diff --git a/core/alien/compiler/compiler.factor b/core/alien/compiler/compiler.factor
index 7e0165cd64..992c7763f2 100755
--- a/core/alien/compiler/compiler.factor
+++ b/core/alien/compiler/compiler.factor
@@ -202,23 +202,6 @@ M: alien-invoke-error summary
 
 : pop-parameters pop-literal nip [ expand-constants ] map ;
 
-\ alien-invoke [
-    ! Four literals
-    4 ensure-values
-    \ alien-invoke empty-node
-    ! Compile-time parameters
-    pop-parameters over set-alien-invoke-parameters
-    pop-literal nip over set-alien-invoke-function
-    pop-literal nip over set-alien-invoke-library
-    pop-literal nip over set-alien-invoke-return
-    ! Quotation which coerces parameters to required types
-    dup make-prep-quot recursive-state get infer-quot
-    ! Add node to IR
-    dup node,
-    ! Magic #: consume exactly the number of inputs
-    0 alien-invoke-stack
-] "infer" set-word-prop
-
 : stdcall-mangle ( symbol node -- symbol )
     "@"
     swap alien-node-parameters parameter-sizes drop
@@ -228,13 +211,38 @@ M: alien-invoke-error summary
     dup alien-invoke-function
     swap alien-invoke-library load-library ;
 
+TUPLE: no-such-symbol ;
+
+M: no-such-symbol summary
+    drop "Symbol not found" ;
+
+: no-such-symbol ( -- )
+    \ no-such-symbol inference-error ;
+
 : alien-invoke-dlsym ( node -- symbol dll )
-    dup (alien-invoke-dlsym)
-    >r over stdcall-mangle r> 2dup dlsym [
-        rot drop
-    ] [
-        2drop (alien-invoke-dlsym)
-    ] if ;
+    dup (alien-invoke-dlsym) 2dup dlsym [
+        >r over stdcall-mangle r> 2dup dlsym
+        [ no-such-symbol ] unless
+    ] unless rot drop ;
+
+\ alien-invoke [
+    ! Four literals
+    4 ensure-values
+    \ alien-invoke empty-node
+    ! Compile-time parameters
+    pop-parameters over set-alien-invoke-parameters
+    pop-literal nip over set-alien-invoke-function
+    pop-literal nip over set-alien-invoke-library
+    pop-literal nip over set-alien-invoke-return
+    ! If symbol doesn't resolve, no stack effect, no compile
+    dup alien-invoke-dlsym 2drop
+    ! Quotation which coerces parameters to required types
+    dup make-prep-quot recursive-state get infer-quot
+    ! Add node to IR
+    dup node,
+    ! Magic #: consume exactly the number of inputs
+    0 alien-invoke-stack
+] "infer" set-word-prop
 
 M: alien-invoke generate-node
     dup alien-invoke-frame [
diff --git a/core/system/system-docs.factor b/core/system/system-docs.factor
index 4e31fd4722..d91a84ec99 100644
--- a/core/system/system-docs.factor
+++ b/core/system/system-docs.factor
@@ -29,7 +29,7 @@ ARTICLE: "os" "System interface"
 { $subsection millis }
 "Exiting the Factor VM:"
 { $subsection exit }
-{ $see-also "file-streams" "network-streams" "io.launcher" } ;
+{ $see-also "file-streams" "network-streams" "io.launcher" "io.mmap" } ;
 
 ABOUT: "os"
 
diff --git a/extra/help/handbook/handbook.factor b/extra/help/handbook/handbook.factor
index c38023e544..749a5ed0ec 100644
--- a/extra/help/handbook/handbook.factor
+++ b/extra/help/handbook/handbook.factor
@@ -131,7 +131,7 @@ ARTICLE: "collections" "Collections"
 { $subsection "graphs" }
 { $subsection "buffers" } ;
 
-USING: io.sockets io.launcher ;
+USING: io.sockets io.launcher io.mmap ;
 
 ARTICLE: "io" "Input and output" 
 { $subsection "streams" }
@@ -145,7 +145,8 @@ ARTICLE: "io" "Input and output"
 { $subsection "stream-binary" }
 { $subsection "styles" }
 { $subsection "network-streams" }
-{ $subsection "io.launcher" } ;
+{ $subsection "io.launcher" }
+{ $subsection "io.mmap" } ;
 
