diff --git a/crates/generate/src/funcs.rs b/crates/generate/src/funcs.rs
index ccf7365eab..de2c7413da 100644
--- a/crates/generate/src/funcs.rs
+++ b/crates/generate/src/funcs.rs
@@ -111,21 +111,25 @@ fn marshal_arg(
     let interface_typename = names.type_ref(&tref);
     let name = names.func_param(&param.name);
 
-    let value_error_handling = if let Some(tref) = error_type {
-        let abi_ret = match tref.type_().passed_by() {
-            witx::TypePassedBy::Value(atom) => names.atom_type(atom),
-            _ => unreachable!("err should always be passed by value"),
-        };
-        let err_typename = names.type_ref(&tref);
-        quote! {
-            let err: #err_typename = ::memory::GuestError::from_value_error(e, ctx);
-            return #abi_ret::from(err);
-        }
-    } else {
-        quote! {
-            panic!("memory error: {:?}", e)
+    let error_handling = |method| -> TokenStream {
+        if let Some(tref) = error_type {
+            let abi_ret = match tref.type_().passed_by() {
+                witx::TypePassedBy::Value(atom) => names.atom_type(atom),
+                _ => unreachable!("err should always be passed by value"),
+            };
+            let err_typename = names.type_ref(&tref);
+            quote! {
+                let err: #err_typename = ::memory::GuestError::#method(e, ctx);
+                return #abi_ret::from(err);
+            }
+        } else {
+            quote! {
+                panic!("error: {:?}", e)
+            }
         }
     };
+    let value_error_handling = error_handling(quote!(from_value_error));
+    let memory_error_handling = error_handling(quote!(from_memory_error));
 
     let try_into_conversion = quote! {
         use ::std::convert::TryInto;
@@ -162,6 +166,28 @@ fn marshal_arg(
             },
             witx::BuiltinType::String => unimplemented!("string types unimplemented"),
         },
-        _ => unimplemented!("only enums and builtins so far"),
+        witx::Type::Pointer(pointee) => {
+            let pointee_type = names.type_ref(pointee);
+            quote! {
+                let #name = match memory.ptr_mut::<#pointee_type>(#name as u32) {
+                    Ok(p) => p,
+                    Err(e) => {
+                        #memory_error_handling
+                    }
+                };
+            }
+        }
+        witx::Type::ConstPointer(pointee) => {
+            let pointee_type = names.type_ref(pointee);
+            quote! {
+                let #name = match memory.ptr::<#pointee_type>(#name as u32) {
+                    Ok(p) => p,
+                    Err(e) => {
+                        #memory_error_handling
+                    }
+                };
+            }
+        }
+        _ => unimplemented!("argument type marshalling"),
     }
 }
diff --git a/crates/generate/src/names.rs b/crates/generate/src/names.rs
index 78cec1e0b8..6465e7f832 100644
--- a/crates/generate/src/names.rs
+++ b/crates/generate/src/names.rs
@@ -50,6 +50,14 @@ impl Names {
             }
             TypeRef::Value(ty) => match &**ty {
                 witx::Type::Builtin(builtin) => self.builtin_type(*builtin),
+                witx::Type::Pointer(pointee) => {
+                    let pointee_type = self.type_ref(&pointee);
+                    quote!(::memory::GuestPtrMut<#pointee_type>)
+                }
+                witx::Type::ConstPointer(pointee) => {
+                    let pointee_type = self.type_ref(&pointee);
+                    quote!(::memory::GuestPtr<#pointee_type>)
+                }
                 _ => unimplemented!("anonymous type ref"),
             },
         }
diff --git a/crates/generate/src/types.rs b/crates/generate/src/types.rs
index 59b99507a5..a03b4003a2 100644
--- a/crates/generate/src/types.rs
+++ b/crates/generate/src/types.rs
@@ -95,12 +95,12 @@ fn define_enum(names: &Names, name: &witx::Id, e: &witx::EnumDatatype) -> TokenS
         }
 
         impl ::memory::GuestTypeCopy for #ident {
-            fn read_val(src: ::memory::GuestPtr<#ident>) -> Result<#ident, ::memory::GuestValueError> {
+            fn read_val<P: ::memory::GuestPtrRead<#ident>>(src: &P) -> Result<#ident, ::memory::GuestValueError> {
                 use ::std::convert::TryInto;
                 let val = unsafe { ::std::ptr::read_unaligned(src.ptr() as *const #repr) };
                 val.try_into()
             }
-            fn write_val(val: #ident, dest: ::memory::GuestPtrMut<#ident>) {
+            fn write_val(val: #ident, dest: &::memory::GuestPtrMut<#ident>) {
                 let val: #repr = val.into();
                 unsafe {
                     ::std::ptr::write_unaligned(dest.ptr_mut() as *mut #repr, val)
diff --git a/crates/memory/src/borrow.rs b/crates/memory/src/borrow.rs
index 50d5656f53..c691888db9 100644
--- a/crates/memory/src/borrow.rs
+++ b/crates/memory/src/borrow.rs
@@ -1,59 +1,66 @@
+use std::collections::HashMap;
+
 use crate::region::Region;
 
