diff --git a/crates/fayalite-proc-macros-impl/src/hdl_type_common.rs b/crates/fayalite-proc-macros-impl/src/hdl_type_common.rs
index 57b1034..0ae6480 100644
--- a/crates/fayalite-proc-macros-impl/src/hdl_type_common.rs
+++ b/crates/fayalite-proc-macros-impl/src/hdl_type_common.rs
@@ -5,7 +5,7 @@ use std::{collections::HashMap, fmt, mem};
 use syn::{
     parse::{Parse, ParseStream},
     parse_quote, parse_quote_spanned,
-    punctuated::Punctuated,
+    punctuated::{Pair, Punctuated},
     spanned::Spanned,
     token::{Brace, Bracket, Paren},
     AngleBracketedGenericArguments, Attribute, ConstParam, Expr, ExprIndex, ExprPath, ExprTuple,
@@ -239,7 +239,34 @@ impl ParseTypes<GenericArgument> for ParsedGenericArgument {
         parser: &mut TypesParser<'_>,
     ) -> Result<Self, ParseFailed> {
         match arg {
-            GenericArgument::Type(ty) => Ok(Self::Type(parser.parse(ty)?)),
+            GenericArgument::Type(ty) => {
+                {
+                    let mut ty = &*ty;
+                    while let Type::Group(TypeGroup { elem, .. }) = ty {
+                        ty = &**elem;
+                    }
+                    if let Type::Path(TypePath { qself: None, path }) = ty {
+                        if let Some(ident) = path.get_ident() {
+                            if let Some(&param_index) =
+                                parser.generics.param_name_to_index_map.get(ident)
+                            {
+                                match parser.generics.params[param_index] {
+                                    ParsedGenericParam::Type(_)
+                                    | ParsedGenericParam::SizeType(_) => {}
+                                    ParsedGenericParam::Const(_) => {
+                                        return Ok(Self::Const(Expr::Path(ExprPath {
+                                            attrs: vec![],
+                                            qself: None,
+                                            path: path.clone(),
+                                        })));
+                                    }
+                                }
+                            }
+                        }
+                    }
+                }
+                Ok(Self::Type(parser.parse(ty)?))
+            }
             GenericArgument::Const(expr) => Ok(Self::Const(expr.clone())),
             _ => parse_failed!(parser, arg, "expected type or const generic argument"),
         }
@@ -1205,6 +1232,13 @@ impl ParseTypes<Path> for ParsedType {
                                 param_index,
                             })
                         }
+                        ParsedGenericParam::Const(ParsedConstParam { ref ident, .. }) => {
+                            parser
+                                .errors
+                                .error(ident, "constant provided when a type was expected");
+                            todo!();
+                            return Err(ParseFailed);
+                        }
                     },
                 ));
             }
@@ -1291,6 +1325,120 @@ impl ParseTypes<Type> for ParsedType {
     }
 }
 
+#[derive(Debug, Clone)]
+pub(crate) enum ParsedConstGenericType {
+    Usize(known_items::usize),
+}
+
+impl_fold! {
+    enum ParsedConstGenericType<> {
+        Usize(known_items::usize),
+    }
+}
+
+impl From<ParsedConstGenericType> for Type {
+    fn from(value: ParsedConstGenericType) -> Self {
+        match value {
+            ParsedConstGenericType::Usize(v) => parse_quote! { #v },
+        }
+    }
+}
+
+impl From<MaybeParsed<ParsedConstGenericType, Type>> for Type {
+    fn from(value: MaybeParsed<ParsedConstGenericType, Type>) -> Self {
+        match value {
+            MaybeParsed::Unrecognized(value) => value,
+            MaybeParsed::Parsed(value) => value.into(),
+        }
+    }
+}
+
+impl From<Box<ParsedConstGenericType>> for Type {
+    fn from(value: Box<ParsedConstGenericType>) -> Self {
+        (*value).into()
+    }
+}
+
+impl ToTokens for ParsedConstGenericType {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        match self {
+            ParsedConstGenericType::Usize(ty) => ty.to_tokens(tokens),
+        }
+    }
+}
+
+impl ParseTypes<Path> for ParsedConstGenericType {
+    fn parse_types(path: &mut Path, parser: &mut TypesParser<'_>) -> Result<Self, ParseFailed> {
+        let Path {
+            leading_colon,
+            ref mut segments,
+        } = *path;
+        if segments.is_empty() {
+            parse_failed!(parser, path, "path must not be empty");
+        }
+        let mut args = None;
+        let segments = Punctuated::from_iter(segments.pairs_mut().map_pair_value_mut(|segment| {
+            let PathSegment { ident, arguments } = segment;
+            if let Some(_) = args {
+                parser
