diff --git a/sys/ia64/include/vmparam.h b/sys/ia64/include/vmparam.h
index 07fd4fbad73..bc0f78d6258 100644
--- a/sys/ia64/include/vmparam.h
+++ b/sys/ia64/include/vmparam.h
@@ -144,6 +144,8 @@
 #define	IA64_ID_PAGE_SIZE	(1<<(LOG2_ID_PAGE_SIZE))
 #define	IA64_ID_PAGE_MASK	(IA64_ID_PAGE_SIZE-1)
 
+#define	IA64_BACKINGSTORE	IA64_RR_BASE(4)
+
 /*
  * Mach derived constants
  */
diff --git a/sys/kern/kern_exec.c b/sys/kern/kern_exec.c
index fde4ee48773..b2ce458aaf0 100644
--- a/sys/kern/kern_exec.c
+++ b/sys/kern/kern_exec.c
@@ -868,7 +868,7 @@ exec_new_vmspace(imgp, sv)
 
 #ifdef __ia64__
 	/* Allocate a new register stack */
-	stack_addr = sv->sv_usrstack - 2 * maxssiz;
+	stack_addr = IA64_BACKINGSTORE;
 	error = vm_map_stack(map, stack_addr, (vm_size_t)maxssiz,
 	    sv->sv_stackprot, VM_PROT_ALL, MAP_STACK_GROWS_UP);
 	if (error)