commit 657e5f87523009cab319c427829543b0231923af
Author: Joel Brobecker <brobecker@adacore.com>
Date:   Wed May 2 09:00:11 2012 -0700

    Segment register reading on Windows targets.
    
    gdb/ChangeLog:
    
            * i386-tdep.h (struct gdbarch_tdep): New field
            `first_segment_regnum'.
            * amd64-tdep.c (amd64_init_abi): Set tdep->first_segment_regnum.
            * i386-tdep.c (i386_gdbarch_init): Likewise.
            * windows-nat.c (do_windows_fetch_inferior_registers): Only
            read the first 16 bits of segment register values.

diff --git a/gdb/amd64-tdep.c b/gdb/amd64-tdep.c
index 685fa48..27988bf 100644
--- a/gdb/amd64-tdep.c
+++ b/gdb/amd64-tdep.c
@@ -2585,6 +2585,8 @@ amd64_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
   struct gdbarch_tdep *tdep = gdbarch_tdep (gdbarch);
   const struct target_desc *tdesc = info.target_desc;
 
+  tdep->first_segment_regnum = AMD64_CS_REGNUM;
+
   /* AMD64 generally uses `fxsave' instead of `fsave' for saving its
      floating-point registers.  */
   tdep->sizeof_fpregset = I387_SIZEOF_FXSAVE;
diff --git a/gdb/i386-tdep.c b/gdb/i386-tdep.c
index 769ef42..e6457f6 100644
--- a/gdb/i386-tdep.c
+++ b/gdb/i386-tdep.c
@@ -7613,6 +7613,8 @@ i386_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
   tdep->gregset_num_regs = I386_NUM_GREGS;
   tdep->sizeof_gregset = 0;
 
+  tdep->first_segment_regnum = I386_CS_REGNUM;
+
   /* Floating-point registers.  */
   tdep->fpregset = NULL;
   tdep->sizeof_fpregset = I387_SIZEOF_FSAVE;
diff --git a/gdb/i386-tdep.h b/gdb/i386-tdep.h
index f297ae7..e2acbbc 100644
--- a/gdb/i386-tdep.h
+++ b/gdb/i386-tdep.h
@@ -75,6 +75,10 @@ struct gdbarch_tdep
   int gregset_num_regs;
   size_t sizeof_gregset;
 
+  /* Register number for the first segment register.  Its value
+     depends on whether we're debugging on an x86 or amd64 target.  */
+  int first_segment_regnum;
+
   /* The general-purpose registers used to pass integers when making
      function calls.  This only applies to amd64, as all parameters
      are passed through the stack on x86.  */
diff --git a/gdb/windows-nat.c b/gdb/windows-nat.c
index f536ed1..a265462 100644
--- a/gdb/windows-nat.c
+++ b/gdb/windows-nat.c
@@ -456,6 +456,15 @@ do_windows_fetch_inferior_registers (struct regcache *regcache, int r)
       l = (*((long *) context_offset) >> 16) & ((1 << 11) - 1);
       regcache_raw_supply (regcache, r, (char *) &l);
     }
+  else if (r >= tdep->first_segment_regnum
+	   && r < tdep->first_segment_regnum + 6)
+    {
+      /* GDB treats segment registers as 32bit registers, but they are
+	 in fact only 16 bits long.  Make sure we do not read extra
+	 bits from our source buffer.  */
+      l = *((long *) context_offset) & 0xffff;
+      regcache_raw_supply (regcache, r, (char *) &l);
+    }
   else if (r >= 0)
     regcache_raw_supply (regcache, r, context_offset);
   else