diff --git a/src/ctx.rs b/src/ctx.rs
index 019239955f..6d7645fcdf 100644
--- a/src/ctx.rs
+++ b/src/ctx.rs
@@ -1,7 +1,7 @@
 use crate::host;
 
 use crate::sys::dev_null;
-use crate::sys::fdmap::{FdEntry, FdMap};
+use crate::sys::fdentry::FdEntry;
 
 use failure::{bail, format_err, Error};
 use std::collections::HashMap;
@@ -11,7 +11,7 @@ use std::io::{stderr, stdin, stdout};
 use std::path::{Path, PathBuf};
 
 pub struct WasiCtxBuilder {
-    fds: FdMap,
+    fds: HashMap<host::__wasi_fd_t, FdEntry>,
     preopens: HashMap<PathBuf, File>,
     args: Vec<CString>,
     env: HashMap<CString, CString>,
@@ -21,15 +21,15 @@ impl WasiCtxBuilder {
     /// Builder for a new `WasiCtx`.
     pub fn new() -> Self {
         let mut builder = Self {
-            fds: FdMap::new(),
+            fds: HashMap::new(),
             preopens: HashMap::new(),
             args: vec![],
             env: HashMap::new(),
         };
 
-        builder.fds.insert_fd_entry_at(0, FdEntry::from_file(dev_null()));
-        builder.fds.insert_fd_entry_at(1, FdEntry::from_file(dev_null()));
-        builder.fds.insert_fd_entry_at(2, FdEntry::from_file(dev_null()));
+        builder.fds.insert(0, FdEntry::from_file(dev_null()));
+        builder.fds.insert(1, FdEntry::from_file(dev_null()));
+        builder.fds.insert(2, FdEntry::from_file(dev_null()));
 
         builder
     }
@@ -62,9 +62,9 @@ impl WasiCtxBuilder {
     }
 
     pub fn inherit_stdio(mut self) -> Self {
-        self.fds.insert_fd_entry_at(0, FdEntry::duplicate(&stdin()));
-        self.fds.insert_fd_entry_at(1, FdEntry::duplicate(&stdout()));
-        self.fds.insert_fd_entry_at(2, FdEntry::duplicate(&stderr()));
+        self.fds.insert(0, FdEntry::duplicate(&stdin()));
+        self.fds.insert(1, FdEntry::duplicate(&stdout()));
+        self.fds.insert(2, FdEntry::duplicate(&stderr()));
         self
     }
 
@@ -106,15 +106,22 @@ impl WasiCtxBuilder {
     }
 
     pub fn build(mut self) -> Result<WasiCtx, Error> {
+        // startup code starts looking at fd 3 for preopens
+        let mut preopen_fd = 3;
         for (guest_path, dir) in self.preopens {
             if !dir.metadata()?.is_dir() {
                 bail!("preopened file is not a directory");
             }
+
+            while self.fds.contains_key(&preopen_fd) {
+                preopen_fd = preopen_fd
+                    .checked_add(1)
+                    .ok_or(format_err!("not enough file handles"))?;
+            }
             let mut fe = FdEntry::from_file(dir);
             fe.preopen_path = Some(guest_path);
-            self.fds
-                .insert_fd_entry(fe)
-                .map_err(|_| format_err!("not enough file handles"))?;
+            self.fds.insert(preopen_fd, fe);
+            preopen_fd += 1;
         }
 
         let env = self
@@ -139,7 +146,7 @@ impl WasiCtxBuilder {
 
 #[derive(Debug)]
 pub struct WasiCtx {
-    pub fds: FdMap,
+    pub fds: HashMap<host::__wasi_fd_t, FdEntry>,
     pub args: Vec<CString>,
     pub env: Vec<CString>,
 }
@@ -167,13 +174,33 @@ impl WasiCtx {
         rights_base: host::__wasi_rights_t,
         rights_inheriting: host::__wasi_rights_t,
     ) -> Result<&FdEntry, host::__wasi_errno_t> {
-        self.fds.get_fd_entry(fd, rights_base, rights_inheriting)
+        if let Some(fe) = self.fds.get(&fd) {
+            // validate rights
+            if !fe.rights_base & rights_base != 0 || !fe.rights_inheriting & rights_inheriting != 0
+            {
+                Err(host::__WASI_ENOTCAPABLE)
+            } else {
+                Ok(fe)
+            }
+        } else {
+            Err(host::__WASI_EBADF)
+        }
     }
 
     pub fn insert_fd_entry(
         &mut self,
         fe: FdEntry,
     ) -> Result<host::__wasi_fd_t, host::__wasi_errno_t> {
-        self.fds.insert_fd_entry(fe)
+        // never insert where stdio handles usually are
+        let mut fd = 3;
+        while self.fds.contains_key(&fd) {
+            if let Some(next_fd) = fd.checked_add(1) {
+                fd = next_fd;
+            } else {
+                return Err(host::__WASI_EMFILE);
+            }
+        }
+        self.fds.insert(fd, fe);
+        Ok(fd)
     }
 }
diff --git a/src/hostcalls/misc.rs b/src/hostcalls.rs
similarity index 81%
rename from src/hostcalls/misc.rs
rename to src/hostcalls.rs
index e24350253d..deb30fe0f6 100644
--- a/src/hostcalls/misc.rs
+++ b/src/hostcalls.rs
@@ -3,12 +3,13 @@ use crate::ctx::WasiCtx;
 use crate::memory::*;
 use crate::wasm32;
 
-use crate::sys::hostcalls as hostcalls_impl;
-
+// NOTE avoid shadowing `std::convert::From` - cf. rust-lang/rfcs#1311
 use cast::From as _0;
 
 use wasi_common_cbindgen::wasi_common_cbindgen;
 
+pub use crate::sys::hostcalls::*;
+
 #[wasi_common_cbindgen]
 pub fn args_get(
     wasi_ctx: &WasiCtx,
@@ -67,25 +68,6 @@ pub fn args_sizes_get(
     wasm32::__WASI_ESUCCESS
 }
 
-#[wasi_common_cbindgen]
-pub fn clock_res_get(
-    memory: &mut [u8],
-    clock_id: wasm32::__wasi_clockid_t,
-    resolution_ptr: wasm32::uintptr_t,
-) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::clock_res_get(memory, clock_id, resolution_ptr)
-}
-
-#[wasi_common_cbindgen]
-pub fn clock_time_get(
-    memory: &mut [u8],
-    clock_id: wasm32::__wasi_clockid_t,
-    precision: wasm32::__wasi_timestamp_t,
-    time_ptr: wasm32::uintptr_t,
-) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::clock_time_get(memory, clock_id, precision, time_ptr)
-}
-
 #[wasi_common_cbindgen]
 pub fn environ_get(
     wasi_ctx: &WasiCtx,
@@ -144,17 +126,6 @@ pub fn environ_sizes_get(
     }
 }
 
-#[wasi_common_cbindgen]
-pub fn poll_oneoff(
-    memory: &mut [u8],
-    input: wasm32::uintptr_t,
-    output: wasm32::uintptr_t,
-    nsubscriptions: wasm32::size_t,
-    nevents: wasm32::uintptr_t,
-) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::poll_oneoff(memory, input, output, nsubscriptions, nevents)
-}
-
 #[wasi_common_cbindgen]
 pub fn proc_exit(rval: wasm32::__wasi_exitcode_t) -> () {
     // TODO: Rather than call std::process::exit here, we should trigger a
@@ -171,11 +142,6 @@ pub fn proc_raise(
     unimplemented!("proc_raise")
 }
 
-#[wasi_common_cbindgen]
-pub fn sched_yield() -> wasm32::__wasi_errno_t {
-    hostcalls_impl::sched_yield()
-}
-
 #[wasi_common_cbindgen]
 pub fn random_get(
     memory: &mut [u8],
diff --git a/src/hostcalls/mod.rs b/src/hostcalls/mod.rs
deleted file mode 100644
index 5832ce7dba..0000000000
--- a/src/hostcalls/mod.rs
+++ /dev/null
@@ -1,7 +0,0 @@
-mod fs;
-mod misc;
-mod sock;
-
-pub use self::fs::*;
-pub use self::misc::*;
-pub use self::sock::*;
diff --git a/src/lib.rs b/src/lib.rs
index f58e918c24..8543a54ab2 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -23,9 +23,9 @@
 mod ctx;
 mod sys;
 
