diff --git a/src/ia64/Gstep-ia64.c b/src/ia64/Gstep-ia64.c
index 20926e2c..408e472b 100644
--- a/src/ia64/Gstep-ia64.c
+++ b/src/ia64/Gstep-ia64.c
@@ -1,5 +1,5 @@
 /* libunwind - a platform-independent unwind library
-   Copyright (C) 2001-2002 Hewlett-Packard Co
+   Copyright (C) 2001-2003 Hewlett-Packard Co
 	Contributed by David Mosberger-Tang <davidm@hpl.hp.com>
 
 This file is part of libunwind.
@@ -31,6 +31,7 @@ static inline int
 update_frame_state (struct cursor *c)
 {
   unw_word_t prev_ip, prev_sp, prev_bsp, ip, pr, num_regs, cfm;
+  unw_word_t bsp, bspstore, rnat_addr, ndirty, loadrs;
   int ret;
 
   prev_ip = c->ip;
@@ -48,6 +49,8 @@ update_frame_state (struct cursor *c)
       if (ret < 0)
 	return ret;
 
+      c->sigcontext_off = c->sigcontext_loc - c->sp;
+
       if (c->ip_loc == c->sigcontext_loc + SIGCONTEXT_BR_OFF + 0*8)
 	{
 	  /* Earlier kernels (before 2.4.19 and 2.5.10) had buggy
@@ -64,6 +67,40 @@ update_frame_state (struct cursor *c)
 	return ret;
 
       num_regs = cfm & 0x7f;		/* size of frame */
+
+      /* When Linux delivers a signal on an alternate stack, it does
+         things a bit differently from what the unwind conventions
+         allow us to describe: instead of saving ar.rnat, ar.bsp, and
+         ar.bspstore, it saves the former two plus the "loadrs" value.
+         Because of this, we need to detect & record a potential
+         rbs-area switch here manually... */
+      if (c->bsp_loc)
+	{
+	  /* If ar.bsp has been saved already AND the current bsp is
+	     not equal to the saved value, then we know for sure that
+	     we're past the point where the backing store has been
+	     switched (and before the point where it's restored).  */
+	  ret = ia64_get (c, c->sigcontext_loc + SIGCONTEXT_AR_BSP_OFF, &bsp);
+	  if (ret < 0)
+	    return ret;
+
+	  if (bsp != c->bsp)
+	    {
+	      assert (c->rnat_loc);
+
+	      ret = ia64_get (c, c->sigcontext_loc + SIGCONTEXT_LOADRS_OFF,
+			      &loadrs);
+	      if (ret < 0)
+		return ret;
+
+	      loadrs >>= 16;
+	      ndirty = ia64_rse_num_regs (c->bsp - loadrs, c->bsp);
+	      bspstore = ia64_rse_skip_regs (bsp, -ndirty);
+	      ret = rbs_record_switch (c, bsp, bspstore, c->rnat_loc);
+	      if (ret < 0)
+		return ret;
+	    }
+	}
     }
   else
     {
@@ -72,7 +109,25 @@ update_frame_state (struct cursor *c)
 	return ret;
       num_regs = (cfm >> 7) & 0x7f;	/* size of locals */
     }
+
+  if (unlikely (c->pi.flags & UNW_PI_FLAG_IA64_RBS_SWITCH))
+    {
+      if ((ret = ia64_get (c, c->bsp_loc, &bsp)) < 0
+	  || (ret = ia64_get (c, c->bspstore_loc, &bspstore)) < 0
+	  || (ret = ia64_get (c, c->rnat_loc, &rnat_addr)) < 0)
+	return ret;
+
+      if (bsp != c->bsp)
+	{
+	  ret = rbs_record_switch (c, bsp, bspstore, rnat_addr);
+	  if (ret < 0)
+	    return ret;
+	}
+    }
+
   c->bsp = ia64_rse_skip_regs (c->bsp, -num_regs);
+  if (c->rbs_area[c->rbs_curr].end - c->bsp > c->rbs_area[c->rbs_curr].size)
+    rbs_underflow (c);
 
   /* update the IP cache: */
   ret = ia64_get (c, c->ip_loc, &ip);