+                    .errors()
+                    .error(&ident, "associated types/consts are not yet implemented");
+            }
+            args = match arguments {
+                PathArguments::None => None,
+                PathArguments::AngleBracketed(args) => parser.parse(args).ok(),
+                PathArguments::Parenthesized(_) => {
+                    parser
+                        .errors()
+                        .error(&segment, "function traits are not allowed");
+                    None
+                }
+            };
+            PathSegment::from(segment.ident.clone())
+        }));
+        let named = ParsedTypeNamed {
+            path: Path {
+                leading_colon,
+                segments,
+            },
+            args,
+        };
+        let named = match known_items::usize::try_from_named(named, parser)? {
+            Ok(v) => return Ok(Self::Usize(v)),
+            Err(named) => named,
+        };
+        parser.errors.error(
+            named,
+            "const parameter types other than `usize` are not yet implemented",
+        );
+        Err(ParseFailed)
+    }
+}
+
+impl ParseTypes<TypePath> for ParsedConstGenericType {
+    fn parse_types(ty: &mut TypePath, parser: &mut TypesParser<'_>) -> Result<Self, ParseFailed> {
+        let TypePath { qself, path } = ty;
+        if let Some(_qself) = qself {
+            parse_failed!(
+                parser,
+                ty,
+                "associated types/consts are not yet implemented"
+            );
+        } else {
+            parser.parse(path)
+        }
+    }
+}
+
+impl ParseTypes<Type> for ParsedConstGenericType {
+    fn parse_types(ty: &mut Type, parser: &mut TypesParser<'_>) -> Result<Self, ParseFailed> {
+        Ok(match ty {
+            Type::Path(ty) => parser.parse(ty)?,
+            _ => parse_failed!(parser, ty, "unsupported const generic type"),
+        })
+    }
+}
+
 pub(crate) struct ParseFailed;
 
 impl fmt::Display for ParseFailed {
@@ -1377,6 +1525,14 @@ pub(crate) enum UnparsedGenericParam {
         colon_token: Token![:],
         bounds: ParsedBounds,
     },
+    Const {
+        attrs: Vec<Attribute>,
+        options: HdlAttr<ConstParamOptions>,
+        const_token: Token![const],
+        ident: Ident,
+        colon_token: Token![:],
+        ty: ParsedConstGenericType,
+    },
 }
 
 pub(crate) mod known_items {
@@ -1390,7 +1546,7 @@ pub(crate) mod known_items {
     macro_rules! impl_known_item_body {
         ($known_item:ident) => {
             #[derive(Clone, Debug)]
-            #[allow(dead_code)]
+            #[allow(dead_code, non_camel_case_types)]
             pub(crate) struct $known_item {
                 pub(crate) path: Path,
                 pub(crate) span: Span,
@@ -1509,6 +1665,7 @@ pub(crate) mod known_items {
     impl_known_item!(::fayalite::ty::StaticType);
     impl_known_item!(::fayalite::ty::Type);
     impl_known_item!(::fayalite::util::ConstUsize);
+    impl_known_item!(::fayalite::__std::primitive::usize);
 }
 
 macro_rules! impl_bounds {
@@ -1865,10 +2022,54 @@ impl ToTokens for ParsedSizeTypeParam {
     }
 }
 
+#[derive(Debug, Clone)]
+pub(crate) struct ParsedConstParamWhereBounds {
+    pub(crate) const_usize: known_items::ConstUsize,
+    pub(crate) lt_token: Token![<],
+    pub(crate) ident: Ident,
+    pub(crate) gt_token: Token![>],
+    pub(crate) colon_token: Token![:],
+    pub(crate) bounds: ParsedSizeTypeBounds,
+}
+
+#[derive(Debug, Clone)]
+pub(crate) struct ParsedConstParam {
+    pub(crate) attrs: Vec<Attribute>,
+    pub(crate) options: HdlAttr<ConstParamOptions>,
+    pub(crate) const_token: Token![const],
+    pub(crate) ident: Ident,
+    pub(crate) colon_token: Token![:],
+    pub(crate) ty: ParsedConstGenericType,
+    pub(crate) bounds: ParsedConstParamWhereBounds,
+}
+
+impl ToTokens for ParsedConstParam {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let Self {
+            attrs,
+            options,
+            const_token,
+            ident,
+            colon_token,
+            ty,
+            bounds: _,
+        } = self;
+        let ConstParamOptions {} = options.body;
+        for attr in attrs {
+            attr.to_tokens(tokens);
+        }
+        const_token.to_tokens(tokens);
+        ident.to_tokens(tokens);
+        colon_token.to_tokens(tokens);
+        ty.to_tokens(tokens);
+    }
+}
+
 #[derive(Debug, Clone)]
 pub(crate) enum ParsedGenericParam {
     Type(ParsedTypeParam),
     SizeType(ParsedSizeTypeParam),
+    Const(ParsedConstParam),
 }
 
 impl ToTokens for ParsedGenericParam {
@@ -1876,6 +2077,7 @@ impl ToTokens for ParsedGenericParam {
         match self {
             ParsedGenericParam::Type(v) => v.to_tokens(tokens),
             ParsedGenericParam::SizeType(v) => v.to_tokens(tokens),