-pub mod memory;
 pub mod host;
 pub mod hostcalls;
+pub mod memory;
 pub mod wasm32;
 
 pub use ctx::{WasiCtx, WasiCtxBuilder};
diff --git a/src/memory.rs b/src/memory.rs
index 98085b780a..c45d77aa20 100644
--- a/src/memory.rs
+++ b/src/memory.rs
@@ -1,9 +1,11 @@
 //! Functions to go back and forth between WASI types in host and wasm32 representations.
+#![allow(unused)]
 use crate::{host, wasm32};
-pub use crate::sys::memory::*;
+
+// NOTE avoid shadowing `std::convert::From` - cf. rust-lang/rfcs#1311
+use cast::From as _0;
 
 use cast;
-use cast::From as _0;
 use std::mem::{align_of, size_of};
 use std::ptr;
 use std::slice;
diff --git a/src/sys/unix/fdmap.rs b/src/sys/unix/fdentry.rs
similarity index 71%
rename from src/sys/unix/fdmap.rs
rename to src/sys/unix/fdentry.rs
index b1aa321486..a2237161a8 100644
--- a/src/sys/unix/fdmap.rs
+++ b/src/sys/unix/fdentry.rs
@@ -1,9 +1,8 @@
 use crate::host;
 
 use std::fs::File;
-use std::os::unix::prelude::{FileTypeExt, FromRawFd, IntoRawFd, RawFd, AsRawFd};
+use std::os::unix::prelude::{AsRawFd, FileTypeExt, FromRawFd, IntoRawFd, RawFd};
 use std::path::PathBuf;
-use std::collections::HashMap;
 
 #[derive(Clone, Debug)]
 pub struct FdObject {
@@ -21,11 +20,6 @@ pub struct FdEntry {
     pub preopen_path: Option<PathBuf>,
 }
 
-#[derive(Debug)]
-pub struct FdMap {
-    entries: HashMap<host::__wasi_fd_t, FdEntry>,
-}
-
 impl Drop for FdObject {
     fn drop(&mut self) {
         if self.needs_close {
@@ -148,63 +142,3 @@ pub unsafe fn determine_type_rights(
     };
     Ok((ty, rights_base, rights_inheriting))
 }
-
-impl FdMap {
-    pub fn new() -> Self {
-        Self {
-            entries: HashMap::new()
-        }
-    }
-
-    pub(crate) fn insert_fd_entry_at(&mut self, fd: host::__wasi_fd_t, fe: FdEntry) {
-        self.entries.insert(fd, fe);
-    }
-
-    pub(crate) fn get(&self, fd: &host::__wasi_fd_t) -> Option<&FdEntry> {
-        self.entries.get(fd)
-    }
-
-    pub(crate) fn get_mut(&mut self, fd: &host::__wasi_fd_t) -> Option<&mut FdEntry> {
-        self.entries.get_mut(fd)
-    }
-
-    pub(crate) fn remove(&mut self, fd: &host::__wasi_fd_t) -> Option<FdEntry> {
-        self.entries.remove(fd)
-    }
-
-    pub fn get_fd_entry(
-        &self,
-        fd: host::__wasi_fd_t,
-        rights_base: host::__wasi_rights_t,
-        rights_inheriting: host::__wasi_rights_t,
-    ) -> Result<&FdEntry, host::__wasi_errno_t> {
-        if let Some(fe) = self.entries.get(&fd) {
-            // validate rights
-            if !fe.rights_base & rights_base != 0 || !fe.rights_inheriting & rights_inheriting != 0
-            {
-                Err(host::__WASI_ENOTCAPABLE)
-            } else {
-                Ok(fe)
-            }
-        } else {
-            Err(host::__WASI_EBADF)
-        }
-    }
-
-    pub fn insert_fd_entry(
-        &mut self,
-        fe: FdEntry,
-    ) -> Result<host::__wasi_fd_t, host::__wasi_errno_t> {
-        // never insert where stdio handles usually are
-        let mut fd = 3;
-        while self.entries.contains_key(&fd) {
-            if let Some(next_fd) = fd.checked_add(1) {
-                fd = next_fd;
-            } else {
-                return Err(host::__WASI_EMFILE);
-            }
-        }
-        self.entries.insert(fd, fe);
-        Ok(fd)
-    }
-}
\ No newline at end of file
diff --git a/src/sys/unix/host.rs b/src/sys/unix/host_impl.rs
similarity index 62%
rename from src/sys/unix/host.rs
rename to src/sys/unix/host_impl.rs
index 6f6914e90d..7e3a1d2a9a 100644
--- a/src/sys/unix/host.rs
+++ b/src/sys/unix/host_impl.rs
@@ -3,26 +3,28 @@
 #![allow(non_snake_case)]
 #![allow(dead_code)]
 use crate::host;
+use crate::memory;
+use crate::wasm32;
 
