diff --git a/library/bn_mul.h b/library/bn_mul.h
index 93dd4b6bb..4ccd7b4b1 100644
--- a/library/bn_mul.h
+++ b/library/bn_mul.h
@@ -248,6 +248,8 @@
 
 #endif /* AMD64 */
 
+// The following assembly code assumes that a pointer will fit in a 64-bit register
+// (including ILP32 __aarch64__ ABIs such as on watchOS, hence the 2^32 - 1)
 #if defined(__aarch64__) && (UINTPTR_MAX == 0xfffffffful || UINTPTR_MAX == 0xfffffffffffffffful)
 
 #define MULADDC_X1_INIT             \