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+pub struct BorrowHandle(usize);
+
 pub struct GuestBorrows {
-    immutable: Vec<Region>,
-    mutable: Vec<Region>,
+    immutable: HashMap<BorrowHandle, Region>,
+    mutable: HashMap<BorrowHandle, Region>,
+    next_handle: BorrowHandle,
 }
 
 impl GuestBorrows {
     pub fn new() -> Self {
         GuestBorrows {
-            immutable: Vec::new(),
-            mutable: Vec::new(),
+            immutable: HashMap::new(),
+            mutable: HashMap::new(),
+            next_handle: BorrowHandle(0),
         }
     }
 
     fn is_borrowed_immut(&self, r: Region) -> bool {
-        !self.immutable.iter().all(|b| !b.overlaps(r))
+        !self.immutable.values().all(|b| !b.overlaps(r))
     }
 
     fn is_borrowed_mut(&self, r: Region) -> bool {
-        !self.mutable.iter().all(|b| !b.overlaps(r))
+        !self.mutable.values().all(|b| !b.overlaps(r))
     }
 
-    pub fn borrow_immut(&mut self, r: Region) -> bool {
+    fn new_handle(&mut self) -> BorrowHandle {
+        let h = self.next_handle;
+        self.next_handle = BorrowHandle(h.0 + 1);
+        h
+    }
+
+    pub fn borrow_immut(&mut self, r: Region) -> Option<BorrowHandle> {
         if self.is_borrowed_mut(r) {
-            return false;
+            return None;
         }
-        self.immutable.push(r);
-        true
+        let h = self.new_handle();
+        self.immutable.insert(h, r);
+        Some(h)
     }
 
-    pub fn unborrow_immut(&mut self, r: Region) {
-        let (ix, _) = self
-            .immutable
-            .iter()
-            .enumerate()
-            .find(|(_, reg)| r == **reg)
-            .expect("region exists in borrows");
-        self.immutable.remove(ix);
+    pub fn unborrow_immut(&mut self, h: BorrowHandle) {
+        self.immutable
+            .remove(&h)
+            .expect("handle exists in immutable borrows");
     }
 
-    pub fn borrow_mut(&mut self, r: Region) -> bool {
+    pub fn borrow_mut(&mut self, r: Region) -> Option<BorrowHandle> {
         if self.is_borrowed_immut(r) || self.is_borrowed_mut(r) {
-            return false;
+            return None;
         }
-        self.mutable.push(r);
-        true
+        let h = self.new_handle();
+        self.mutable.insert(h, r);
+        Some(h)
     }
 
-    pub fn unborrow_mut(&mut self, r: Region) {
-        let (ix, _) = self
-            .mutable
-            .iter()
-            .enumerate()
-            .find(|(_, reg)| r == **reg)
-            .expect("region exists in borrows");
-        self.mutable.remove(ix);
+    pub fn unborrow_mut(&mut self, h: BorrowHandle) {
+        self.mutable
+            .remove(&h)
+            .expect("handle exists in mutable borrows");
     }
 }
diff --git a/crates/memory/src/guest_type.rs b/crates/memory/src/guest_type.rs
index 9410e6054d..324ca5bf09 100644
--- a/crates/memory/src/guest_type.rs
+++ b/crates/memory/src/guest_type.rs
@@ -1,4 +1,4 @@
-use crate::{GuestPtr, GuestPtrMut, MemoryError};
+use crate::{GuestPtrMut, GuestPtrRead, MemoryError};
 use thiserror::Error;
 
 pub trait GuestType: Sized {
@@ -13,25 +13,25 @@ pub enum GuestValueError {
 }
 
 pub trait GuestTypeCopy: GuestType + Copy {
-    fn read_val(src: GuestPtr<Self>) -> Result<Self, GuestValueError>;
-    fn write_val(val: Self, dest: GuestPtrMut<Self>);
+    fn read_val<P: GuestPtrRead<Self>>(src: &P) -> Result<Self, GuestValueError>;
+    fn write_val(val: Self, dest: &GuestPtrMut<Self>);
 }
 
