diff --git a/include/llvm/MC/MCAssembler.h b/include/llvm/MC/MCAssembler.h
index bcd678d..aae5f08 100644
--- a/include/llvm/MC/MCAssembler.h
+++ b/include/llvm/MC/MCAssembler.h
@@ -317,6 +317,13 @@
   /// unit.
   unsigned IsExternal : 1;
 
+  /// IsPrivateExtern - True if this symbol is private extern.
+  unsigned IsPrivateExtern : 1;
+
+  /// Flags - The Flags field is used by object file implementations to store
+  /// additional per symbol information which is not easily classified.
+  uint32_t Flags;
+
 public:
   // Only for use as sentinel.
   MCSymbolData();
@@ -334,13 +341,22 @@
   uint64_t getOffset() const { return Offset; }
   void setOffset(uint64_t Value) { Offset = Value; }
 
-    /// @}
-    /// @name Symbol Attributes
-    /// @{
+  /// @}
+  /// @name Symbol Attributes
+  /// @{
+  
+  bool isExternal() const { return IsExternal; }
+  void setExternal(bool Value) { IsExternal = Value; }
+  
+  bool isPrivateExtern() const { return IsPrivateExtern; }
+  void setPrivateExtern(bool Value) { IsPrivateExtern = Value; }
+  
+  /// getFlags - Get the (implementation defined) symbol flags.
+  uint32_t getFlags() const { return Flags; }
 
-    bool isExternal() const { return IsExternal; }
-    void setExternal(bool Value) { IsExternal = Value; }
-
+  /// setFlags - Set the (implementation defined) symbol flags.
+  void setFlags(uint32_t Value) { Flags = Value; }
+  
   /// @}  
 };
 
diff --git a/lib/MC/MCAssembler.cpp b/lib/MC/MCAssembler.cpp
index 80e1b9e..9315422 100644
--- a/lib/MC/MCAssembler.cpp
+++ b/lib/MC/MCAssembler.cpp
@@ -291,7 +291,8 @@
 
     // FIXME: Set STAB bits.
 
-    // FIXME: Set private external bit.
+    if (MSD.SymbolData->isPrivateExtern())
+      Type |= STF_PrivateExtern;
 
     // Set external bit.
     if (MSD.SymbolData->isExternal() || Symbol.isUndefined())
@@ -302,7 +303,11 @@
     Write32(MSD.StringIndex);
     Write8(Type);
     Write8(MSD.SectionIndex);
-    Write16(0); // FIXME: Desc
+    
+    // The Mach-O streamer uses the lowest 16-bits of the flags for the 'desc'
+    // value.
+    Write16(MSD.SymbolData->getFlags() & 0xFFFF);
+
     Write32(0); // FIXME: Value
   }
 
@@ -544,7 +549,7 @@
 MCSymbolData::MCSymbolData(MCSymbol &_Symbol, MCFragment *_Fragment,
                            uint64_t _Offset, MCAssembler *A)
   : Symbol(_Symbol), Fragment(_Fragment), Offset(_Offset),
-    IsExternal(false)
+    IsExternal(false), IsPrivateExtern(false), Flags(0)
 {
   if (A)
     A->getSymbolList().push_back(this);
diff --git a/lib/MC/MCMachOStreamer.cpp b/lib/MC/MCMachOStreamer.cpp
index caff125..9d60e3e 100644
--- a/lib/MC/MCMachOStreamer.cpp
+++ b/lib/MC/MCMachOStreamer.cpp
@@ -19,6 +19,27 @@
 namespace {
 
 class MCMachOStreamer : public MCStreamer {
+  /// SymbolFlags - We store the value for the 'desc' symbol field in the lowest
+  /// 16 bits of the implementation defined flags.
+  enum SymbolFlags { // See <mach-o/nlist.h>.
+    SF_DescFlagsMask                        = 0xFFFF,
+
+    // Reference type flags.
+    SF_ReferenceTypeMask                    = 0x0007,
+    SF_ReferenceTypeUndefinedNonLazy        = 0x0000,
+    SF_ReferenceTypeUndefinedLazy           = 0x0001,
+    SF_ReferenceTypeDefined                 = 0x0002,
+    SF_ReferenceTypePrivateDefined          = 0x0003,
+    SF_ReferenceTypePrivateUndefinedNonLazy = 0x0004,
+    SF_ReferenceTypePrivateUndefinedLazy    = 0x0005,
+
+    // Other 'desc' flags.
+    SF_NoDeadStrip                          = 0x0020,
+    SF_WeakReference                        = 0x0040,
+    SF_WeakDefinition                       = 0x0080
+  };
+
+private:
   MCAssembler Assembler;
 