+            ParsedGenericParam::Const(v) => v.to_tokens(tokens),
         }
     }
 }
@@ -1885,6 +2087,7 @@ impl ParsedGenericParam {
         match self {
             ParsedGenericParam::Type(v) => &v.ident,
             ParsedGenericParam::SizeType(v) => &v.ident,
+            ParsedGenericParam::Const(v) => &v.ident,
         }
     }
 }
@@ -1911,6 +2114,11 @@ impl ParsedGenerics {
                         .KnownSize
                         .get_or_insert_with(|| known_items::KnownSize(ident.span()));
                 }
+                ParsedGenericParam::Const(ParsedConstParam { ident, bounds, .. }) => {
+                    bounds
+                        .KnownSize
+                        .get_or_insert_with(|| known_items::KnownSize(ident.span()));
+                }
             }
         }
         self
@@ -1982,6 +2190,13 @@ impl ParsedGenerics {
                                     <#ident as ::fayalite::int::Size>::SizeType
                                 }
                             }
+                            ParsedGenericParam::Const(param) => {
+                                let ident = &param.ident;
+                                parse_quote_spanned! {span=>
+                                    <::fayalite::util::ConstUsize<#ident>
+                                        as ::fayalite::int::Size>::SizeType
+                                }
+                            }
                         })
                         .collect(),
                 }
@@ -2254,98 +2469,117 @@ impl ParsedGenerics {
                         todo!();
                     }
                 }
+                ParsedGenericParam::Const(_) => todo!(),
             };
         }
     }
-    pub(crate) fn parse(generics: &mut Generics) -> syn::Result<Self> {
+    pub(crate) fn parse<'a>(generics: &'a mut Generics) -> syn::Result<Self> {
         let Generics {
             lt_token,
-            params,
+            params: input_params,
             gt_token,
             where_clause,
         } = generics;
         let mut errors = Errors::new();
-        let mut predicates: Vec<WherePredicate> = Vec::with_capacity(params.len());
-        let mut defaults = Vec::with_capacity(params.len());
-        let mut params = Punctuated::<UnparsedGenericParam, _>::from_iter(
-            params.pairs_mut().filter_map_pair_value_mut(|param| {
-                let (param, default) = match param {
-                    GenericParam::Lifetime(param) => {
-                        errors.unwrap_or_default(
-                            HdlAttr::<LifetimeParamOptions>::parse_and_take_attr(&mut param.attrs),
-                        );
-                        errors.error(param, "lifetime generics are not supported by #[hdl]");
-                        return None;
+        let mut predicates: Vec<WherePredicate> = Vec::with_capacity(input_params.len());
+        struct LateParsedParam<'a> {
+            default: Option<(Token![=], &'a mut Type)>,
+            const_param_type: Option<&'a mut Type>,
+        }
+        let mut late_parsed_params: Vec<LateParsedParam<'a>> =
+            Vec::with_capacity(input_params.len());
+        let mut unparsed_params: Punctuated<UnparsedGenericParam, _> = Punctuated::new();
+        for input_param in input_params.pairs_mut() {
+            let (input_param, punct) = input_param.into_tuple();
+            let (unparsed_param, late_parsed_param) = match input_param {
+                GenericParam::Lifetime(param) => {
+                    errors.unwrap_or_default(HdlAttr::<LifetimeParamOptions>::parse_and_take_attr(
+                        &mut param.attrs,
+                    ));
+                    errors.error(param, "lifetime generics are not supported by #[hdl]");
+                    continue;
+                }
+                GenericParam::Type(TypeParam {
+                    attrs,
+                    ident,
+                    colon_token,
+                    bounds,
+                    eq_token,
+                    default,
+                }) => {
+                    let span = ident.span();
+                    let options = errors
+                        .unwrap_or_default(HdlAttr::<TypeParamOptions>::parse_and_take_attr(attrs))
+                        .unwrap_or_default();
+                    let colon_token = colon_token.unwrap_or_else(|| Token![:](span));
+                    if !bounds.is_empty() {
+                        predicates.push(WherePredicate::Type(PredicateType {
+                            lifetimes: None,
+                            bounded_ty: parse_quote! { #ident },
+                            colon_token,
+                            bounds: bounds.clone(),
+                        }));
                     }
-                    GenericParam::Type(TypeParam {
-                        attrs,