 pub fn errno_from_nix(errno: nix::errno::Errno) -> host::__wasi_errno_t {
     match errno {
-        nix::errno::Errno::EPERM  => host::__WASI_EPERM,
+        nix::errno::Errno::EPERM => host::__WASI_EPERM,
         nix::errno::Errno::ENOENT => host::__WASI_ENOENT,
-        nix::errno::Errno::ESRCH  => host::__WASI_ESRCH,
-        nix::errno::Errno::EINTR  => host::__WASI_EINTR,
-        nix::errno::Errno::EIO    => host::__WASI_EIO,
-        nix::errno::Errno::ENXIO  => host::__WASI_ENXIO,
-        nix::errno::Errno::E2BIG  => host::__WASI_E2BIG,
+        nix::errno::Errno::ESRCH => host::__WASI_ESRCH,
+        nix::errno::Errno::EINTR => host::__WASI_EINTR,
+        nix::errno::Errno::EIO => host::__WASI_EIO,
+        nix::errno::Errno::ENXIO => host::__WASI_ENXIO,
+        nix::errno::Errno::E2BIG => host::__WASI_E2BIG,
         nix::errno::Errno::ENOEXEC => host::__WASI_ENOEXEC,
-        nix::errno::Errno::EBADF  => host::__WASI_EBADF,
+        nix::errno::Errno::EBADF => host::__WASI_EBADF,
         nix::errno::Errno::ECHILD => host::__WASI_ECHILD,
         nix::errno::Errno::EAGAIN => host::__WASI_EAGAIN,
         nix::errno::Errno::ENOMEM => host::__WASI_ENOMEM,
         nix::errno::Errno::EACCES => host::__WASI_EACCES,
         nix::errno::Errno::EFAULT => host::__WASI_EFAULT,
-        nix::errno::Errno::EBUSY  => host::__WASI_EBUSY,
+        nix::errno::Errno::EBUSY => host::__WASI_EBUSY,
         nix::errno::Errno::EEXIST => host::__WASI_EEXIST,
-        nix::errno::Errno::EXDEV  => host::__WASI_EXDEV,
+        nix::errno::Errno::EXDEV => host::__WASI_EXDEV,
         nix::errno::Errno::ENODEV => host::__WASI_ENODEV,
         nix::errno::Errno::ENOTDIR => host::__WASI_ENOTDIR,
         nix::errno::Errno::EISDIR => host::__WASI_EISDIR,
@@ -31,60 +33,62 @@ pub fn errno_from_nix(errno: nix::errno::Errno) -> host::__wasi_errno_t {
         nix::errno::Errno::EMFILE => host::__WASI_EMFILE,
         nix::errno::Errno::ENOTTY => host::__WASI_ENOTTY,
         nix::errno::Errno::ETXTBSY => host::__WASI_ETXTBSY,
-        nix::errno::Errno::EFBIG  => host::__WASI_EFBIG,
+        nix::errno::Errno::EFBIG => host::__WASI_EFBIG,
         nix::errno::Errno::ENOSPC => host::__WASI_ENOSPC,
         nix::errno::Errno::ESPIPE => host::__WASI_ESPIPE,
-        nix::errno::Errno::EROFS  => host::__WASI_EROFS,
+        nix::errno::Errno::EROFS => host::__WASI_EROFS,
         nix::errno::Errno::EMLINK => host::__WASI_EMLINK,
-        nix::errno::Errno::EPIPE  => host::__WASI_EPIPE,
-        nix::errno::Errno::EDOM   => host::__WASI_EDOM,
+        nix::errno::Errno::EPIPE => host::__WASI_EPIPE,
+        nix::errno::Errno::EDOM => host::__WASI_EDOM,
         nix::errno::Errno::ERANGE => host::__WASI_ERANGE,
-        nix::errno::Errno::EDEADLK      => host::__WASI_EDEADLK,
+        nix::errno::Errno::EDEADLK => host::__WASI_EDEADLK,
         nix::errno::Errno::ENAMETOOLONG => host::__WASI_ENAMETOOLONG,
-        nix::errno::Errno::ENOLCK       => host::__WASI_ENOLCK,
-        nix::errno::Errno::ENOSYS       => host::__WASI_ENOSYS,
-        nix::errno::Errno::ENOTEMPTY    => host::__WASI_ENOTEMPTY,
-        nix::errno::Errno::ELOOP        => host::__WASI_ELOOP,
-        nix::errno::Errno::ENOMSG       => host::__WASI_ENOMSG,
-        nix::errno::Errno::EIDRM        => host::__WASI_EIDRM,
-        nix::errno::Errno::ENOLINK      => host::__WASI_ENOLINK,
-        nix::errno::Errno::EPROTO       => host::__WASI_EPROTO,
-        nix::errno::Errno::EMULTIHOP    => host::__WASI_EMULTIHOP,
-        nix::errno::Errno::EBADMSG      => host::__WASI_EBADMSG,
-        nix::errno::Errno::EOVERFLOW    => host::__WASI_EOVERFLOW,
-        nix::errno::Errno::EILSEQ       => host::__WASI_EILSEQ,
-        nix::errno::Errno::ENOTSOCK     => host::__WASI_ENOTSOCK,
+        nix::errno::Errno::ENOLCK => host::__WASI_ENOLCK,
+        nix::errno::Errno::ENOSYS => host::__WASI_ENOSYS,
+        nix::errno::Errno::ENOTEMPTY => host::__WASI_ENOTEMPTY,
+        nix::errno::Errno::ELOOP => host::__WASI_ELOOP,
+        nix::errno::Errno::ENOMSG => host::__WASI_ENOMSG,
+        nix::errno::Errno::EIDRM => host::__WASI_EIDRM,
+        nix::errno::Errno::ENOLINK => host::__WASI_ENOLINK,
+        nix::errno::Errno::EPROTO => host::__WASI_EPROTO,
+        nix::errno::Errno::EMULTIHOP => host::__WASI_EMULTIHOP,
+        nix::errno::Errno::EBADMSG => host::__WASI_EBADMSG,
+        nix::errno::Errno::EOVERFLOW => host::__WASI_EOVERFLOW,
+        nix::errno::Errno::EILSEQ => host::__WASI_EILSEQ,
+        nix::errno::Errno::ENOTSOCK => host::__WASI_ENOTSOCK,
         nix::errno::Errno::EDESTADDRREQ => host::__WASI_EDESTADDRREQ,
-        nix::errno::Errno::EMSGSIZE     => host::__WASI_EMSGSIZE,
-        nix::errno::Errno::EPROTOTYPE   => host::__WASI_EPROTOTYPE,
-        nix::errno::Errno::ENOPROTOOPT  => host::__WASI_ENOPROTOOPT,
+        nix::errno::Errno::EMSGSIZE => host::__WASI_EMSGSIZE,
+        nix::errno::Errno::EPROTOTYPE => host::__WASI_EPROTOTYPE,
+        nix::errno::Errno::ENOPROTOOPT => host::__WASI_ENOPROTOOPT,
         nix::errno::Errno::EPROTONOSUPPORT => host::__WASI_EPROTONOSUPPORT,
-        nix::errno::Errno::EAFNOSUPPORT  => host::__WASI_EAFNOSUPPORT,
-        nix::errno::Errno::EADDRINUSE    => host::__WASI_EADDRINUSE,
+        nix::errno::Errno::EAFNOSUPPORT => host::__WASI_EAFNOSUPPORT,
+        nix::errno::Errno::EADDRINUSE => host::__WASI_EADDRINUSE,
         nix::errno::Errno::EADDRNOTAVAIL => host::__WASI_EADDRNOTAVAIL,
-        nix::errno::Errno::ENETDOWN      => host::__WASI_ENETDOWN,
-        nix::errno::Errno::ENETUNREACH   => host::__WASI_ENETUNREACH,
-        nix::errno::Errno::ENETRESET     => host::__WASI_ENETRESET,
-        nix::errno::Errno::ECONNABORTED  => host::__WASI_ECONNABORTED,
-        nix::errno::Errno::ECONNRESET    => host::__WASI_ECONNRESET,
-        nix::errno::Errno::ENOBUFS       => host::__WASI_ENOBUFS,
-        nix::errno::Errno::EISCONN       => host::__WASI_EISCONN,
-        nix::errno::Errno::ENOTCONN      => host::__WASI_ENOTCONN,
-        nix::errno::Errno::ETIMEDOUT     => host::__WASI_ETIMEDOUT,
-        nix::errno::Errno::ECONNREFUSED  => host::__WASI_ECONNREFUSED,
-        nix::errno::Errno::EHOSTUNREACH  => host::__WASI_EHOSTUNREACH,
-        nix::errno::Errno::EALREADY      => host::__WASI_EALREADY,
-        nix::errno::Errno::EINPROGRESS   => host::__WASI_EINPROGRESS,
-        nix::errno::Errno::ESTALE        => host::__WASI_ESTALE,
-        nix::errno::Errno::EDQUOT        => host::__WASI_EDQUOT,
-        nix::errno::Errno::ECANCELED     => host::__WASI_ECANCELED,
-        nix::errno::Errno::EOWNERDEAD    => host::__WASI_EOWNERDEAD,
+        nix::errno::Errno::ENETDOWN => host::__WASI_ENETDOWN,
+        nix::errno::Errno::ENETUNREACH => host::__WASI_ENETUNREACH,
+        nix::errno::Errno::ENETRESET => host::__WASI_ENETRESET,
+        nix::errno::Errno::ECONNABORTED => host::__WASI_ECONNABORTED,
+        nix::errno::Errno::ECONNRESET => host::__WASI_ECONNRESET,
+        nix::errno::Errno::ENOBUFS => host::__WASI_ENOBUFS,
+        nix::errno::Errno::EISCONN => host::__WASI_EISCONN,
+        nix::errno::Errno::ENOTCONN => host::__WASI_ENOTCONN,
+        nix::errno::Errno::ETIMEDOUT => host::__WASI_ETIMEDOUT,
+        nix::errno::Errno::ECONNREFUSED => host::__WASI_ECONNREFUSED,
+        nix::errno::Errno::EHOSTUNREACH => host::__WASI_EHOSTUNREACH,
+        nix::errno::Errno::EALREADY => host::__WASI_EALREADY,
+        nix::errno::Errno::EINPROGRESS => host::__WASI_EINPROGRESS,
+        nix::errno::Errno::ESTALE => host::__WASI_ESTALE,
+        nix::errno::Errno::EDQUOT => host::__WASI_EDQUOT,
+        nix::errno::Errno::ECANCELED => host::__WASI_ECANCELED,
+        nix::errno::Errno::EOWNERDEAD => host::__WASI_EOWNERDEAD,
         nix::errno::Errno::ENOTRECOVERABLE => host::__WASI_ENOTRECOVERABLE,
         _ => host::__WASI_ENOSYS,
     }
 }
 
-pub unsafe fn ciovec_to_nix<'a>(ciovec: &'a host::__wasi_ciovec_t) -> nix::sys::uio::IoVec<&'a [u8]> {
+pub unsafe fn ciovec_to_nix<'a>(
+    ciovec: &'a host::__wasi_ciovec_t,
+) -> nix::sys::uio::IoVec<&'a [u8]> {
     let slice = std::slice::from_raw_parts(ciovec.buf as *const u8, ciovec.buf_len);
     nix::sys::uio::IoVec::from_slice(slice)
 }
@@ -231,10 +235,40 @@ pub fn filestat_from_nix(filestat: nix::sys::stat::FileStat) -> host::__wasi_fil
         st_dev: dev,
         st_ino: ino,
         st_nlink: filestat.st_nlink as host::__wasi_linkcount_t,
-        st_size: filestat.st_size as  host::__wasi_filesize_t,
+        st_size: filestat.st_size as host::__wasi_filesize_t,
         st_atim: filestat.st_atime as host::__wasi_timestamp_t,
         st_ctim: filestat.st_ctime as host::__wasi_timestamp_t,
         st_mtim: filestat.st_mtime as host::__wasi_timestamp_t,
         st_filetype: filetype_from_nix(filetype),
     }
 }
+
+#[cfg(target_os = "linux")]
+pub fn dirent_from_host(
+    host_entry: &nix::libc::dirent,
+) -> Result<wasm32::__wasi_dirent_t, host::__wasi_errno_t> {
+    let mut entry = unsafe { std::mem::zeroed::<wasm32::__wasi_dirent_t>() };
+    let d_namlen = unsafe { std::ffi::CStr::from_ptr(host_entry.d_name.as_ptr()) }
+        .to_bytes()
+        .len();
+    if d_namlen > u32::max_value() as usize {
+        return Err(host::__WASI_EIO);
+    }
+    entry.d_ino = memory::enc_inode(host_entry.d_ino);
+    entry.d_next = memory::enc_dircookie(host_entry.d_off as u64);
+    entry.d_namlen = memory::enc_u32(d_namlen as u32);
+    entry.d_type = memory::enc_filetype(host_entry.d_type);
+    Ok(entry)
+}
+
+#[cfg(not(target_os = "linux"))]
+pub fn dirent_from_host(
+    host_entry: &nix::libc::dirent,
+) -> Result<wasm32::__wasi_dirent_t, host::__wasi_errno_t> {
+    let mut entry = unsafe { std::mem::zeroed::<wasm32::__wasi_dirent_t>() };
+    entry.d_ino = memory::enc_inode(host_entry.d_ino);
+    entry.d_next = memory::enc_dircookie(host_entry.d_seekoff);
+    entry.d_namlen = memory::enc_u32(u32::from(host_entry.d_namlen));
+    entry.d_type = memory::enc_filetype(host_entry.d_type);
+    Ok(entry)
+}
diff --git a/src/sys/unix/hostcalls/fs.rs b/src/sys/unix/hostcalls/fs.rs
index a922061914..af3b3c1ae7 100644
--- a/src/sys/unix/hostcalls/fs.rs
+++ b/src/sys/unix/hostcalls/fs.rs
@@ -1,18 +1,19 @@
 #![allow(non_camel_case_types)]
 #![allow(unused_unsafe)]
-use crate::sys::fdmap::{determine_type_rights, FdEntry};
-use crate::sys::host as host_impl;
+use super::fdentry::{determine_type_rights, FdEntry};
+use super::fs_helpers::*;
+use super::host_impl;
 
