diff --git a/src/platform-freebsd.cc b/src/platform-freebsd.cc
index ad1e499ac5..7174c7f9cb 100644
--- a/src/platform-freebsd.cc
+++ b/src/platform-freebsd.cc
@@ -215,6 +215,7 @@ class PosixMemoryMappedFile : public OS::MemoryMappedFile {
     : file_(file), memory_(memory), size_(size) { }
   virtual ~PosixMemoryMappedFile();
   virtual void* memory() { return memory_; }
+  virtual int size() { return size_; }
  private:
   FILE* file_;
   void* memory_;
@@ -222,6 +223,19 @@ class PosixMemoryMappedFile : public OS::MemoryMappedFile {
 };
 
 
+OS::MemoryMappedFile* OS::MemoryMappedFile::open(const char* name) {
+  FILE* file = fopen(name, "w+");
+  if (file == NULL) return NULL;
+
+  fseek(file, 0, SEEK_END);
+  int size = ftell(file);
+
+  void* memory =
+      mmap(0, size, PROT_READ | PROT_WRITE, MAP_SHARED, fileno(file), 0);
+  return new PosixMemoryMappedFile(file, memory, size);
+}
+
+
 OS::MemoryMappedFile* OS::MemoryMappedFile::create(const char* name, int size,
     void* initial) {
   FILE* file = fopen(name, "w+");
diff --git a/src/platform-linux.cc b/src/platform-linux.cc
index 755e8cdaf6..761ff7e207 100644
--- a/src/platform-linux.cc
+++ b/src/platform-linux.cc
@@ -318,6 +318,7 @@ class PosixMemoryMappedFile : public OS::MemoryMappedFile {
     : file_(file), memory_(memory), size_(size) { }
   virtual ~PosixMemoryMappedFile();
   virtual void* memory() { return memory_; }
+  virtual int size() { return size_; }
  private:
   FILE* file_;
   void* memory_;
@@ -325,6 +326,19 @@ class PosixMemoryMappedFile : public OS::MemoryMappedFile {
 };
 
 
+OS::MemoryMappedFile* OS::MemoryMappedFile::open(const char* name) {
+  FILE* file = fopen(name, "w+");
+  if (file == NULL) return NULL;
+
+  fseek(file, 0, SEEK_END);
+  int size = ftell(file);
+
+  void* memory =
+      mmap(0, size, PROT_READ | PROT_WRITE, MAP_SHARED, fileno(file), 0);
+  return new PosixMemoryMappedFile(file, memory, size);
+}
+
+
 OS::MemoryMappedFile* OS::MemoryMappedFile::create(const char* name, int size,
     void* initial) {
   FILE* file = fopen(name, "w+");
diff --git a/src/platform-macos.cc b/src/platform-macos.cc
index ce53305173..ea35c1b130 100644
--- a/src/platform-macos.cc
+++ b/src/platform-macos.cc
@@ -196,6 +196,7 @@ class PosixMemoryMappedFile : public OS::MemoryMappedFile {
     : file_(file), memory_(memory), size_(size) { }
   virtual ~PosixMemoryMappedFile();
   virtual void* memory() { return memory_; }
+  virtual int size() { return size_; }
  private:
   FILE* file_;
   void* memory_;
@@ -203,6 +204,19 @@ class PosixMemoryMappedFile : public OS::MemoryMappedFile {
 };
 
 
+OS::MemoryMappedFile* OS::MemoryMappedFile::open(const char* name) {
+  FILE* file = fopen(name, "w+");
+  if (file == NULL) return NULL;
+
+  fseek(file, 0, SEEK_END);
+  int size = ftell(file);
+
+  void* memory =
+      mmap(0, size, PROT_READ | PROT_WRITE, MAP_SHARED, fileno(file), 0);
+  return new PosixMemoryMappedFile(file, memory, size);
+}
+
+
 OS::MemoryMappedFile* OS::MemoryMappedFile::create(const char* name, int size,
     void* initial) {
   FILE* file = fopen(name, "w+");
diff --git a/src/platform-nullos.cc b/src/platform-nullos.cc
index f1b7695981..49d3dd988d 100644
--- a/src/platform-nullos.cc
+++ b/src/platform-nullos.cc
@@ -242,6 +242,12 @@ void OS::DebugBreak() {
 }
 