-                        ident,
-                        colon_token,
-                        bounds,
-                        eq_token,
-                        default,
-                    }) => {
-                        let span = ident.span();
-                        let options = errors
-                            .unwrap_or_default(HdlAttr::<TypeParamOptions>::parse_and_take_attr(
-                                attrs,
-                            ))
-                            .unwrap_or_default();
-                        let colon_token = colon_token.unwrap_or_else(|| Token![:](span));
-                        if !bounds.is_empty() {
-                            predicates.push(WherePredicate::Type(PredicateType {
-                                lifetimes: None,
-                                bounded_ty: parse_quote! { #ident },
-                                colon_token,
-                                bounds: bounds.clone(),
-                            }));
-                        }
-                        (
-                            UnparsedGenericParam::Type {
-                                attrs: attrs.clone(),
-                                options,
-                                ident: ident.clone(),
-                                colon_token,
-                                bounds: ParsedBounds::default(),
-                            },
-                            default
-                                .clone()
+                    (
+                        UnparsedGenericParam::Type {
+                            attrs: attrs.clone(),
+                            options,
+                            ident: ident.clone(),
+                            colon_token,
+                            bounds: ParsedBounds::default(),
+                        },
+                        LateParsedParam {
+                            default: default
+                                .as_mut()
                                 .map(|v| (eq_token.unwrap_or_else(|| Token![=](span)), v)),
-                        )
-                    }
-                    GenericParam::Const(ConstParam {
-                        attrs,
-                        const_token,
-                        ident,
-                        colon_token,
-                        ty,
-                        eq_token,
-                        default,
-                    }) => {
-                        let options = errors
-                            .unwrap_or_default(HdlAttr::<ConstParamOptions>::parse_and_take_attr(
-                                attrs,
-                            ))
-                            .unwrap_or_default();
-                        let _ = const_token;
-                        let _ = ident;
-                        let _ = colon_token;
-                        let _ = ty;
+                            const_param_type: None,
+                        },
+                    )
+                }
+                GenericParam::Const(ConstParam {
+                    attrs,
+                    const_token,
+                    ident,
+                    colon_token,
+                    ty,
+                    eq_token,
+                    default,
+                }) => {
+                    let options = errors
+                        .unwrap_or_default(HdlAttr::<ConstParamOptions>::parse_and_take_attr(attrs))
+                        .unwrap_or_default();
+                    if let Some(default) = default {
                         let _ = eq_token;
-                        let _ = default;
-                        let _ = options;
-                        todo!()
+                        errors.error(
+                            default,
+                            "const generics' default values are not yet implemented",
+                        );
                     }
-                };
-                defaults.push(default);
-                Some(param)
-            }),
-        );
-        let param_name_to_index_map: HashMap<Ident, usize> = params
+                    (
+                        UnparsedGenericParam::Const {
+                            attrs: attrs.clone(),
+                            options,
+                            const_token: *const_token,
+                            ident: ident.clone(),
+                            colon_token: *colon_token,
+                            ty: ParsedConstGenericType::Usize(known_items::usize(ident.span())),
+                        },
+                        LateParsedParam {
+                            default: None,
+                            const_param_type: Some(ty),
+                        },
+                    )
+                }
+            };
+            late_parsed_params.push(late_parsed_param);