   MCSectionData *CurSectionData;
@@ -121,6 +142,9 @@
   assert(!SD.getFragment() && "Unexpected fragment on symbol data!");
   SD.setFragment(F);
   SD.setOffset(F->getContents().size());
+
+  // This causes the reference type and weak reference flags to be cleared.
+  SD.setFlags(SD.getFlags() & ~(SF_WeakReference | SF_ReferenceTypeMask));
   
   Symbol->setSection(*CurSection);
 }
@@ -141,18 +165,71 @@
 
 void MCMachOStreamer::EmitSymbolAttribute(MCSymbol *Symbol,
                                           SymbolAttr Attribute) {
+  // Adding a symbol attribute always introduces the symbol, note that an
+  // important side effect of calling getSymbolData here is to register the
+  // symbol with the assembler.
+  MCSymbolData &SD = getSymbolData(*Symbol);
+
+  // The implementation of symbol attributes is designed to match 'as', but it
+  // leaves much to desired. It doesn't really make sense to arbitrarily add and
+  // remove flags, but 'as' allows this (in particular, see .desc).
+  //
+  // In the future it might be worth trying to make these operations more well
+  // defined.
   switch (Attribute) {
-  default:
-    llvm_unreachable("FIXME: Not yet implemented!");
+  case MCStreamer::Hidden:
+  case MCStreamer::Internal:
+  case MCStreamer::Protected:
+  case MCStreamer::Weak:
+    assert(0 && "Invalid symbol attribute for Mach-O!");
+    break;
 
   case MCStreamer::Global:
     getSymbolData(*Symbol).setExternal(true);
     break;
+
+  case MCStreamer::LazyReference:
+    // FIXME: This requires -dynamic.
+    SD.setFlags(SD.getFlags() | SF_NoDeadStrip);
+    if (Symbol->isUndefined())
+      SD.setFlags(SD.getFlags() | SF_ReferenceTypeUndefinedLazy);
+    break;
+
+  case MCStreamer::IndirectSymbol:
+    llvm_unreachable("FIXME: Not yet implemented!");
+    break;
+
+    // Since .reference sets the no dead strip bit, it is equivalent to
+    // .no_dead_strip in practice.
+  case MCStreamer::Reference:
+  case MCStreamer::NoDeadStrip:
+    SD.setFlags(SD.getFlags() | SF_NoDeadStrip);
+    break;
+
+  case MCStreamer::PrivateExtern:
+    SD.setExternal(true);
+    SD.setPrivateExtern(true);
+    break;
+
+  case MCStreamer::WeakReference:
+    // FIXME: This requires -dynamic.
+    if (Symbol->isUndefined())
+      SD.setFlags(SD.getFlags() | SF_WeakReference);
+    break;
+
+  case MCStreamer::WeakDefinition:
+    // FIXME: 'as' enforces that this is defined and global. The manual claims
+    // it has to be in a coalesced section, but this isn't enforced.
+    SD.setFlags(SD.getFlags() | SF_WeakDefinition);
+    break;
   }
 }
 
 void MCMachOStreamer::EmitSymbolDesc(MCSymbol *Symbol, unsigned DescValue) {
-  llvm_unreachable("FIXME: Not yet implemented!");
+  // Encode the 'desc' value into the lowest implementation defined bits.
+  assert(DescValue == (DescValue & SF_DescFlagsMask) && 
+         "Invalid .desc value!");
+  getSymbolData(*Symbol).setFlags(DescValue & SF_DescFlagsMask);
 }
 
