--- xnu/xnu-123.5/iokit/Kernel/IOMultiMemoryDescriptor.cpp
+++ xnu/xnu-792/iokit/Kernel/IOMultiMemoryDescriptor.cpp
@@ -87,7 +87,7 @@
                                   IOMemoryDescriptor ** descriptors,
                                   UInt32                withCount,
                                   IODirection           withDirection,
-                                  bool                  asReference = false )
+                                  bool                  asReference )
 {
     //
     // Create a new IOMultiMemoryDescriptor.  The "buffer" is made up of several
@@ -118,7 +118,7 @@
                                   IOMemoryDescriptor ** descriptors,
                                   UInt32                withCount,
                                   IODirection           withDirection,
-                                  bool                  asReference = false )
+                                  bool                  asReference )
 {
     //
     // Initialize an IOMultiMemoryDescriptor. The "buffer" is made up of several
@@ -131,10 +131,19 @@
     assert(descriptors);
     assert(withCount);
 
-    // Ask our superclass' opinion.
-
-    if ( super::init() == false )  return false;
-
+    // Release existing descriptors, if any
+    if ( _descriptors )
+    {
+        for ( unsigned index = 0; index < _descriptorsCount; index++ ) 
+            _descriptors[index]->release();
+
+        if ( _descriptorsIsAllocated )
+            IODelete(_descriptors, IOMemoryDescriptor *, _descriptorsCount);
+    } else {
+        // Ask our superclass' opinion.
+        if ( super::init() == false )  return false;
+    }
+    
     // Initialize our minimal state.
 
     _descriptors            = 0;
@@ -289,6 +298,34 @@
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 
+IOPhysicalAddress IOMultiMemoryDescriptor::getSourceSegment(
+                                                       IOByteCount   offset,
+                                                       IOByteCount * length )
+{
+    //
+    // This method returns the physical address of the byte at the given offset
+    // into the memory,  and optionally the length of the physically contiguous
+    // segment from that offset.
+    //
+
+    assert(offset <= _length);
+
+    for ( unsigned index = 0; index < _descriptorsCount; index++ ) 
+    {
+        if ( offset < _descriptors[index]->getLength() )
+        {
+            return _descriptors[index]->getSourceSegment(offset, length);
+        }
+        offset -= _descriptors[index]->getLength();
+    }
+
+    if ( length )  *length = 0;
+
+    return 0;
+}
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
 void * IOMultiMemoryDescriptor::getVirtualSegment( IOByteCount   /* offset */ ,
                                                    IOByteCount * /* length */ )
 {