diff --git a/kernel/riscv.h b/kernel/riscv.h
index 1f8a503..7e358f1 100644
--- a/kernel/riscv.h
+++ b/kernel/riscv.h
@@ -183,6 +183,7 @@ w_mtvec(uint64 x)
   asm volatile("csrw mtvec, %0" : : "r" (x));
 }
 
+// Physical Memory Protection
 static inline void
 w_pmpcfg0(uint64 x)
 {