+            unparsed_params.extend([Pair::new(unparsed_param, punct.cloned())]);
+        }
+        let param_name_to_index_map: HashMap<Ident, usize> = unparsed_params
             .iter()
             .enumerate()
             .map(|(index, param)| {
-                let UnparsedGenericParam::Type { ident, .. } = param;
+                let (UnparsedGenericParam::Type { ident, .. }
+                | UnparsedGenericParam::Const { ident, .. }) = param;
                 (ident.clone(), index)
             })
             .collect();
@@ -2367,80 +2601,131 @@ impl ParsedGenerics {
                 errors.error(predicate, "unsupported where predicate kind");
                 continue;
             };
+            ParsedTypeNamed {
+                path: todo!(),
+                args: todo!(),
+            };
             let Some(&index) = bounded_ty
                 .get_ident()
                 .and_then(|bounded_ty| param_name_to_index_map.get(bounded_ty))
             else {
                 errors.error(
                     bounded_ty,
-                    "where predicate bounded type must be one of the generic parameters",
+                    "where predicate bounded type must be one of the generic type \
+                    parameters or `ConstUsize<A_CONST_PARAM>`",
                 );
                 continue;
             };
-            let UnparsedGenericParam::Type {
-                bounds: parsed_bounds,
-                ..
-            } = &mut params[index];
+            let parsed_bounds = match &mut unparsed_params[index] {
+                UnparsedGenericParam::Type { bounds, .. } => bounds,
+                UnparsedGenericParam::Const { ident, .. } => {
+                    errors.error(
+                        bounded_ty,
+                        format_args!("expected type, found const parameter `{ident}`"),
+                    );
+                    continue;
+                }
+            };
             parsed_bounds.extend(errors.ok(syn::parse2::<ParsedBounds>(
                 unparsed_bounds.to_token_stream(),
             )));
         }
         let params =
-            Punctuated::from_iter(params.into_pairs().map_pair_value(|param| match param {
-                UnparsedGenericParam::Type {
-                    attrs,
-                    options,
-                    ident,
-                    colon_token,
-                    mut bounds,
-                } => {
-                    bounds.add_implied_bounds();
-                    match bounds.categorize(&mut errors, ident.span()) {
-                        ParsedBoundsCategory::Type(bounds) => {
-                            ParsedGenericParam::Type(ParsedTypeParam {
-                                attrs,
-                                options,
-                                ident,
-                                colon_token,
-                                bounds,
-                                default: None,
-                            })
-                        }
-                        ParsedBoundsCategory::SizeType(bounds) => {
-                            ParsedGenericParam::SizeType(ParsedSizeTypeParam {
-                                attrs,
-                                options,
-                                ident,
-                                colon_token,
-                                bounds,
-                                default: None,
-                            })
+            Punctuated::from_iter(unparsed_params.into_pairs().map_pair_value(
+                |param| match param {
+                    UnparsedGenericParam::Type {
+                        attrs,
+                        options,
+                        ident,
+                        colon_token,
+                        mut bounds,
+                    } => {
+                        bounds.add_implied_bounds();
+                        match bounds.categorize(&mut errors, ident.span()) {
+                            ParsedBoundsCategory::Type(bounds) => {
+                                ParsedGenericParam::Type(ParsedTypeParam {
+                                    attrs,
+                                    options,
+                                    ident,
+                                    colon_token,
+                                    bounds,
+                                    default: None,
+                                })
+                            }
+                            ParsedBoundsCategory::SizeType(bounds) => {
+                                ParsedGenericParam::SizeType(ParsedSizeTypeParam {
+                                    attrs,
+                                    options,
+                                    ident,
+                                    colon_token,
+                                    bounds,
+                                    default: None,
