Index: kernel/arch/ia32/src/userspace.c
===================================================================
--- kernel/arch/ia32/src/userspace.c	(revision b5a1730ff354c85c5705e67a0537b3a6768217ac)
+++ kernel/arch/ia32/src/userspace.c	(revision 61bacded878f459ea118aa64c4d85fac0742a54a)
@@ -71,4 +71,8 @@
 		"pushl %4\n"
 		"movl %5, %%eax\n"
+
+		/* %ebx is defined to hold pcb_ptr - set it to 0 */
+		"xorl %%ebx, %%ebx\n"	
+
 		"iret\n"
 		: