diff --git a/src/stub/src/arch/amd64/regs.h b/src/stub/src/arch/amd64/regs.h
index a99ce818..5e2de60b 100644
--- a/src/stub/src/arch/amd64/regs.h
+++ b/src/stub/src/arch/amd64/regs.h
@@ -5,9 +5,9 @@
 #define arg2l  esi
 #define arg3  rdx
 #define arg3l  edx
-#define arg4  rcx
+#define arg4  rcx   /* clobbered by syscall */
 #define arg4l  ecx
-#define sys4   r10  # 4th arg to syscall is not in %rcx
+#define sys4   r10  /* 4th arg to syscall is not in %rcx */
 #define sys4l  r10d
 #define arg5  r8
 #define arg5l  r8d