diff --git a/sys/powerpc/include/reg.h b/sys/powerpc/include/reg.h
index e188f02f7b08..640365546e7c 100644
--- a/sys/powerpc/include/reg.h
+++ b/sys/powerpc/include/reg.h
@@ -4,6 +4,7 @@
 #ifndef _POWERPC_REG_H_
 #define	_POWERPC_REG_H_
 
+/* Must match struct trapframe */
 struct reg {
 	register_t fixreg[32];
 	register_t lr;
@@ -13,6 +14,7 @@ struct reg {
 	register_t pc;
 };
 
+/* Must match pcb.pcb_fpu */
 struct fpreg {
 	double fpreg[32];
 	double fpscr;