 
+OS::MemoryMappedFile* OS::MemoryMappedFile::open(const char* name) {
+  UNIMPLEMENTED();
+  return NULL;
+}
+
+
 OS::MemoryMappedFile* OS::MemoryMappedFile::create(const char* name, int size,
     void* initial) {
   UNIMPLEMENTED();
diff --git a/src/platform-openbsd.cc b/src/platform-openbsd.cc
index 5de6081907..0002dd7620 100644
--- a/src/platform-openbsd.cc
+++ b/src/platform-openbsd.cc
@@ -213,6 +213,7 @@ class PosixMemoryMappedFile : public OS::MemoryMappedFile {
     : file_(file), memory_(memory), size_(size) { }
   virtual ~PosixMemoryMappedFile();
   virtual void* memory() { return memory_; }
+  virtual int size() { return size_; }
  private:
   FILE* file_;
   void* memory_;
@@ -220,6 +221,19 @@ class PosixMemoryMappedFile : public OS::MemoryMappedFile {
 };
 
 
+OS::MemoryMappedFile* OS::MemoryMappedFile::open(const char* name) {
+  FILE* file = fopen(name, "w+");
+  if (file == NULL) return NULL;
+
+  fseek(file, 0, SEEK_END);
+  int size = ftell(file);
+
+  void* memory =
+      mmap(0, size, PROT_READ | PROT_WRITE, MAP_SHARED, fileno(file), 0);
+  return new PosixMemoryMappedFile(file, memory, size);
+}
+
+
 OS::MemoryMappedFile* OS::MemoryMappedFile::create(const char* name, int size,
     void* initial) {
   FILE* file = fopen(name, "w+");
diff --git a/src/platform-posix.cc b/src/platform-posix.cc
index ab5c0a3768..256dc75f6b 100644
--- a/src/platform-posix.cc
+++ b/src/platform-posix.cc
@@ -122,6 +122,11 @@ FILE* OS::FOpen(const char* path, const char* mode) {
 }
 
 
+bool OS::Remove(const char* path) {
+  return (remove(path) == 0);
+}
+
+
 const char* OS::LogFileOpenMode = "w";
 
 
diff --git a/src/platform-solaris.cc b/src/platform-solaris.cc
index dc4493aab9..556e26be21 100644
--- a/src/platform-solaris.cc
+++ b/src/platform-solaris.cc
@@ -226,6 +226,7 @@ class PosixMemoryMappedFile : public OS::MemoryMappedFile {
     : file_(file), memory_(memory), size_(size) { }
   virtual ~PosixMemoryMappedFile();
   virtual void* memory() { return memory_; }
+  virtual int size() { return size_; }
  private:
   FILE* file_;
   void* memory_;
@@ -233,6 +234,19 @@ class PosixMemoryMappedFile : public OS::MemoryMappedFile {
 };
 
 
+OS::MemoryMappedFile* OS::MemoryMappedFile::open(const char* name) {
+  FILE* file = fopen(name, "w+");
+  if (file == NULL) return NULL;
+
+  fseek(file, 0, SEEK_END);
+  int size = ftell(file);
+
+  void* memory =
+      mmap(0, size, PROT_READ | PROT_WRITE, MAP_SHARED, fileno(file), 0);
+  return new PosixMemoryMappedFile(file, memory, size);
+}
+
+
 OS::MemoryMappedFile* OS::MemoryMappedFile::create(const char* name, int size,
     void* initial) {
   FILE* file = fopen(name, "w+");
diff --git a/src/platform-win32.cc b/src/platform-win32.cc
index bf1737a54b..9ebf2c7e98 100644
--- a/src/platform-win32.cc
+++ b/src/platform-win32.cc
@@ -670,6 +670,11 @@ FILE* OS::FOpen(const char* path, const char* mode) {
 }
 