 pub trait GuestTypeClone: GuestType + Clone {
-    fn read_ref(src: GuestPtr<Self>, dest: &mut Self) -> Result<(), GuestValueError>;
-    fn write_ref(val: &Self, dest: GuestPtrMut<Self>);
+    fn read_ref<P: GuestPtrRead<Self>>(src: &P, dest: &mut Self) -> Result<(), GuestValueError>;
+    fn write_ref(val: &Self, dest: &GuestPtrMut<Self>);
 }
 
 impl<T> GuestTypeClone for T
 where
     T: GuestTypeCopy,
 {
-    fn read_ref(src: GuestPtr<T>, dest: &mut T) -> Result<(), GuestValueError> {
+    fn read_ref<P: GuestPtrRead<Self>>(src: &P, dest: &mut T) -> Result<(), GuestValueError> {
         let val = GuestTypeCopy::read_val(src)?;
         *dest = val;
         Ok(())
     }
-    fn write_ref(val: &T, dest: GuestPtrMut<T>) {
+    fn write_ref(val: &T, dest: &GuestPtrMut<T>) {
         GuestTypeCopy::write_val(*val, dest)
     }
 }
@@ -49,12 +49,12 @@ macro_rules! builtin_copy {
         }
 
         impl GuestTypeCopy for $t {
-            fn read_val(src: GuestPtr<$t>) -> Result<$t, GuestValueError> {
+            fn read_val<P: GuestPtrRead<$t>>(src: &P) -> Result<$t, GuestValueError> {
                 Ok(unsafe {
                     ::std::ptr::read_unaligned(src.ptr() as *const $t)
                 })
             }
-            fn write_val(val: $t, dest: GuestPtrMut<$t>) {
+            fn write_val(val: $t, dest: &GuestPtrMut<$t>) {
                 unsafe {
                     ::std::ptr::write_unaligned(dest.ptr_mut() as *mut $t, val)
                 }
diff --git a/crates/memory/src/lib.rs b/crates/memory/src/lib.rs
index 872c556b15..1099776f95 100644
--- a/crates/memory/src/lib.rs
+++ b/crates/memory/src/lib.rs
@@ -4,5 +4,5 @@ mod memory;
 mod region;
 
 pub use guest_type::{GuestError, GuestType, GuestTypeClone, GuestTypeCopy, GuestValueError};
-pub use memory::{GuestMemory, GuestPtr, GuestPtrMut, MemoryError};
+pub use memory::{GuestMemory, GuestPtr, GuestPtrMut, GuestPtrRead, MemoryError};
 pub use region::Region;
diff --git a/crates/memory/src/memory.rs b/crates/memory/src/memory.rs
index efb027550e..64fa192953 100644
--- a/crates/memory/src/memory.rs
+++ b/crates/memory/src/memory.rs
@@ -3,7 +3,7 @@ use std::marker::PhantomData;
 use std::rc::Rc;
 use thiserror::Error;
 
-use crate::borrow::GuestBorrows;
+use crate::borrow::{BorrowHandle, GuestBorrows};
 use crate::guest_type::GuestType;
 use crate::region::Region;
 
@@ -31,80 +31,85 @@ impl<'a> GuestMemory<'a> {
     }
 
     pub fn ptr<T: GuestType>(&'a self, at: u32) -> Result<GuestPtr<'a, T>, MemoryError> {
-        let r = Region {
+        let region = Region {
             start: at,
             len: T::size(),
         };
-        let mut borrows = self.borrows.borrow_mut();
-        if !self.contains(r) {
-            Err(MemoryError::OutOfBounds(r))?;
+        if !self.contains(region) {
+            Err(MemoryError::OutOfBounds(region))?;
         }
-        if borrows.borrow_immut(r) {
+        let mut borrows = self.borrows.borrow_mut();
+        if let Some(handle) = borrows.borrow_immut(region) {
             Ok(GuestPtr {
                 mem: &self,
-                region: r,
+                region,
+                handle,
                 type_: PhantomData,
             })
         } else {
-            Err(MemoryError::Borrowed(r))
+            Err(MemoryError::Borrowed(region))
         }
     }
 
     pub fn ptr_mut<T: GuestType>(&'a self, at: u32) -> Result<GuestPtrMut<'a, T>, MemoryError> {
-        let r = Region {
+        let region = Region {
             start: at,
             len: T::size(),
         };
-        let mut borrows = self.borrows.borrow_mut();
-        if !self.contains(r) {
-            Err(MemoryError::OutOfBounds(r))?;
+        if !self.contains(region) {
+            Err(MemoryError::OutOfBounds(region))?;
         }
-        if borrows.borrow_mut(r) {
+        let mut borrows = self.borrows.borrow_mut();
+        if let Some(handle) = borrows.borrow_mut(region) {
             Ok(GuestPtrMut {
                 mem: &self,
-                region: r,
+                region,
+                handle,
                 type_: PhantomData,
             })
         } else {
-            Err(MemoryError::Borrowed(r))
+            Err(MemoryError::Borrowed(region))
         }
     }
 }
 
