diff --git a/Exception.h b/Exception.h
index cf9ffb7..cd23c37 100644
--- a/Exception.h
+++ b/Exception.h
@@ -11,20 +11,36 @@ class Exception : public std::exception
 public:
     Exception(const char *cp);
     Exception(const std::string &str);
-    Exception(const char *cp, int error);
-    Exception(const std::string& string, int error);
+
     
     virtual ~Exception() throw ();
     
     virtual const char *what();
     
-    int error() const;
+    int error() const { return _error; }
+
+protected:
+    Exception(const char *cp, int error);
+    Exception(const std::string& string, int error);
+
 private:
     int _error;
     std::string _string;
 
 };
 
+class POSIXException : public Exception {
+public:
+    POSIXException(const char *cp, int error);
+    POSIXException(const std::string& string, int error);
+};
+
+class ProDOSException : public Exception {
+public:
+    ProDOSException(const char *cp, int error);
+    ProDOSException(const std::string& string, int error);
+};
+
 inline Exception::Exception(const char *cp):
     _error(0),
     _string(cp)
@@ -49,11 +65,28 @@ inline Exception::Exception(const std::string& string, int error):
 {
 }
 
-inline int Exception::error() const
+inline POSIXException::POSIXException(const char *cp, int error) :
+    Exception(cp, error)
 {
-    return _error;
 }
 
+inline POSIXException::POSIXException(const std::string& string, int error) :
+    Exception(string, error)
+{
+}
+
+inline ProDOSException::ProDOSException(const char *cp, int error) :
+    Exception(cp, error)
+{
+}
+
+inline ProDOSException::ProDOSException(const std::string& string, int error) :
+    Exception(string, error)
+{
+}
+
+
+
 }
 
 #endif
\ No newline at end of file
diff --git a/MappedFile.cpp b/MappedFile.cpp
index b8b055d..7b078b3 100644
--- a/MappedFile.cpp
+++ b/MappedFile.cpp
@@ -36,7 +36,7 @@ MappedFile::MappedFile(const char *name, bool readOnly)
      
     if (fd < 0)
     {
-        throw Exception(__METHOD__ ": Unable to open file.", errno);
+        throw POSIXException(__METHOD__ ": Unable to open file.", errno);
     }
     // init may throw; auto_fd guarantees the file will be closed if that happens.
     init(fd, readOnly);
@@ -63,12 +63,12 @@ MappedFile::MappedFile(const char *name, size_t size)
     auto_fd fd(::open(name, O_CREAT | O_TRUNC | O_RDWR, 0644));
     
     if (fd < 0)
-        throw Exception(__METHOD__ ": Unable to create file.", errno);
+        throw POSIXException(__METHOD__ ": Unable to create file.", errno);
     
     // TODO -- is ftruncate portable?
     if (::ftruncate(fd, _size) < 0)
     {
-        throw Exception(__METHOD__ ": Unable to truncate file.", errno);    
+        throw POSIXException(__METHOD__ ": Unable to truncate file.", errno);    
     }
     
     //_map = ::mmap(NULL, _size, PROT_READ | PROT_WRITE, MAP_FILE | MAP_SHARED, _fd, 0);
@@ -80,7 +80,7 @@ MappedFile::MappedFile(const char *name, size_t size)
         MAP_FILE | MAP_SHARED
     );
     
-    if (map == MAP_FAILED) throw Exception(__METHOD__ ": Unable to map file.", errno);
+    if (map == MAP_FAILED) throw POSIXException(__METHOD__ ": Unable to map file.", errno);
 
     _fd = fd.release();
     _map = map.release();
@@ -110,7 +110,7 @@ void MappedFile::init(int f, bool readOnly)
     _size = ::lseek(f, 0, SEEK_END);
     
     if (_size < 0)
-        throw Exception(__METHOD__ ": Unable to determine file size.", errno);
+        throw POSIXException(__METHOD__ ": Unable to determine file size.", errno);
         
 /*
     _map = ::mmap(NULL, _size, readOnly ? PROT_READ : PROT_READ | PROT_WRITE, 
@@ -123,7 +123,7 @@ void MappedFile::init(int f, bool readOnly)
         readOnly ? MAP_FILE : MAP_FILE |  MAP_SHARED
     );
   