 void MCMachOStreamer::EmitLocalSymbol(MCSymbol *Symbol, const MCValue &Value) {
diff --git a/test/MC/MachO/symbol-flags.s b/test/MC/MachO/symbol-flags.s
new file mode 100644
index 0000000..e82b0a0
--- /dev/null
+++ b/test/MC/MachO/symbol-flags.s
@@ -0,0 +1,254 @@
+// RUN: llvm-mc -triple i386-apple-darwin9 %s -filetype=obj -o - | macho-dump | FileCheck %s
+
+        .reference sym_ref_A
+        .reference sym_ref_def_A
+sym_ref_def_A:
+sym_ref_def_C:  
+        .reference sym_ref_def_C
+        
+        .weak_reference sym_weak_ref_A
+        .weak_reference sym_weak_ref_def_A
+sym_weak_ref_def_A:        
+sym_weak_ref_def_B:
+        .weak_reference sym_weak_ref_def_B
+
+        .data
+        .globl sym_weak_def_A
+        .weak_definition sym_weak_def_A        
+sym_weak_def_A:
+
+        .lazy_reference sym_lazy_ref_A
+        .lazy_reference sym_lazy_ref_B
+sym_lazy_ref_B:
+sym_lazy_ref_C:
+        .lazy_reference sym_lazy_ref_C
+
+        .private_extern sym_private_ext_A
+        .private_extern sym_private_ext_B
+sym_private_ext_B:
+sym_private_ext_C:
+        .private_extern sym_private_ext_C
+        .private_extern sym_private_ext_D
+        .globl sym_private_ext_D
+
+        .no_dead_strip sym_no_dead_strip_A
+
+        .reference sym_ref_A
+        .desc sym_ref_A, 1
+        .desc sym_ref_A, 0x1234
+
+        .desc sym_desc_flags,0x47
+sym_desc_flags:
+
+// CHECK: ('cputype', 7)
+// CHECK: ('cpusubtype', 3)
+// CHECK: ('filetype', 1)
+// CHECK: ('num_load_commands', 1)
+// CHECK: ('load_commands_size', 296)
+// CHECK: ('flag', 0)
+// CHECK: ('load_commands', [
+// CHECK:   # Load Command 0
+// CHECK:  (('command', 1)
+// CHECK:   ('size', 192)
+// CHECK:   ('segment_name', '\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00')
+// CHECK:   ('vm_addr', 0)
+// CHECK:   ('vm_size', 0)
+// CHECK:   ('file_offset', 324)
+// CHECK:   ('file_size', 0)
+// CHECK:   ('maxprot', 7)
+// CHECK:   ('initprot', 7)
+// CHECK:   ('num_sections', 2)
+// CHECK:   ('flags', 0)
+// CHECK:   ('sections', [
+// CHECK:     # Section 0
+// CHECK:    (('section_name', '__text\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00')
+// CHECK:     ('segment_name', '__TEXT\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00')
+// CHECK:     ('address', 0)
+// CHECK:     ('size', 0)
+// CHECK:     ('offset', 324)
+// CHECK:     ('alignment', 0)
+// CHECK:     ('reloc_offset', 0)
+// CHECK:     ('num_reloc', 0)
+// CHECK:     ('flags', 0x80000000)
+// CHECK:     ('reserved1', 0)
+// CHECK:     ('reserved2', 0)
+// CHECK:    ),
+// CHECK:     # Section 1
+// CHECK:    (('section_name', '__data\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00')
+// CHECK:     ('segment_name', '__DATA\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00')