 use crate::ctx::WasiCtx;
 use crate::memory::*;
 use crate::{host, wasm32};
 
-use super::fs_helpers::*;
-
 use nix::libc::{self, c_long, c_void, off_t};
 use std::ffi::OsStr;
 use std::os::unix::prelude::{FromRawFd, OsStrExt};
+use wasi_common_cbindgen::wasi_common_cbindgen;
 
+#[wasi_common_cbindgen]
 pub fn fd_close(wasi_ctx: &mut WasiCtx, fd: wasm32::__wasi_fd_t) -> wasm32::__wasi_errno_t {
     let fd = dec_fd(fd);
     if let Some(fdent) = wasi_ctx.fds.get(&fd) {
@@ -32,6 +33,7 @@ pub fn fd_close(wasi_ctx: &mut WasiCtx, fd: wasm32::__wasi_fd_t) -> wasm32::__wa
     }
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_datasync(wasi_ctx: &WasiCtx, fd: wasm32::__wasi_fd_t) -> wasm32::__wasi_errno_t {
     let host_fd = dec_fd(fd);
     let rights = host::__WASI_RIGHT_FD_DATASYNC;
@@ -57,6 +59,7 @@ pub fn fd_datasync(wasi_ctx: &WasiCtx, fd: wasm32::__wasi_fd_t) -> wasm32::__was
     wasm32::__WASI_ESUCCESS
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_pread(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -106,6 +109,7 @@ pub fn fd_pread(
         .unwrap_or_else(|e| e)
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_pwrite(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -147,6 +151,7 @@ pub fn fd_pwrite(
         .unwrap_or_else(|e| e)
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_read(
     wasi_ctx: &mut WasiCtx,
     memory: &mut [u8],
@@ -189,6 +194,7 @@ pub fn fd_read(
         .unwrap_or_else(|e| e)
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_renumber(
     wasi_ctx: &mut WasiCtx,
     from: wasm32::__wasi_fd_t,
@@ -213,6 +219,7 @@ pub fn fd_renumber(
     wasm32::__WASI_ESUCCESS
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_seek(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -253,6 +260,7 @@ pub fn fd_seek(
         .unwrap_or_else(|e| e)
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_tell(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -279,6 +287,7 @@ pub fn fd_tell(
         .unwrap_or_else(|e| e)
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_fdstat_get(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -313,6 +322,7 @@ pub fn fd_fdstat_get(
     errno
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_fdstat_set_flags(
     wasi_ctx: &WasiCtx,
     fd: wasm32::__wasi_fd_t,
@@ -332,6 +342,7 @@ pub fn fd_fdstat_set_flags(
     }
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_fdstat_set_rights(
     wasi_ctx: &mut WasiCtx,
     fd: wasm32::__wasi_fd_t,
@@ -354,6 +365,7 @@ pub fn fd_fdstat_set_rights(
     wasm32::__WASI_ESUCCESS
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_sync(wasi_ctx: &WasiCtx, fd: wasm32::__wasi_fd_t) -> wasm32::__wasi_errno_t {
     let host_fd = dec_fd(fd);
     let rights = host::__WASI_RIGHT_FD_SYNC;
@@ -368,6 +380,7 @@ pub fn fd_sync(wasi_ctx: &WasiCtx, fd: wasm32::__wasi_fd_t) -> wasm32::__wasi_er
     wasm32::__WASI_ESUCCESS
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_write(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -404,6 +417,7 @@ pub fn fd_write(
         .unwrap_or_else(|e| e)
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_advise(
     wasi_ctx: &WasiCtx,
     fd: wasm32::__wasi_fd_t,
@@ -462,6 +476,7 @@ pub fn fd_advise(
     wasm32::__WASI_ESUCCESS
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_allocate(
     wasi_ctx: &WasiCtx,
     fd: wasm32::__wasi_fd_t,
@@ -514,6 +529,7 @@ pub fn fd_allocate(
     wasm32::__WASI_ESUCCESS
 }
 
+#[wasi_common_cbindgen]
 pub fn path_create_directory(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -552,6 +568,7 @@ pub fn path_create_directory(
     }
 }
 
+#[wasi_common_cbindgen]
 pub fn path_link(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -613,6 +630,7 @@ pub fn path_link(
     wasm32::__WASI_ESUCCESS
 }
 
+#[wasi_common_cbindgen]
 pub fn path_open(
     wasi_ctx: &mut WasiCtx,
     memory: &mut [u8],
@@ -770,6 +788,7 @@ pub fn path_open(
         .unwrap_or_else(|e| e)
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_readdir(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -817,10 +836,11 @@ pub fn fd_readdir(
         if host_entry.is_null() {
             break;
         }
-        let entry: wasm32::__wasi_dirent_t = match dirent_from_host(&unsafe { *host_entry }) {
-            Ok(entry) => entry,
-            Err(e) => return enc_errno(e),
-        };
+        let entry: wasm32::__wasi_dirent_t =
+            match host_impl::dirent_from_host(&unsafe { *host_entry }) {
+                Ok(entry) => entry,
+                Err(e) => return enc_errno(e),
+            };
         let name_len = entry.d_namlen as usize;
         let required_space = std::mem::size_of_val(&entry) + name_len;
         if required_space > left {
@@ -849,6 +869,7 @@ pub fn fd_readdir(
         .unwrap_or_else(|e| e)
 }
 
+#[wasi_common_cbindgen]
 pub fn path_readlink(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -892,6 +913,7 @@ pub fn path_readlink(
     wasm32::__WASI_ESUCCESS
 }
 
+#[wasi_common_cbindgen]
 pub fn path_rename(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -948,6 +970,7 @@ pub fn path_rename(
     wasm32::__WASI_ESUCCESS
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_filestat_get(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -974,6 +997,7 @@ pub fn fd_filestat_get(
     errno
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_filestat_set_times(
     wasi_ctx: &WasiCtx,
     fd: wasm32::__wasi_fd_t,
@@ -1030,6 +1054,7 @@ pub fn fd_filestat_set_times(
     wasm32::__WASI_ESUCCESS
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_filestat_set_size(
     wasi_ctx: &WasiCtx,
     fd: wasm32::__wasi_fd_t,
@@ -1053,6 +1078,7 @@ pub fn fd_filestat_set_size(
     wasm32::__WASI_ESUCCESS
 }
 
+#[wasi_common_cbindgen]
 pub fn path_filestat_get(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -1098,6 +1124,7 @@ pub fn path_filestat_get(
     }
 }
 
+#[wasi_common_cbindgen]
 pub fn path_filestat_set_times(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -1171,6 +1198,7 @@ pub fn path_filestat_set_times(
     wasm32::__WASI_ESUCCESS
 }
 
+#[wasi_common_cbindgen]
 pub fn path_symlink(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -1211,6 +1239,7 @@ pub fn path_symlink(
     wasm32::__WASI_ESUCCESS
 }
 
+#[wasi_common_cbindgen]
 pub fn path_unlink_file(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -1249,6 +1278,7 @@ pub fn path_unlink_file(
     }
 }
 
+#[wasi_common_cbindgen]
 pub fn path_remove_directory(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -1280,6 +1310,7 @@ pub fn path_remove_directory(
     }
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_prestat_get(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -1316,6 +1347,7 @@ pub fn fd_prestat_get(
     }
 }
 