+pub trait GuestPtrRead<T> {
+    fn ptr(&self) -> *const u8;
+}
+
 pub struct GuestPtr<'a, T> {
     mem: &'a GuestMemory<'a>,
     region: Region,
+    handle: BorrowHandle,
     type_: PhantomData<T>,
 }
 
-impl<'a, T: GuestType> GuestPtr<'a, T> {
-    pub fn ptr(&self) -> *const u8 {
+impl<'a, T: GuestType> GuestPtrRead<T> for GuestPtr<'a, T> {
+    fn ptr(&self) -> *const u8 {
         (self.mem.ptr as usize + self.region.start as usize) as *const u8
     }
-
-    pub unsafe fn downcast<Q: GuestType>(self) -> GuestPtr<'a, Q> {
-        debug_assert!(T::size() == Q::size(), "downcast to type of same size");
-        GuestPtr {
-            mem: self.mem,
-            region: self.region,
-            type_: PhantomData,
-        }
-    }
 }
 
 impl<'a, T> Drop for GuestPtr<'a, T> {
     fn drop(&mut self) {
         let mut borrows = self.mem.borrows.borrow_mut();
-        borrows.unborrow_immut(self.region);
+        borrows.unborrow_immut(self.handle);
     }
 }
 
 pub struct GuestPtrMut<'a, T> {
     mem: &'a GuestMemory<'a>,
     region: Region,
+    handle: BorrowHandle,
     type_: PhantomData<T>,
 }
 
+impl<'a, T: GuestType> GuestPtrRead<T> for GuestPtrMut<'a, T> {
+    fn ptr(&self) -> *const u8 {
+        (self.mem.ptr as usize + self.region.start as usize) as *const u8
+    }
+}
+
 impl<'a, T> GuestPtrMut<'a, T> {
     pub fn ptr_mut(&self) -> *mut u8 {
         (self.mem.ptr as usize + self.region.start as usize) as *mut u8
@@ -113,7 +118,7 @@ impl<'a, T> GuestPtrMut<'a, T> {
 impl<'a, T> Drop for GuestPtrMut<'a, T> {
     fn drop(&mut self) {
         let mut borrows = self.mem.borrows.borrow_mut();
-        borrows.unborrow_mut(self.region);
+        borrows.unborrow_mut(self.handle);
     }
 }
 
diff --git a/src/lib.rs b/src/lib.rs
index f6500df8c0..2f8f782965 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -12,8 +12,29 @@ pub mod test {
             println!("BAR: {} {}", an_int, an_float);
             Ok(())
         }
-        fn baz(&mut self, excuse: types::Excuse) -> Result<(), types::Errno> {
-            println!("BAZ: {:?}", excuse);
+        fn baz(
+            &mut self,
+            excuse: types::Excuse,
+            a_better_excuse_by_reference: ::memory::GuestPtrMut<types::Excuse>,
+            a_lamer_excuse_by_reference: ::memory::GuestPtr<types::Excuse>,
+        ) -> Result<(), types::Errno> {
+            use memory::GuestTypeCopy;
+            let a_better_excuse =
+                types::Excuse::read_val(&a_better_excuse_by_reference).map_err(|val_err| {
+                    eprintln!("a_better_excuse_by_reference value error: {:?}", val_err);
+                    types::Errno::InvalidArg
+                })?;
+            let a_lamer_excuse =
+                types::Excuse::read_val(&a_lamer_excuse_by_reference).map_err(|val_err| {
+                    eprintln!("a_lamer_excuse_by_reference value error: {:?}", val_err);
+                    types::Errno::InvalidArg
+                })?;
+            types::Excuse::write_val(a_lamer_excuse, &a_better_excuse_by_reference);
+
+            println!(
+                "BAZ: {:?} {:?} {:?}",
+                excuse, a_better_excuse, a_lamer_excuse
+            );
             Ok(())
         }
     }
diff --git a/test.witx b/test.witx
index 9611b9203b..d8e8848cc5 100644
--- a/test.witx
+++ b/test.witx
@@ -21,5 +21,7 @@
     (result $error $errno))
   (@interface func (export "baz")
     (param $an_excuse $excuse)
+    (param $an_excuse_by_reference (@witx pointer $excuse))
+    (param $a_lamer_excuse (@witx const_pointer $excuse))
     (result $error $errno))
 )