+// CHECK:     ('address', 0)
+// CHECK:     ('size', 0)
+// CHECK:     ('offset', 324)
+// CHECK:     ('alignment', 0)
+// CHECK:     ('reloc_offset', 0)
+// CHECK:     ('num_reloc', 0)
+// CHECK:     ('flags', 0x0)
+// CHECK:     ('reserved1', 0)
+// CHECK:     ('reserved2', 0)
+// CHECK:    ),
+// CHECK:   ])
+// CHECK:  ),
+// CHECK:   # Load Command 1
+// CHECK:  (('command', 2)
+// CHECK:   ('size', 24)
+// CHECK:   ('symoff', 324)
+// CHECK:   ('nsyms', 16)
+// CHECK:   ('stroff', 516)
+// CHECK:   ('strsize', 260)
+// CHECK:   ('_string_data', '\x00sym_ref_A\x00sym_weak_ref_A\x00sym_weak_def_A\x00sym_lazy_ref_A\x00sym_private_ext_A\x00sym_private_ext_B\x00sym_private_ext_C\x00sym_private_ext_D\x00sym_no_dead_strip_A\x00sym_ref_def_A\x00sym_ref_def_C\x00sym_weak_ref_def_A\x00sym_weak_ref_def_B\x00sym_lazy_ref_B\x00sym_lazy_ref_C\x00sym_desc_flags\x00\x00')
+// CHECK:   ('_symbols', [
+// CHECK:     # Symbol 0
+// CHECK:    (('n_strx', 148)
+// CHECK:     ('n_type', 0xe)
+// CHECK:     ('n_sect', 1)
+// CHECK:     ('n_desc', 32)
+// CHECK:     ('n_value', 0)
+// CHECK:     ('_string', 'sym_ref_def_A')
+// CHECK:    ),
+// CHECK:     # Symbol 1
+// CHECK:    (('n_strx', 162)
+// CHECK:     ('n_type', 0xe)
+// CHECK:     ('n_sect', 1)
+// CHECK:     ('n_desc', 32)
+// CHECK:     ('n_value', 0)
+// CHECK:     ('_string', 'sym_ref_def_C')
+// CHECK:    ),
+// CHECK:     # Symbol 2
+// CHECK:    (('n_strx', 176)
+// CHECK:     ('n_type', 0xe)
+// CHECK:     ('n_sect', 1)
+// CHECK:     ('n_desc', 0)
+// CHECK:     ('n_value', 0)
+// CHECK:     ('_string', 'sym_weak_ref_def_A')
+// CHECK:    ),
+// CHECK:     # Symbol 3
+// CHECK:    (('n_strx', 195)
+// CHECK:     ('n_type', 0xe)
+// CHECK:     ('n_sect', 1)
+// CHECK:     ('n_desc', 0)
+// CHECK:     ('n_value', 0)
+// CHECK:     ('_string', 'sym_weak_ref_def_B')
+// CHECK:    ),
+// CHECK:     # Symbol 4
+// CHECK:    (('n_strx', 214)
+// CHECK:     ('n_type', 0xe)
+// CHECK:     ('n_sect', 2)
+// CHECK:     ('n_desc', 32)
+// CHECK:     ('n_value', 0)
+// CHECK:     ('_string', 'sym_lazy_ref_B')
+// CHECK:    ),
+// CHECK:     # Symbol 5
+// CHECK:    (('n_strx', 229)
+// CHECK:     ('n_type', 0xe)
+// CHECK:     ('n_sect', 2)
+// CHECK:     ('n_desc', 32)
+// CHECK:     ('n_value', 0)
+// CHECK:     ('_string', 'sym_lazy_ref_C')
+// CHECK:    ),
+// CHECK:     # Symbol 6
+// CHECK:    (('n_strx', 244)
+// CHECK:     ('n_type', 0xe)
+// CHECK:     ('n_sect', 2)
+// CHECK:     ('n_desc', 0)
+// CHECK:     ('n_value', 0)
+// CHECK:     ('_string', 'sym_desc_flags')
+// CHECK:    ),
+// CHECK:     # Symbol 7
+// CHECK:    (('n_strx', 74)
+// CHECK:     ('n_type', 0x1f)
+// CHECK:     ('n_sect', 2)
+// CHECK:     ('n_desc', 0)