+#[wasi_common_cbindgen]
 pub fn fd_prestat_dir_name(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
diff --git a/src/sys/unix/hostcalls/fs_helpers.rs b/src/sys/unix/hostcalls/fs_helpers.rs
index 10f5b43e68..5d1c70bd88 100644
--- a/src/sys/unix/hostcalls/fs_helpers.rs
+++ b/src/sys/unix/hostcalls/fs_helpers.rs
@@ -1,9 +1,9 @@
 #![allow(non_camel_case_types)]
 #![allow(unused_unsafe)]
 
+use super::host_impl;
 use crate::ctx::WasiCtx;
 use crate::host;
-use crate::sys::host as host_impl;
 
 use nix::libc::{self, c_long};
 use std::ffi::{OsStr, OsString};
diff --git a/src/sys/unix/hostcalls/misc.rs b/src/sys/unix/hostcalls/misc.rs
index bad79caca1..3c3982bab3 100644
--- a/src/sys/unix/hostcalls/misc.rs
+++ b/src/sys/unix/hostcalls/misc.rs
@@ -1,15 +1,17 @@
 #![allow(non_camel_case_types)]
 #![allow(unused_unsafe)]
+use super::host_impl;
 
 use crate::memory::*;
 use crate::{host, wasm32};
-use crate::sys::host as host_impl;
 
 use nix::convert_ioctl_res;
 use nix::libc::{self, c_int};
 use std::cmp;
 use std::time::SystemTime;
+use wasi_common_cbindgen::wasi_common_cbindgen;
 
+#[wasi_common_cbindgen]
 pub fn clock_res_get(
     memory: &mut [u8],
     clock_id: wasm32::__wasi_clockid_t,
@@ -49,6 +51,7 @@ pub fn clock_res_get(
         })
 }
 
+#[wasi_common_cbindgen]
 pub fn clock_time_get(
     memory: &mut [u8],
     clock_id: wasm32::__wasi_clockid_t,
@@ -85,6 +88,7 @@ pub fn clock_time_get(
         })
 }
 
+#[wasi_common_cbindgen]
 pub fn poll_oneoff(
     memory: &mut [u8],
     input: wasm32::uintptr_t,
@@ -176,6 +180,7 @@ pub fn poll_oneoff(
     wasm32::__WASI_ESUCCESS
 }
 
+#[wasi_common_cbindgen]
 pub fn sched_yield() -> wasm32::__wasi_errno_t {
     unsafe { libc::sched_yield() };
     wasm32::__WASI_ESUCCESS
diff --git a/src/sys/unix/hostcalls/mod.rs b/src/sys/unix/hostcalls/mod.rs
index a0ce8f8ecc..481e90dee9 100644
--- a/src/sys/unix/hostcalls/mod.rs
+++ b/src/sys/unix/hostcalls/mod.rs
@@ -1,10 +1,13 @@
-//! Hostcalls that implement
+//! Unix-specific hostcalls that implement
 //! [WASI](https://github.com/CraneStation/wasmtime-wasi/blob/wasi/docs/WASI-overview.md).
 mod fs;
 mod fs_helpers;
 mod misc;
 mod sock;
 
+use super::fdentry;
+use super::host_impl;
+
 pub use self::fs::*;
 pub use self::misc::*;
 pub use self::sock::*;
diff --git a/src/sys/unix/hostcalls/sock.rs b/src/sys/unix/hostcalls/sock.rs
index 2dbef5a69f..0d38a3f611 100644
--- a/src/sys/unix/hostcalls/sock.rs
+++ b/src/sys/unix/hostcalls/sock.rs
@@ -6,6 +6,7 @@ use crate::ctx::WasiCtx;
 use crate::wasm32;
 use wasi_common_cbindgen::wasi_common_cbindgen;
 
+#[wasi_common_cbindgen]
 pub fn sock_recv(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -19,6 +20,7 @@ pub fn sock_recv(
     unimplemented!("sock_recv")
 }
 
+#[wasi_common_cbindgen]
 pub fn sock_send(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -31,6 +33,7 @@ pub fn sock_send(
     unimplemented!("sock_send")
 }
 
+#[wasi_common_cbindgen]
 pub fn sock_shutdown(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
@@ -38,4 +41,4 @@ pub fn sock_shutdown(
     how: wasm32::__wasi_sdflags_t,
 ) -> wasm32::__wasi_errno_t {
     unimplemented!("sock_shutdown")
-}
\ No newline at end of file
+}
diff --git a/src/sys/unix/mod.rs b/src/sys/unix/mod.rs
index 5b200f9fc4..8d9f70ad3c 100644
--- a/src/sys/unix/mod.rs
+++ b/src/sys/unix/mod.rs
@@ -1,41 +1,6 @@
-pub mod host;
+pub mod fdentry;
+mod host_impl;
 pub mod hostcalls;
-pub mod fdmap;
-
-pub mod memory {
-    use crate::{host, wasm32};
-    use crate::memory::*;
-
-    #[cfg(target_os = "linux")]
-    pub fn dirent_from_host(
-        host_entry: &nix::libc::dirent,
-    ) -> Result<wasm32::__wasi_dirent_t, host::__wasi_errno_t> {
-        let mut entry = unsafe { std::mem::zeroed::<wasm32::__wasi_dirent_t>() };
-        let d_namlen = unsafe { std::ffi::CStr::from_ptr(host_entry.d_name.as_ptr()) }
-            .to_bytes()
-            .len();
-        if d_namlen > u32::max_value() as usize {
-            return Err(host::__WASI_EIO);
-        }
-        entry.d_ino = enc_inode(host_entry.d_ino);
-        entry.d_next = enc_dircookie(host_entry.d_off as u64);
-        entry.d_namlen = enc_u32(d_namlen as u32);
-        entry.d_type = enc_filetype(host_entry.d_type);
-        Ok(entry)
-    }
-
-    #[cfg(not(target_os = "linux"))]
-    pub fn dirent_from_host(
-        host_entry: &nix::libc::dirent,
-    ) -> Result<wasm32::__wasi_dirent_t, host::__wasi_errno_t> {
-        let mut entry = unsafe { std::mem::zeroed::<wasm32::__wasi_dirent_t>() };
-        entry.d_ino = enc_inode(host_entry.d_ino);
-        entry.d_next = enc_dircookie(host_entry.d_seekoff);
-        entry.d_namlen = enc_u32(u32::from(host_entry.d_namlen));
-        entry.d_type = enc_filetype(host_entry.d_type);
-        Ok(entry)
-    }
-}
 