+                                })
+                            }
                         }
                     }
-                }
-            }));
+                    UnparsedGenericParam::Const {
+                        attrs,
+                        options,
+                        const_token,
+                        ident,
+                        colon_token,
+                        ty,
+                    } => ParsedGenericParam::Const(ParsedConstParam {
+                        bounds: ParsedSizeTypeBounds {
+                            KnownSize: None,
+                            Size: Some(known_items::Size(ident.span())),
+                        },
+                        attrs,
+                        options,
+                        const_token,
+                        ident,
+                        colon_token,
+                        ty,
+                    }),
+                },
+            ));
         let mut retval = Self {
             lt_token: *lt_token,
             params,
             gt_token: *gt_token,
             param_name_to_index_map,
         };
-        for (cur_param_index, default) in defaults.into_iter().enumerate() {
-            let Some((eq, mut ty)) = default else {
-                continue;
-            };
-            let Ok(ty) = TypesParser {
+        for (
+            cur_param_index,
+            LateParsedParam {
+                default,
+                const_param_type,
+            },
+        ) in late_parsed_params.into_iter().enumerate()
+        {
+            let mut parser = TypesParser {
                 generics: &retval,
                 cur_param_index: Some(cur_param_index),
                 errors: &mut errors,
-            }
-            .parse(&mut ty) else {
-                continue;
             };
+            let parsed_default = default.and_then(|(eq, ty)| {
+                let ty = parser.parse(ty).ok()?;
+                Some((eq, ty))
+            });
+            let parsed_const_param_type = const_param_type.and_then(|ty| parser.parse(ty).ok());
             match &mut retval.params[cur_param_index] {
                 ParsedGenericParam::Type(ParsedTypeParam { default, .. })
                 | ParsedGenericParam::SizeType(ParsedSizeTypeParam { default, .. }) => {
-                    *default = Some((eq, ty))
+                    *default = parsed_default;
+                }
+                ParsedGenericParam::Const(ParsedConstParam {
+                    attrs: _,
+                    options: _,
+                    const_token: _,
+                    ident: _,
+                    colon_token: _,
+                    ty,
+                    bounds: _,
+                }) => {
+                    if let Some(parsed_const_param_type) = parsed_const_param_type {
+                        *ty = parsed_const_param_type;
+                    }
                 }
             }
         }
@@ -2517,6 +2802,21 @@ impl ToTokens for ParsedGenericsImplGenerics<'_> {
                     colon_token.to_tokens(tokens);
                     bounds.to_tokens(tokens);
                 }
+                ParsedGenericParam::Const(ParsedConstParam {
+                    attrs: _,
+                    options,
+                    const_token,
+                    ident,
+                    colon_token,
+                    ty,
+                    bounds: _,
+                }) => {
+                    let ConstParamOptions {} = options.body;
+                    const_token.to_tokens(tokens);
+                    ident.to_tokens(tokens);
+                    colon_token.to_tokens(tokens);
+                    ty.to_tokens(tokens);
+                }
             }
             punct.to_tokens(tokens);
         }
@@ -2609,6 +2909,27 @@ impl ToTokens for ParsedGenericsWhereClause<'_> {
                     }
                 }
                 ParsedGenericParam::SizeType(_) => {}
+                ParsedGenericParam::Const(ParsedConstParam {
+                    ident: _,
+                    bounds:
+                        ParsedConstParamWhereBounds {
+                            const_usize,
+                            lt_token,
+                            ident,
+                            gt_token,
+                            colon_token,
+                            bounds,
+                        },
+                    ..
+                }) => {
+                    where_token(ident.span()).to_tokens(tokens);
+                    const_usize.to_tokens(tokens);
+                    lt_token.to_tokens(tokens);
+                    ident.to_tokens(tokens);
+                    gt_token.to_tokens(tokens);
+                    colon_token.to_tokens(tokens);
+                    bounds.to_tokens(tokens);
+                }
             }
         }
     }