        assert_eq!(
+            Length::new(N::from_usize(Expr::ty(elements).len())),
+            Expr::ty(len),
+            "len type mismatch",
+        );
+        #[hdl]
+        Self { elements, len }
+    }
+    pub fn len_ty(self) -> Length<N> {
+        self.len
+    }
+    pub fn len(this: impl ToExpr<Type = Self>) -> Expr<Length<N>> {
+        this.to_expr().len
+    }
+    pub fn is_empty(this: impl ToExpr<Type = Self>) -> Expr<Bool> {
+        let len = Self::len(this);
+        len.cmp_eq(Expr::ty(len).zero())
+    }
+    pub fn capacity(self) -> usize {
+        self.elements.len()
+    }
+    pub fn get_unchecked<Idx>(this: impl ToExpr<Type = Self>, index: Idx) -> Expr<T>
+    where
+        ArrayType<T, N>: ExprIndex<Idx, Output = T>,
+    {
+        this.to_expr().elements[index]
+    }
+    #[hdl]
+    pub fn for_each(this: impl ToExpr<Type = Self>, mut f: impl FnMut(usize, Expr<T>)) {
+        let this = this.to_expr();
+        for (index, element) in this.elements.into_iter().enumerate() {
+            #[hdl]
+            if index.cmp_lt(Length::as_uint(this.len)) {
+                f(index, element);
+            }
+        }
+    }
+    pub fn mapped_ty<U: Type>(self, new_element_ty: U) -> ArrayVec<U, N> {
+        ArrayVec {
+            elements: ArrayType[new_element_ty][N::from_usize(self.elements.len())],
+            len: self.len,
+        }
+    }
+    #[hdl]
+    pub fn map<U: Type>(
+        this: impl ToExpr<Type = Self>,
+        new_element_ty: U,
+        mut f: impl FnMut(usize, Expr<T>) -> Expr<U>,
+    ) -> Expr<ArrayVec<U, N>> {
+        let this = this.to_expr();
+        #[hdl]
+        let mapped_array_vec = wire(Expr::ty(this).mapped_ty(new_element_ty));
+        connect(mapped_array_vec.len, this.len);
+        Self::for_each(this, |index, element| {
+            connect(mapped_array_vec[index], f(index, element));
+        });
+        mapped_array_vec
+    }
+    #[hdl]
+    pub fn as_array_of_options(this: impl ToExpr<Type = Self>) -> Expr<ArrayType<HdlOption<T>, N>> {
+        let this = this.to_expr();
+        #[hdl]
+        let array_vec_as_array_of_options = wire(
+            ArrayType[HdlOption[Expr::ty(this).element()]]
+                [N::from_usize(Expr::ty(this).capacity())],
+        );
+        for element in array_vec_as_array_of_options {
+            connect(element, Expr::ty(element).HdlNone());
+        }
+        Self::for_each(this, |index, element| {
+            connect(array_vec_as_array_of_options[index], HdlSome(element))
+        });
+        array_vec_as_array_of_options
+    }
+}
+
+impl<T: Type, N: Size, Idx, IdxWidth: Size> ExprIndex<Idx> for ArrayVec<T, N>
+where
+    ArrayType<T, N>: ExprIndex<Idx, Output = T>,
+    Idx: ToExpr<Type = UIntType<IdxWidth>>,
+{
+    type Output = T;
+
+    fn expr_index(this: &Expr<Self>, index: Idx) -> &Expr<Self::Output> {
+        // TODO: add assert that index is in-bounds
+        <ArrayType<T, N> as ExprIndex<Idx>>::expr_index(&this.elements, index)
+    }
+}