 ARTICLE: "tools" "Developer tools"
 { $subsection "tools.annotations" }
diff --git a/extra/io/mmap/mmap-docs.factor b/extra/io/mmap/mmap-docs.factor
new file mode 100644
index 0000000000..22e403ed31
--- /dev/null
+++ b/extra/io/mmap/mmap-docs.factor
@@ -0,0 +1,38 @@
+USING: help.markup help.syntax alien math ;
+IN: io.mmap
+
+HELP: mapped-file
+{ $class-description "The class of memory-mapped files, opened by " { $link <mapped-file> } " and closed by " { $link close-mapped-file } ". The following two slots are of interest to users:"
+    { $list
+        { { $link mapped-file-length } " - the length of the mapped file area, in bytes" }
+        { { $link mapped-file-address } " - an " { $link alien } " pointing at the file's memory area" }
+    }
+} ;
+
+HELP: <mapped-file>
+{ $values { "path" "a pathname string" } { "length" integer } { "mmap" mapped-file } }
+{ $contract "Opens a file and maps the first " { $snippet "length" } " bytes into memory. The length is permitted to exceed the length of the file on disk, in which case the remaining space is padded with zero bytes." }
+{ $notes "You must call " { $link close-mapped-file } " when you are finished working with the returned object, to reclaim resources. The " { $link with-mapped-file } " provides an abstraction which can close the mapped file for you." }
+{ $errors "Throws an error if a memory mapping could not be established." } ;
+
+HELP: (close-mapped-file)
+{ $values { "mmap" mapped-file } }
+{ $contract "Releases system resources associated with the mapped file. This word should not be called by user code; use " { $link close-mapped-file } " instead." }
+{ $errors "Throws an error if a memory mapping could not be established." } ;
+
+HELP: close-mapped-file
+{ $values { "mmap" mapped-file } }
+{ $description "Releases system resources associated with the mapped file." }
+{ $errors "Throws an error if a memory mapping could not be established." } ;
+
+ARTICLE: "io.mmap" "Memory-mapped files"
+"The " { $vocab-link "io.mmap" } " vocabulary implements support for memory-mapped files."
+{ $subsection <mapped-file> }
+{ $subsection close-mapped-file }
+"A combinator which wraps the above two words:"
+{ $subsection with-mapped-file }
+"Memory mapped files implement the " { $link "sequence-protocol" } " and present themselves as a sequence of bytes. The underlying memory area can also be accessed directly:"
+{ $subsection mapped-file-address }
+"Data can be read and written from the memory area using alien words. See " { $link "reading-writing-memory" } "." ;
+
+ABOUT: "io.mmap"
diff --git a/extra/io/mmap/mmap.factor b/extra/io/mmap/mmap.factor
index f9ccd61423..aaa786f6a4 100755
--- a/extra/io/mmap/mmap.factor
+++ b/extra/io/mmap/mmap.factor
@@ -4,20 +4,31 @@ USING: continuations io.backend kernel quotations sequences
 system alien sequences.private ;
 IN: io.mmap
 
-TUPLE: mapped-file length address handle ;
+TUPLE: mapped-file length address handle closed? ;
 
-M: mapped-file length mapped-file-length ;
+: check-closed ( mapped-file -- mapped-file )
+    dup mapped-file-closed? [
+        "Mapped file is closed" throw
+    ] when ; inline
+
+M: mapped-file length check-closed mapped-file-length ;
 
 M: mapped-file nth-unsafe
-    mapped-file-address swap alien-unsigned-1 ;
+    check-closed mapped-file-address swap alien-unsigned-1 ;
 
 M: mapped-file set-nth-unsafe
-    mapped-file-address swap set-alien-unsigned-1 ;
+    check-closed mapped-file-address swap set-alien-unsigned-1 ;
 
 INSTANCE: mapped-file sequence
 
 HOOK: <mapped-file> io-backend ( path length -- mmap )
-HOOK: close-mapped-file io-backend ( mmap -- )
+
+HOOK: (close-mapped-file) io-backend ( mmap -- )
+
+: close-mapped-file ( mmap -- )
+    check-closed
+    t over set-mapped-file-closed?
+    (close-mapped-file) ;
 
 : with-mapped-file ( path length quot -- )
     >r <mapped-file> r>
diff --git a/extra/io/unix/mmap/mmap.factor b/extra/io/unix/mmap/mmap.factor
index 50e928f16d..d7dcad67d9 100644
--- a/extra/io/unix/mmap/mmap.factor
+++ b/extra/io/unix/mmap/mmap.factor
@@ -15,7 +15,7 @@ M: unix-io <mapped-file> ( path length -- obj )
     dup PROT_READ PROT_WRITE bitor MAP_FILE MAP_SHARED bitor
     r> mmap-open \ mapped-file construct-boa ;
 
-M: unix-io close-mapped-file ( mmap -- )
+M: unix-io (close-mapped-file) ( mmap -- )
     [ mapped-file-address ] keep
     [ mapped-file-length munmap ] keep
     mapped-file-handle close
diff --git a/extra/io/windows/mmap/mmap.factor b/extra/io/windows/mmap/mmap.factor
index b291bb6bcd..20c6a6fc22 100755
--- a/extra/io/windows/mmap/mmap.factor
+++ b/extra/io/windows/mmap/mmap.factor
@@ -81,7 +81,7 @@ M: windows-io <mapped-file> ( path length -- mmap )
         \ mapped-file construct-boa
     ] with-destructors ;
 
-M: windows-io close-mapped-file ( mapped-file -- )
+M: windows-io (close-mapped-file) ( mapped-file -- )
     [
         dup mapped-file-handle [ close-always ] each
         mapped-file-address UnmapViewOfFile win32-error=0/f
diff --git a/extra/io/windows/windows-tests.factor b/extra/io/windows/windows-tests.factor
new file mode 100755
index 0000000000..4c090590df
--- /dev/null
+++ b/extra/io/windows/windows-tests.factor
@@ -0,0 +1,16 @@
+USING: io.files kernel tools.test ;
+IN: temporary
+
+[ "c:\\foo\\" ] [ "c:\\foo\\bar" parent-directory ] unit-test
+[ "c:\\" ] [ "c:\\foo\\" parent-directory ] unit-test
+[ "c:\\" ] [ "c:\\foo" parent-directory ] unit-test
+! { "c:" "c:\\" "c:/" } [ directory ] each -- all do the same thing
+[ "c:\\" ] [ "c:\\" parent-directory ] unit-test
+[ "Z:\\" ] [ "Z:\\" parent-directory ] unit-test
+[ "c:" ] [ "c:" parent-directory ] unit-test
+[ "Z:" ] [ "Z:" parent-directory ] unit-test
+[ t ] [ "c:\\" root-directory? ] unit-test
+[ t ] [ "Z:\\" root-directory? ] unit-test
+[ f ] [ "c:\\foo" root-directory? ] unit-test
+[ f ] [ "." root-directory? ] unit-test
+[ f ] [ ".." root-directory? ] unit-test