-    if (map == MAP_FAILED) throw Exception(__METHOD__ ": Unable to map file.", errno);
+    if (map == MAP_FAILED) throw POSIXException(__METHOD__ ": Unable to map file.", errno);
     
     _fd = f;
     _map = map.release();
@@ -227,7 +227,7 @@ void MappedFile::sync()
     if (_readOnly) return;
     
     if (::msync(_map, _size, MS_SYNC) < 0)
-        throw Exception(__METHOD__ ": msync error.", errno);
+        throw POSIXException(__METHOD__ ": msync error.", errno);
 }
 
 void MappedFile::reset()
diff --git a/RawDevice.cpp b/RawDevice.cpp
index 2635143..bc19ce5 100644
--- a/RawDevice.cpp
+++ b/RawDevice.cpp
@@ -36,7 +36,7 @@ void RawDevice::devSize(int fd)
     struct dk_minfo minfo;
 
     if (::ioctl(fd, DKIOCGMEDIAINFO, &minfo) < 0)
-        throw Exception(__METHOD__ ": Unable to determine device size.", errno);
+        throw POSIXException(__METHOD__ ": Unable to determine device size.", errno);
     
     _size = minfo.dki_lbsize * minfo.dki_capacity;
     _blockSize = 512; // not really, but whatever.
@@ -55,11 +55,11 @@ void RawDevice::devSize(int fd)
     uint64_t blockCount; // 64 bit
     
     if (::ioctl(fd, DKIOCGETBLOCKSIZE, &blockSize) < 0)
-        throw Exception(__METHOD__ ": Unable to determine block size.", errno);
+        throw POSIXException(__METHOD__ ": Unable to determine block size.", errno);
     
     
     if (::ioctl(fd, DKIOCGETBLOCKCOUNT, &blockCount) < 0)
-        throw Exception(__METHOD__ ": Unable to determine block count.", errno);
+        throw POSIXException(__METHOD__ ": Unable to determine block count.", errno);
         
     _blockSize = blockSize;
     _size = _blockSize * blockCount;
@@ -79,7 +79,7 @@ void RawDevice::devSize(int fd)
     int blocks;
 
     if (::ioctl(fd, BLKGETSIZE, &blocks) < 0)
-        throw Exception(__METHOD__ ": Unable to determine device size.", errno);
+        throw POSIXException(__METHOD__ ": Unable to determine device size.", errno);
     
     _size = 512 * blocks;
     _blockSize = 512; // 
@@ -110,7 +110,7 @@ RawDevice::RawDevice(const char *name, bool readOnly)
     }
      
     if (fd < 0)
-        throw Exception(__METHOD__ ": Unable to open device.", errno);
+        throw POSIXException(__METHOD__ ": Unable to open device.", errno);
 
     _fd = -1;
 
@@ -147,8 +147,10 @@ void RawDevice::read(unsigned block, void *bp)
     
     // TODO -- EINTR?
     if (ok != 512)
-        throw Exception(__METHOD__ ": Error reading block.", 
-            ok < 0 ? errno : 0);
+        throw ok < 0
+            ? POSIXException(__METHOD__ ": Error reading block.", errno)
+            : Exception(__METHOD__ ": Error reading block.");
+
     
     
 }
@@ -166,8 +168,9 @@ void RawDevice::write(unsigned block, const void *bp)
     size_t ok = ::pwrite(_fd, bp, 512, block * 512);
     
     if (ok != 512)
-        throw Exception(__METHOD__ ": Error writing block.", 
-            ok < 0 ? errno : 0);
+        throw ok < 0 
+            ? POSIXException(__METHOD__ ": Error writing block.", errno)
+            : Exception(__METHOD__ ": Error writing block.");
 }
 
 
@@ -184,5 +187,5 @@ void RawDevice::sync()
     if (_readOnly) return;
     
     if (::fsync(_fd) < 0)
-        throw Exception(__METHOD__ ": fsync error.", errno);
+        throw POSIXException(__METHOD__ ": fsync error.", errno);
 }
\ No newline at end of file