 pub fn dev_null() -> std::fs::File {
     std::fs::File::open("/dev/null").expect("failed to open /dev/null")
diff --git a/src/sys/windows/fdentry.rs b/src/sys/windows/fdentry.rs
new file mode 100644
index 0000000000..c662cf2898
--- /dev/null
+++ b/src/sys/windows/fdentry.rs
@@ -0,0 +1,89 @@
+use crate::host;
+
+use std::fs::File;
+use std::os::windows::prelude::{AsRawHandle, FromRawHandle, IntoRawHandle, RawHandle};
+use std::path::PathBuf;
+use winapi::shared::minwindef::FALSE;
+use winapi::um::handleapi::DuplicateHandle;
+use winapi::um::processthreadsapi::GetCurrentProcess;
+use winapi::um::winnt::DUPLICATE_SAME_ACCESS;
+
+#[derive(Clone, Debug)]
+pub struct FdObject {
+    pub ty: host::__wasi_filetype_t,
+    pub raw_handle: RawHandle,
+    pub needs_close: bool,
+    // TODO: directories
+}
+
+#[derive(Clone, Debug)]
+pub struct FdEntry {
+    pub fd_object: FdObject,
+    pub rights_base: host::__wasi_rights_t,
+    pub rights_inheriting: host::__wasi_rights_t,
+    pub preopen_path: Option<PathBuf>,
+}
+
+impl Drop for FdObject {
+    fn drop(&mut self) {
+        if self.needs_close {
+            unsafe {
+                if winapi::um::handleapi::CloseHandle(self.raw_handle) == 0 {
+                    // TODO: use DWORD WINAPI GetLastError(void) to get error
+                    eprintln!("FdObject::drop(): couldn't close raw Handle");
+                }
+            }
+        }
+    }
+}
+
+impl FdEntry {
+    pub fn from_file(file: File) -> Self {
+        unsafe { Self::from_raw_handle(file.into_raw_handle()) }
+    }
+
+    pub fn duplicate<F: AsRawHandle>(fd: &F) -> Self {
+        unsafe {
+            let source = fd.as_raw_handle();
+            let mut dest = 0 as RawHandle;
+
+            let cur_proc = GetCurrentProcess();
+            if DuplicateHandle(
+                cur_proc,
+                source,
+                cur_proc,
+                &mut dest,
+                0, // dwDesiredAccess; this flag is ignored if DUPLICATE_SAME_ACCESS is specified
+                FALSE,
+                DUPLICATE_SAME_ACCESS,
+            ) == FALSE
+            {
+                panic!("Couldn't duplicate handle");
+            }
+
+            Self::from_raw_handle(dest)
+        }
+    }
+}
+
+impl FromRawHandle for FdEntry {
+    // TODO: implement
+    unsafe fn from_raw_handle(raw_handle: RawHandle) -> Self {
+        let (ty, rights_base, rights_inheriting) = (
+            host::__WASI_FILETYPE_REGULAR_FILE,
+            host::RIGHTS_REGULAR_FILE_BASE,
+            host::RIGHTS_REGULAR_FILE_INHERITING,
+        );
+
+        Self {
+            fd_object: FdObject {
+                ty,
+                raw_handle,
+                needs_close: true,
+            },
+            rights_base,
+            rights_inheriting,
+            preopen_path: None,
+        }
+    }
+}
diff --git a/src/sys/windows/host_impl.rs b/src/sys/windows/host_impl.rs
new file mode 100644
index 0000000000..63f34166ab
--- /dev/null
+++ b/src/sys/windows/host_impl.rs
@@ -0,0 +1,84 @@
+//! WASI host types specific to Windows host.
+#![allow(non_camel_case_types)]
+#![allow(non_snake_case)]
+#![allow(unused)]
+use crate::host;
+
+use std::marker::PhantomData;
+use std::slice;
+use winapi::shared::{ntdef, ws2def};
+
+// these will be obsolete once https://github.com/rust-lang/rust/pull/60334
+// lands in stable
+pub struct IoVec<'a> {
+    vec: ws2def::WSABUF,
+    _p: PhantomData<&'a [u8]>,
+}
+
+pub struct IoVecMut<'a> {
+    vec: ws2def::WSABUF,
+    _p: PhantomData<&'a mut [u8]>,
+}
+
+impl<'a> IoVec<'a> {
+    #[inline]
+    pub fn new(buf: &'a [u8]) -> Self {
+        assert!(buf.len() <= ntdef::ULONG::max_value() as usize);
+        Self {
+            vec: ws2def::WSABUF {
+                len: buf.len() as ntdef::ULONG,
+                buf: buf.as_ptr() as *mut u8 as *mut ntdef::CHAR,
+            },
+            _p: PhantomData,
+        }
+    }
+
+    #[inline]
+    pub fn as_slice(&self) -> &[u8] {
+        unsafe { slice::from_raw_parts(self.vec.buf as *mut u8, self.vec.len as usize) }
+    }
+}
+
+impl<'a> IoVecMut<'a> {
+    #[inline]
+    pub fn new(buf: &'a mut [u8]) -> Self {
+        assert!(buf.len() <= ntdef::ULONG::max_value() as usize);
+        Self {
+            vec: ws2def::WSABUF {
+                len: buf.len() as ntdef::ULONG,
+                buf: buf.as_mut_ptr() as *mut u8 as *mut ntdef::CHAR,
+            },
+            _p: PhantomData,
+        }
+    }
+
+    #[inline]
+    pub fn as_slice(&self) -> &[u8] {
+        unsafe { slice::from_raw_parts(self.vec.buf as *mut u8, self.vec.len as usize) }
+    }
+
+    #[inline]
+    pub fn as_mut_slice(&mut self) -> &mut [u8] {
+        unsafe { slice::from_raw_parts_mut(self.vec.buf as *mut u8, self.vec.len as usize) }
+    }
+}
+
+pub unsafe fn ciovec_to_win<'a>(ciovec: &'a host::__wasi_ciovec_t) -> IoVec<'a> {
+    let slice = slice::from_raw_parts(ciovec.buf as *const u8, ciovec.buf_len);
+    IoVec::new(slice)
+}
+
+pub unsafe fn ciovec_to_win_mut<'a>(ciovec: &'a mut host::__wasi_ciovec_t) -> IoVecMut<'a> {
+    let slice = slice::from_raw_parts_mut(ciovec.buf as *mut u8, ciovec.buf_len);
+    IoVecMut::new(slice)
+}
+
+pub unsafe fn iovec_to_win<'a>(iovec: &'a host::__wasi_iovec_t) -> IoVec<'a> {
+    let slice = slice::from_raw_parts(iovec.buf as *const u8, iovec.buf_len);
+    IoVec::new(slice)
+}
+
+pub unsafe fn iovec_to_win_mut<'a>(iovec: &'a mut host::__wasi_iovec_t) -> IoVecMut<'a> {
+    let slice = slice::from_raw_parts_mut(iovec.buf as *mut u8, iovec.buf_len);
+    IoVecMut::new(slice)
+}
diff --git a/src/hostcalls/fs.rs b/src/sys/windows/hostcalls/fs.rs
similarity index 61%
rename from src/hostcalls/fs.rs
rename to src/sys/windows/hostcalls/fs.rs
index 9aa3c55660..75437fa82d 100644
--- a/src/hostcalls/fs.rs
+++ b/src/sys/windows/hostcalls/fs.rs
@@ -1,19 +1,25 @@
 #![allow(non_camel_case_types)]
+#![allow(unused_unsafe)]
+#![allow(unused)]
+use super::host_impl;
+use super::host_impl::IoVec;
+
 use crate::ctx::WasiCtx;
-use crate::wasm32;
-
-use crate::sys::hostcalls as hostcalls_impl;
+use crate::memory::*;
+use crate::{host, wasm32};
 
+use std::cmp;
+use std::os::windows::prelude::OsStrExt;
 use wasi_common_cbindgen::wasi_common_cbindgen;
 
 #[wasi_common_cbindgen]
 pub fn fd_close(wasi_ctx: &mut WasiCtx, fd: wasm32::__wasi_fd_t) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_close(wasi_ctx, fd)
+    unimplemented!("fd_close")
 }
 
 #[wasi_common_cbindgen]
 pub fn fd_datasync(wasi_ctx: &WasiCtx, fd: wasm32::__wasi_fd_t) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_datasync(wasi_ctx, fd)
+    unimplemented!("fd_datasync")
 }
 
 #[wasi_common_cbindgen]
@@ -26,7 +32,7 @@ pub fn fd_pread(
     offset: wasm32::__wasi_filesize_t,
     nread: wasm32::uintptr_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_pread(wasi_ctx, memory, fd, iovs_ptr, iovs_len, offset, nread)
+    unimplemented!("fd_pread")
 }
 
 #[wasi_common_cbindgen]
@@ -39,7 +45,7 @@ pub fn fd_pwrite(
     offset: wasm32::__wasi_filesize_t,
     nwritten: wasm32::uintptr_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_pwrite(wasi_ctx, memory, fd, iovs_ptr, iovs_len, offset, nwritten)
+    unimplemented!("fd_pwrite")
 }
 
 #[wasi_common_cbindgen]
@@ -51,7 +57,7 @@ pub fn fd_read(
     iovs_len: wasm32::size_t,
     nread: wasm32::uintptr_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_read(wasi_ctx, memory, fd, iovs_ptr, iovs_len, nread)
+    unimplemented!("fd_read")
 }
 
 #[wasi_common_cbindgen]
@@ -60,7 +66,7 @@ pub fn fd_renumber(
     from: wasm32::__wasi_fd_t,
     to: wasm32::__wasi_fd_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_renumber(wasi_ctx, from, to)
+    unimplemented!("fd_renumber")
 }
 
 #[wasi_common_cbindgen]
@@ -72,7 +78,7 @@ pub fn fd_seek(
     whence: wasm32::__wasi_whence_t,
     newoffset: wasm32::uintptr_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_seek(wasi_ctx, memory, fd, offset, whence, newoffset)
+    unimplemented!("fd_seek")
 }
 
 #[wasi_common_cbindgen]
@@ -82,7 +88,7 @@ pub fn fd_tell(
     fd: wasm32::__wasi_fd_t,
     newoffset: wasm32::uintptr_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_tell(wasi_ctx, memory, fd, newoffset)
+    unimplemented!("fd_tell")
 }
 
 #[wasi_common_cbindgen]
@@ -90,9 +96,9 @@ pub fn fd_fdstat_get(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
     fd: wasm32::__wasi_fd_t,
-    fdstat_ptr: wasm32::uintptr_t,
+    fdstat_ptr: wasm32::uintptr_t, // *mut wasm32::__wasi_fdstat_t
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_fdstat_get(wasi_ctx, memory, fd, fdstat_ptr)
+    unimplemented!("fd_fdstat_get")
 }
 