+// CHECK:     ('n_value', 0)
+// CHECK:     ('_string', 'sym_private_ext_B')
+// CHECK:    ),
+// CHECK:     # Symbol 8
+// CHECK:    (('n_strx', 92)
+// CHECK:     ('n_type', 0x1f)
+// CHECK:     ('n_sect', 2)
+// CHECK:     ('n_desc', 0)
+// CHECK:     ('n_value', 0)
+// CHECK:     ('_string', 'sym_private_ext_C')
+// CHECK:    ),
+// CHECK:     # Symbol 9
+// CHECK:    (('n_strx', 26)
+// CHECK:     ('n_type', 0xf)
+// CHECK:     ('n_sect', 2)
+// CHECK:     ('n_desc', 128)
+// CHECK:     ('n_value', 0)
+// CHECK:     ('_string', 'sym_weak_def_A')
+// CHECK:    ),
+// CHECK:     # Symbol 10
+// CHECK:    (('n_strx', 41)
+// CHECK:     ('n_type', 0x1)
+// CHECK:     ('n_sect', 0)
+// CHECK:     ('n_desc', 33)
+// CHECK:     ('n_value', 0)
+// CHECK:     ('_string', 'sym_lazy_ref_A')
+// CHECK:    ),
+// CHECK:     # Symbol 11
+// CHECK:    (('n_strx', 128)
+// CHECK:     ('n_type', 0x1)
+// CHECK:     ('n_sect', 0)
+// CHECK:     ('n_desc', 32)
+// CHECK:     ('n_value', 0)
+// CHECK:     ('_string', 'sym_no_dead_strip_A')
+// CHECK:    ),
+// CHECK:     # Symbol 12
+// CHECK:    (('n_strx', 56)
+// CHECK:     ('n_type', 0x11)
+// CHECK:     ('n_sect', 0)
+// CHECK:     ('n_desc', 0)
+// CHECK:     ('n_value', 0)
+// CHECK:     ('_string', 'sym_private_ext_A')
+// CHECK:    ),
+// CHECK:     # Symbol 13
+// CHECK:    (('n_strx', 110)
+// CHECK:     ('n_type', 0x11)
+// CHECK:     ('n_sect', 0)
+// CHECK:     ('n_desc', 0)
+// CHECK:     ('n_value', 0)
+// CHECK:     ('_string', 'sym_private_ext_D')
+// CHECK:    ),
+// CHECK:     # Symbol 14
+// CHECK:    (('n_strx', 1)
+// CHECK:     ('n_type', 0x1)
+// CHECK:     ('n_sect', 0)
+// CHECK:     ('n_desc', 4660)
+// CHECK:     ('n_value', 0)
+// CHECK:     ('_string', 'sym_ref_A')
+// CHECK:    ),
+// CHECK:     # Symbol 15
+// CHECK:    (('n_strx', 11)
+// CHECK:     ('n_type', 0x1)
+// CHECK:     ('n_sect', 0)
+// CHECK:     ('n_desc', 64)
+// CHECK:     ('n_value', 0)
+// CHECK:     ('_string', 'sym_weak_ref_A')
+// CHECK:    ),
+// CHECK:   ])
+// CHECK:  ),
+// CHECK:   # Load Command 2
+// CHECK:  (('command', 11)
+// CHECK:   ('size', 80)
+// CHECK:   ('ilocalsym', 0)
+// CHECK:   ('nlocalsym', 7)
+// CHECK:   ('iextdefsym', 7)
+// CHECK:   ('nextdefsym', 3)
+// CHECK:   ('iundefsym', 10)
+// CHECK:   ('nundefsym', 6)
+// CHECK:   ('tocoff', 0)
+// CHECK:   ('ntoc', 0)
+// CHECK:   ('modtaboff', 0)
+// CHECK:   ('nmodtab', 0)
+// CHECK:   ('extrefsymoff', 0)
+// CHECK:   ('nextrefsyms', 0)
+// CHECK:   ('indirectsymoff', 0)
+// CHECK:   ('nindirectsyms', 0)
+// CHECK:   ('extreloff', 0)
+// CHECK:   ('nextrel', 0)
+// CHECK:   ('locreloff', 0)
+// CHECK:   ('nlocrel', 0)
+// CHECK:   ('_indirect_symbols', [
+// CHECK:   ])
+// CHECK:  ),
+// CHECK: ])