 
+bool OS::Remove(const char* path) {
+  return (DeleteFile(path) != 0);
+}
+
+
 // Open log file in binary mode to avoid /n -> /r/n conversion.
 const char* OS::LogFileOpenMode = "wb";
 
@@ -911,17 +916,44 @@ void OS::DebugBreak() {
 
 class Win32MemoryMappedFile : public OS::MemoryMappedFile {
  public:
-  Win32MemoryMappedFile(HANDLE file, HANDLE file_mapping, void* memory)
-    : file_(file), file_mapping_(file_mapping), memory_(memory) { }
+  Win32MemoryMappedFile(HANDLE file,
+                        HANDLE file_mapping,
+                        void* memory,
+                        int size)
+      : file_(file),
+        file_mapping_(file_mapping),
+        memory_(memory),
+        size_(size) { }
   virtual ~Win32MemoryMappedFile();
   virtual void* memory() { return memory_; }
+  virtual void* size() { return size_; }
  private:
   HANDLE file_;
   HANDLE file_mapping_;
   void* memory_;
+  int size_;
 };
 
 
+OS::MemoryMappedFile* OS::MemoryMappedFile::open(const char* name) {
+  // Open a physical file
+  HANDLE file = CreateFileA(name, GENERIC_READ | GENERIC_WRITE,
+      FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, 0, NULL);
+  if (file == NULL) return NULL;
+
+  int size = static_cast<int>(GetFileSize(file, NULL));
+
+  // Create a file mapping for the physical file
+  HANDLE file_mapping = CreateFileMapping(file, NULL,
+      PAGE_READWRITE, 0, static_cast<DWORD>(size), NULL);
+  if (file_mapping == NULL) return NULL;
+
+  // Map a view of the file into memory
+  void* memory = MapViewOfFile(file_mapping, FILE_MAP_ALL_ACCESS, 0, 0, size);
+  return new Win32MemoryMappedFile(file, file_mapping, memory, size);
+}
+
+
 OS::MemoryMappedFile* OS::MemoryMappedFile::create(const char* name, int size,
     void* initial) {
   // Open a physical file
@@ -935,7 +967,7 @@ OS::MemoryMappedFile* OS::MemoryMappedFile::create(const char* name, int size,
   // Map a view of the file into memory
   void* memory = MapViewOfFile(file_mapping, FILE_MAP_ALL_ACCESS, 0, 0, size);
   if (memory) memmove(memory, initial, size);
-  return new Win32MemoryMappedFile(file, file_mapping, memory);
+  return new Win32MemoryMappedFile(file, file_mapping, memory, size);
 }
 
 
diff --git a/src/platform.h b/src/platform.h
index 7b17067477..0d7d2e9cb8 100644
--- a/src/platform.h
+++ b/src/platform.h
@@ -174,6 +174,7 @@ class OS {
   static int GetLastError();
 
   static FILE* FOpen(const char* path, const char* mode);
+  static bool Remove(const char* path);
 
   // Log file open mode is platform-dependent due to line ends issues.
   static const char* LogFileOpenMode;
@@ -251,9 +252,11 @@ class OS {
 
   class MemoryMappedFile {
    public:
+    static MemoryMappedFile* open(const char* name);
     static MemoryMappedFile* create(const char* name, int size, void* initial);
     virtual ~MemoryMappedFile() { }
     virtual void* memory() = 0;
+    virtual int size() = 0;
   };
 
   // Safe formatting print. Ensures that str is always null-terminated.
diff --git a/src/utils.cc b/src/utils.cc
index d0ec4ef528..5d70866eb7 100644
--- a/src/utils.cc
+++ b/src/utils.cc
@@ -276,4 +276,96 @@ char* StringBuilder::Finalize() {
 }
 