 #[wasi_common_cbindgen]
@@ -101,7 +107,7 @@ pub fn fd_fdstat_set_flags(
     fd: wasm32::__wasi_fd_t,
     fdflags: wasm32::__wasi_fdflags_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_fdstat_set_flags(wasi_ctx, fd, fdflags)
+    unimplemented!("fd_fdstat_set_flags")
 }
 
 #[wasi_common_cbindgen]
@@ -111,12 +117,12 @@ pub fn fd_fdstat_set_rights(
     fs_rights_base: wasm32::__wasi_rights_t,
     fs_rights_inheriting: wasm32::__wasi_rights_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_fdstat_set_rights(wasi_ctx, fd, fs_rights_base, fs_rights_inheriting)
+    unimplemented!("fd_fdstat_set_rights")
 }
 
 #[wasi_common_cbindgen]
 pub fn fd_sync(wasi_ctx: &WasiCtx, fd: wasm32::__wasi_fd_t) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_sync(wasi_ctx, fd)
+    unimplemented!("fd_sync")
 }
 
 #[wasi_common_cbindgen]
@@ -128,7 +134,46 @@ pub fn fd_write(
     iovs_len: wasm32::size_t,
     nwritten: wasm32::uintptr_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_write(wasi_ctx, memory, fd, iovs_ptr, iovs_len, nwritten)
+    use winapi::shared::minwindef::{DWORD, LPVOID};
+    use winapi::shared::ws2def::WSABUF;
+    use winapi::um::fileapi::WriteFile;
+
+    let fd = dec_fd(fd);
+    let mut iovs = match dec_iovec_slice(memory, iovs_ptr, iovs_len) {
+        Ok(iovs) => iovs,
+        Err(e) => return enc_errno(e),
+    };
+
+    let fe = match wasi_ctx.get_fd_entry(fd, host::__WASI_RIGHT_FD_WRITE.into(), 0) {
+        Ok(fe) => fe,
+        Err(e) => return enc_errno(e),
+    };
+
+    let iovs: Vec<IoVec> = iovs
+        .iter()
+        .map(|iov| unsafe { host_impl::iovec_to_win(iov) })
+        .collect();
+
+    let buf = iovs
+        .iter()
+        .find(|b| !b.as_slice().is_empty())
+        .map_or(&[][..], |b| b.as_slice());
+
+    let mut host_nwritten = 0;
+    let len = cmp::min(buf.len(), <DWORD>::max_value() as usize) as DWORD;
+    unsafe {
+        WriteFile(
+            fe.fd_object.raw_handle,
+            buf.as_ptr() as LPVOID,
+            len,
+            &mut host_nwritten,
+            std::ptr::null_mut(),
+        )
+    };
+
+    enc_usize_byref(memory, nwritten, host_nwritten as usize)
+        .map(|_| wasm32::__WASI_ESUCCESS)
+        .unwrap_or_else(|e| e)
 }
 
 #[wasi_common_cbindgen]
@@ -139,7 +184,7 @@ pub fn fd_advise(
     len: wasm32::__wasi_filesize_t,
     advice: wasm32::__wasi_advice_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_advise(wasi_ctx, fd, offset, len, advice)
+    unimplemented!("fd_advise")
 }
 
 #[wasi_common_cbindgen]
@@ -149,7 +194,7 @@ pub fn fd_allocate(
     offset: wasm32::__wasi_filesize_t,
     len: wasm32::__wasi_filesize_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_allocate(wasi_ctx, fd, offset, len)
+    unimplemented!("fd_allocate")
 }
 
 #[wasi_common_cbindgen]
@@ -160,7 +205,7 @@ pub fn path_create_directory(
     path_ptr: wasm32::uintptr_t,
     path_len: wasm32::size_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::path_create_directory(wasi_ctx, memory, dirfd, path_ptr, path_len)
+    unimplemented!("path_create_directory")
 }
 
 #[wasi_common_cbindgen]
@@ -168,24 +213,14 @@ pub fn path_link(
     wasi_ctx: &WasiCtx,
     memory: &mut [u8],
     old_dirfd: wasm32::__wasi_fd_t,
-    old_flags: wasm32::__wasi_lookupflags_t,
+    _old_flags: wasm32::__wasi_lookupflags_t,
     old_path_ptr: wasm32::uintptr_t,
     old_path_len: wasm32::size_t,
     new_dirfd: wasm32::__wasi_fd_t,
     new_path_ptr: wasm32::uintptr_t,
     new_path_len: wasm32::size_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::path_link(
-        wasi_ctx,
-        memory,
-        old_dirfd,
-        old_flags,
-        old_path_ptr,
-        old_path_len,
-        new_dirfd,
-        new_path_ptr,
-        new_path_len,
-    )
+    unimplemented!("path_link")
 }
 
 #[wasi_common_cbindgen]
@@ -202,19 +237,7 @@ pub fn path_open(
     fs_flags: wasm32::__wasi_fdflags_t,
     fd_out_ptr: wasm32::uintptr_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::path_open(
-        wasi_ctx,
-        memory,
-        dirfd,
-        dirflags,
-        path_ptr,
-        path_len,
-        oflags,
-        fs_rights_base,
-        fs_rights_inheriting,
-        fs_flags,
-        fd_out_ptr,
-    )
+    unimplemented!("path_open")
 }
 
 #[wasi_common_cbindgen]
@@ -227,7 +250,7 @@ pub fn fd_readdir(
     cookie: wasm32::__wasi_dircookie_t,
     buf_used: wasm32::uintptr_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_readdir(wasi_ctx, memory, fd, buf, buf_len, cookie, buf_used)
+    unimplemented!("fd_readdir")
 }
 
 #[wasi_common_cbindgen]
@@ -241,9 +264,7 @@ pub fn path_readlink(
     buf_len: wasm32::size_t,
     buf_used: wasm32::uintptr_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::path_readlink(
-        wasi_ctx, memory, dirfd, path_ptr, path_len, buf_ptr, buf_len, buf_used,
-    )
+    unimplemented!("path_readlink")
 }
 
 #[wasi_common_cbindgen]
@@ -257,16 +278,7 @@ pub fn path_rename(
     new_path_ptr: wasm32::uintptr_t,
     new_path_len: wasm32::size_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::path_rename(
-        wasi_ctx,
-        memory,
-        old_dirfd,
-        old_path_ptr,
-        old_path_len,
-        new_dirfd,
-        new_path_ptr,
-        new_path_len,
-    )
+    unimplemented!("path_rename")
 }
 
 #[wasi_common_cbindgen]
@@ -276,7 +288,7 @@ pub fn fd_filestat_get(
     fd: wasm32::__wasi_fd_t,
     filestat_ptr: wasm32::uintptr_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_filestat_get(wasi_ctx, memory, fd, filestat_ptr)
+    unimplemented!("fd_filestat_get")
 }
 
 #[wasi_common_cbindgen]
@@ -287,7 +299,7 @@ pub fn fd_filestat_set_times(
     st_mtim: wasm32::__wasi_timestamp_t,
     fst_flags: wasm32::__wasi_fstflags_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_filestat_set_times(wasi_ctx, fd, st_atim, st_mtim, fst_flags)
+    unimplemented!("fd_filestat_set_times")
 }
 
 #[wasi_common_cbindgen]
@@ -296,7 +308,7 @@ pub fn fd_filestat_set_size(
     fd: wasm32::__wasi_fd_t,
     st_size: wasm32::__wasi_filesize_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_filestat_set_size(wasi_ctx, fd, st_size)
+    unimplemented!("fd_filestat_set_size")
 }
 
 #[wasi_common_cbindgen]
@@ -309,15 +321,7 @@ pub fn path_filestat_get(
     path_len: wasm32::size_t,
     filestat_ptr: wasm32::uintptr_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::path_filestat_get(
-        wasi_ctx,
-        memory,
-        dirfd,
-        dirflags,
-        path_ptr,
-        path_len,
-        filestat_ptr,
-    )
+    unimplemented!("path_filestat_get")
 }
 
 #[wasi_common_cbindgen]
@@ -332,9 +336,7 @@ pub fn path_filestat_set_times(
     st_mtim: wasm32::__wasi_timestamp_t,
     fst_flags: wasm32::__wasi_fstflags_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::path_filestat_set_times(
-        wasi_ctx, memory, dirfd, dirflags, path_ptr, path_len, st_atim, st_mtim, fst_flags,
-    )
+    unimplemented!("path_filestat_set_times")
 }
 
 #[wasi_common_cbindgen]
@@ -347,15 +349,7 @@ pub fn path_symlink(
     new_path_ptr: wasm32::uintptr_t,
     new_path_len: wasm32::size_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::path_symlink(
-        wasi_ctx,
-        memory,
-        old_path_ptr,
-        old_path_len,
-        dirfd,
-        new_path_ptr,
-        new_path_len,
-    )
+    unimplemented!("path_symlink")
 }
 
 #[wasi_common_cbindgen]
@@ -366,7 +360,7 @@ pub fn path_unlink_file(
     path_ptr: wasm32::uintptr_t,
     path_len: wasm32::size_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::path_unlink_file(wasi_ctx, memory, dirfd, path_ptr, path_len)
+    unimplemented!("path_unlink_file")
 }
 
 #[wasi_common_cbindgen]
@@ -377,7 +371,7 @@ pub fn path_remove_directory(
     path_ptr: wasm32::uintptr_t,
     path_len: wasm32::size_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::path_remove_directory(wasi_ctx, memory, dirfd, path_ptr, path_len)
+    unimplemented!("path_remove_directory")
 }
 
 #[wasi_common_cbindgen]
@@ -387,7 +381,35 @@ pub fn fd_prestat_get(
     fd: wasm32::__wasi_fd_t,
     prestat_ptr: wasm32::uintptr_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_prestat_get(wasi_ctx, memory, fd, prestat_ptr)
+    let fd = dec_fd(fd);
+    // TODO: is this the correct right for this?
+    match wasi_ctx.get_fd_entry(fd, host::__WASI_RIGHT_PATH_OPEN.into(), 0) {
+        Ok(fe) => {
+            if let Some(po_path) = &fe.preopen_path {
+                if fe.fd_object.ty != host::__WASI_FILETYPE_DIRECTORY {
+                    return wasm32::__WASI_ENOTDIR;
+                }
+                enc_prestat_byref(
+                    memory,
+                    prestat_ptr,
+                    host::__wasi_prestat_t {
+                        pr_type: host::__WASI_PREOPENTYPE_DIR,
+                        u: host::__wasi_prestat_t___wasi_prestat_u {
+                            dir: host::__wasi_prestat_t___wasi_prestat_u___wasi_prestat_u_dir_t {
+                                // TODO: clean up
+                                pr_name_len: po_path.as_os_str().encode_wide().count() * 2,
+                            },
+                        },
+                    },
+                )
+                .map(|_| wasm32::__WASI_ESUCCESS)
+                .unwrap_or_else(|e| e)
+            } else {
+                wasm32::__WASI_ENOTSUP
+            }
+        }
+        Err(e) => enc_errno(e),
+    }
 }
 
 #[wasi_common_cbindgen]
@@ -398,5 +420,33 @@ pub fn fd_prestat_dir_name(
     path_ptr: wasm32::uintptr_t,
     path_len: wasm32::size_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::fd_prestat_dir_name(wasi_ctx, memory, fd, path_ptr, path_len)
+    let fd = dec_fd(fd);
+
+    match wasi_ctx.get_fd_entry(fd, host::__WASI_RIGHT_PATH_OPEN.into(), 0) {
+        Ok(fe) => {
+            if let Some(po_path) = &fe.preopen_path {
+                if fe.fd_object.ty != host::__WASI_FILETYPE_DIRECTORY {
+                    return wasm32::__WASI_ENOTDIR;
+                }
+                // TODO: clean up
+                let path_bytes = &po_path
+                    .as_os_str()
+                    .encode_wide()
+                    .map(u16::to_le_bytes)
+                    .fold(Vec::new(), |mut acc, bytes| {
+                        acc.extend_from_slice(&bytes);
+                        acc
+                    });
+                if path_bytes.len() > dec_usize(path_len) {
+                    return wasm32::__WASI_ENAMETOOLONG;
+                }
+                enc_slice_of(memory, path_bytes, path_ptr)
+                    .map(|_| wasm32::__WASI_ESUCCESS)
+                    .unwrap_or_else(|e| e)
+            } else {
+                wasm32::__WASI_ENOTSUP
+            }
+        }
+        Err(e) => enc_errno(e),
+    }
 }
diff --git a/src/sys/windows/hostcalls/misc.rs b/src/sys/windows/hostcalls/misc.rs
new file mode 100644
index 0000000000..8bdc8969ef
--- /dev/null
+++ b/src/sys/windows/hostcalls/misc.rs
@@ -0,0 +1,46 @@
+#![allow(non_camel_case_types)]
+#![allow(unused_unsafe)]
+#![allow(unused)]
+use super::host_impl;
+
+use crate::memory::*;
+use crate::{host, wasm32};
+
+use std::cmp;
+use std::time::SystemTime;
+use wasi_common_cbindgen::wasi_common_cbindgen;
+
+#[wasi_common_cbindgen]
+pub fn clock_res_get(
+    memory: &mut [u8],
+    clock_id: wasm32::__wasi_clockid_t,
+    resolution_ptr: wasm32::uintptr_t,
+) -> wasm32::__wasi_errno_t {
+    unimplemented!("clock_res_get")
+}
+
+#[wasi_common_cbindgen]
+pub fn clock_time_get(
+    memory: &mut [u8],
+    clock_id: wasm32::__wasi_clockid_t,
+    precision: wasm32::__wasi_timestamp_t,
+    time_ptr: wasm32::uintptr_t,
+) -> wasm32::__wasi_errno_t {
+    unimplemented!("clock_time_get")
+}
+
+#[wasi_common_cbindgen]
+pub fn poll_oneoff(
+    memory: &mut [u8],
+    input: wasm32::uintptr_t,
+    output: wasm32::uintptr_t,
+    nsubscriptions: wasm32::size_t,
+    nevents: wasm32::uintptr_t,
+) -> wasm32::__wasi_errno_t {
+    unimplemented!("poll_oneoff")
+}
+
+#[wasi_common_cbindgen]
+pub fn sched_yield() -> wasm32::__wasi_errno_t {
+    unimplemented!("sched_yield")
+}
diff --git a/src/sys/windows/hostcalls/mod.rs b/src/sys/windows/hostcalls/mod.rs
new file mode 100644
index 0000000000..58c6f13021
--- /dev/null
+++ b/src/sys/windows/hostcalls/mod.rs
@@ -0,0 +1,11 @@
+//! Windows-specific hostcalls that implement
+//! [WASI](https://github.com/CraneStation/wasmtime-wasi/blob/wasi/docs/WASI-overview.md).
+mod fs;
+mod misc;
+mod sock;
+
+use super::host_impl;
+
+pub use self::fs::*;
+pub use self::misc::*;
+pub use self::sock::*;
diff --git a/src/hostcalls/sock.rs b/src/sys/windows/hostcalls/sock.rs
similarity index 67%
rename from src/hostcalls/sock.rs
rename to src/sys/windows/hostcalls/sock.rs
index e3bd80a9c3..0d38a3f611 100644
--- a/src/hostcalls/sock.rs
+++ b/src/sys/windows/hostcalls/sock.rs
@@ -1,9 +1,9 @@
 #![allow(non_camel_case_types)]
+#![allow(unused_unsafe)]
+#![allow(unused)]
+
 use crate::ctx::WasiCtx;
 use crate::wasm32;
-
-use crate::sys::hostcalls as hostcalls_impl;
-
 use wasi_common_cbindgen::wasi_common_cbindgen;
 
 #[wasi_common_cbindgen]
@@ -17,16 +17,7 @@ pub fn sock_recv(
     ro_datalen: wasm32::uintptr_t,
     ro_flags: wasm32::uintptr_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::sock_recv(
-        wasi_ctx,
-        memory,
-        sock,
-        ri_data,
-        ri_data_len,
-        ri_flags,
-        ro_datalen,
-        ro_flags,
-    )
+    unimplemented!("sock_recv")
 }
 
 #[wasi_common_cbindgen]
@@ -39,15 +30,7 @@ pub fn sock_send(
     si_flags: wasm32::__wasi_siflags_t,
     so_datalen: wasm32::uintptr_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::sock_send(
-        wasi_ctx,
-        memory,
-        sock,
-        si_data,
-        si_data_len,
-        si_flags,
-        so_datalen,
-    )
+    unimplemented!("sock_send")
 }
 
 #[wasi_common_cbindgen]
@@ -57,5 +40,5 @@ pub fn sock_shutdown(
     sock: wasm32::__wasi_fd_t,
     how: wasm32::__wasi_sdflags_t,
 ) -> wasm32::__wasi_errno_t {
-    hostcalls_impl::sock_shutdown(wasi_ctx, memory, sock, how)
+    unimplemented!("sock_shutdown")
 }
diff --git a/src/sys/windows/mod.rs b/src/sys/windows/mod.rs
index e69de29bb2..fe2c757617 100644
--- a/src/sys/windows/mod.rs
+++ b/src/sys/windows/mod.rs
@@ -0,0 +1,9 @@
+pub mod fdentry;
+mod host_impl;
+pub mod hostcalls;
+
+use std::fs::File;
+
+pub fn dev_null() -> File {
+    File::open("NUL").expect("failed to open NUL")
+}