diff --git a/repos/base-hw/src/core/spec/x86_64/mode_transition.s b/repos/base-hw/src/core/spec/x86_64/mode_transition.s
index 08c200b53..78ca7d409 100644
--- a/repos/base-hw/src/core/spec/x86_64/mode_transition.s
+++ b/repos/base-hw/src/core/spec/x86_64/mode_transition.s
@@ -14,6 +14,9 @@
 
 .include "macros.s"
 
+/* size of pointer to CPU context */
+.set CONTEXT_PTR_SIZE, 1 * 8
+
 .section .text
 
 	/*
@@ -44,6 +47,7 @@
 	.p2align 2
 	.global _mt_client_context_ptr
 	_mt_client_context_ptr:
+	.space CONTEXT_PTR_SIZE
 
 	/* a globally mapped buffer per CPU */
 	.p2align 2