diff --git a/src/P503/ARM64/fp_arm64.c b/src/P503/ARM64/fp_arm64.c
index c8fcf75..521d85a 100644
--- a/src/P503/ARM64/fp_arm64.c
+++ b/src/P503/ARM64/fp_arm64.c
@@ -84,9 +84,9 @@ void mp_mul(const digit_t* a, const digit_t* b, digit_t* c, const unsigned int n
 
 
 void rdc_mont(const digit_t* ma, digit_t* mc)
-{ // Montgomery reduction
-  // mc = ma*R^-1 mod p503x2, where R = 2^768.
-  // If ma < 2^768*p503, the output mc is in the range [0, 2*p503-1].
+{ // Montgomery reduction exploiting special form of the prime.
+  // mc = ma*R^-1 mod p503x2, where R = 2^512.
+  // If ma < 2^512*p503, the output mc is in the range [0, 2*p503-1].
   // ma is assumed to be in Montgomery representation.
   
     rdc503_asm(ma, mc);