diff --git a/include/macros.h b/include/macros.h
index f8189f3..bf7627f 100644
--- a/include/macros.h
+++ b/include/macros.h
@@ -21,6 +21,7 @@
 #define ROUND_DOWN(x, k) ((x) & ~((k) - 1))
 #define ROUND_UP(x, k) (((x) + (k) - 1) & ~((k) - 1))
 
+#define mask(x, k) ((x) & ((1 << (k)) - 1))
 #define align(x, k) ((x) & ~((1 << (k)) - 1))
 #define align_up(x, k) align(x + ((1 << (k)) - 1), k)
 #define is_aligned(x, k) (!((x) & ((1 << (k)) - 1)))