 
+MemoryMappedExternalResource::MemoryMappedExternalResource(const char* filename)
+    : filename_(NULL),
+      data_(NULL),
+      length_(0),
+      remove_file_on_cleanup_(false) {
+  Init(filename);
+}
+
+
+MemoryMappedExternalResource::
+    MemoryMappedExternalResource(const char* filename,
+                                 bool remove_file_on_cleanup)
+    : filename_(NULL),
+      data_(NULL),
+      length_(0),
+      remove_file_on_cleanup_(remove_file_on_cleanup) {
+  Init(filename);
+}
+
+
+MemoryMappedExternalResource::~MemoryMappedExternalResource() {
+  // Release the resources if we had successfully acquired them:
+  if (file_ != NULL) {
+    delete file_;
+    if (remove_file_on_cleanup_) {
+      OS::Remove(filename_);
+    }
+    DeleteArray<char>(filename_);
+  }
+}
+
+
+void MemoryMappedExternalResource::Init(const char* filename) {
+  file_ = OS::MemoryMappedFile::open(filename);
+  if (file_ != NULL) {
+    filename_ = StrDup(filename);
+    data_ = reinterpret_cast<char*>(file_->memory());
+    length_ = file_->size();
+  }
+}
+
+
+bool MemoryMappedExternalResource::EnsureIsAscii(bool abort_if_failed) const {
+  bool is_ascii = true;
+
+  int line_no = 1;
+  const char* start_of_line = data_;
+  const char* end = data_ + length_;
+  for (const char* p = data_; p < end; p++) {
+    char c = *p;
+    if ((c & 0x80) != 0) {
+      // Non-ascii detected:
+      is_ascii = false;
+
+      // Report the error and abort if appropriate:
+      if (abort_if_failed) {
+        int char_no = (p - start_of_line) - 1;
+
+        ASSERT(filename_ != NULL);
+        PrintF("\n\n\n"
+               "Abort: Non-Ascii character 0x%.2x in file %s line %d char %d",
+               c, filename_, line_no, char_no);
+
+        // Allow for some context up to kNumberOfLeadingContextChars chars
+        // before the offending non-ascii char to help the user see where
+        // the offending char is.
+        const int kNumberOfLeadingContextChars = 10;
+        const char* err_context = p - kNumberOfLeadingContextChars;
+        if (err_context < data_) {
+          err_context = data_;
+        }
+        // Compute the length of the error context and print it.
+        int err_context_length = p - err_context;
+        if (err_context_length != 0) {
+          PrintF(" after \"%.*s\"", err_context_length, err_context);
+        }
+        PrintF(".\n\n\n");
+        OS::Abort();
+      }
+
+      break;  // Non-ascii detected.  No need to continue scanning.
+    }
+    if (c == '\n') {
+      start_of_line = p;
+      line_no++;
+    }
+  }
+
+  return is_ascii;
+}
+
+
 } }  // namespace v8::internal
diff --git a/src/v8utils.h b/src/v8utils.h
index e9623be62e..0aa53cac59 100644
--- a/src/v8utils.h
+++ b/src/v8utils.h
@@ -316,6 +316,39 @@ static inline void CopyChars(sinkchar* dest, const sourcechar* src, int chars) {
   }
 }
 
+
+// A resource for using mmapped files to back external strings that are read
+// from files.
+class MemoryMappedExternalResource: public
+    v8::String::ExternalAsciiStringResource {
+ public:
+  explicit MemoryMappedExternalResource(const char* filename);
+  MemoryMappedExternalResource(const char* filename,
+                               bool remove_file_on_cleanup);
+  virtual ~MemoryMappedExternalResource();
+
+  virtual const char* data() const { return data_; }
+  virtual size_t length() const { return length_; }
+
+  bool exists() const { return file_ != NULL; }
+  bool is_empty() const { return length_ == 0; }
+
+  bool EnsureIsAscii(bool abort_if_failed) const;
+  bool EnsureIsAscii() const { return EnsureIsAscii(true); }
+  bool IsAscii() const { return EnsureIsAscii(false); }
+
+ private:
+  void Init(const char* filename);
+
+  char* filename_;
+  OS::MemoryMappedFile* file_;
+
+  const char* data_;
+  size_t length_;
+  bool remove_file_on_cleanup_;
+};
+
+
 } }  // namespace v8::internal
 
 #endif  // V8_V8UTILS_H_