diff --git a/wolfcrypt/src/sp_int.c b/wolfcrypt/src/sp_int.c
index cc6d89915..eecdd8c88 100644
--- a/wolfcrypt/src/sp_int.c
+++ b/wolfcrypt/src/sp_int.c
@@ -77,6 +77,7 @@ This library provides single precision (SP) integer math functions.
  *      called again until complete.
  * WOLFSSL_SP_FAST_NCT_EXPTMOD  Enables the faster non-constant time modular
  *      exponentation implementation.
+ * WOLFSSL_SP_INT_NEGATIVE      Enables negative values to be used.
  * WOLFSSL_SP_INT_DIGIT_ALIGN   Enable when unaligned access of sp_int_digit
  *                              pointer is not allowed.
  */
@@ -3107,25 +3108,33 @@ int sp_add_d(sp_int* a, sp_int_digit d, sp_int* r)
 {
     int err = MP_OKAY;
 
+    /* Check validity of parameters. */
     if ((a == NULL) || (r == NULL)) {
         err = MP_VAL;
     }
     else
     {
     #ifndef WOLFSSL_SP_INT_NEGATIVE
+        /* Positive only so just use internal function. */
         err = _sp_add_d(a, d, r);
     #else
         if (a->sign == MP_ZPOS) {
+            /* Positive so use interal function. */
             r->sign = MP_ZPOS;
             err = _sp_add_d(a, d, r);
         }
         else if ((a->used > 1) || (a->dp[0] > d)) {
+            /* Negative value bigger than digit so subtract digit. */
             r->sign = MP_NEG;
             _sp_sub_d(a, d, r);
         }
         else {
+            /* Negative value smaller or equal to digit. */
             r->sign = MP_ZPOS;
+            /* Subtract negative value from digit. */
             r->dp[0] = d - a->dp[0];
+            /* Result is a digit equal to or greater than zero. */
+            r->used = ((r->dp[0] == 0) ? 0 : 1);
         }
     #endif
     }
@@ -3149,25 +3158,32 @@ int sp_sub_d(sp_int* a, sp_int_digit d, sp_int* r)
 {
     int err = MP_OKAY;
 
+    /* Check validity of parameters. */
     if ((a == NULL) || (r == NULL)) {
         err = MP_VAL;
     }
     else {
     #ifndef WOLFSSL_SP_INT_NEGATIVE
+        /* Positive only so just use internal function. */
         _sp_sub_d(a, d, r);
     #else
         if (a->sign == MP_NEG) {
+            /* Subtracting from negative use interal add. */
             r->sign = MP_NEG;
             err = _sp_add_d(a, d, r);
         }
         else if ((a->used > 1) || (a->dp[0] >= d)) {
+            /* Positive number greater than digit so add digit. */
             r->sign = MP_ZPOS;
             _sp_sub_d(a, d, r);
         }
         else {
+            /* Negative value smaller than digit. */
             r->sign = MP_NEG;
+            /* Subtract positive value from digit. */
             r->dp[0] = d - a->dp[0];
-            r->used = r->dp[0] > 0;
+            /* Result is a digit equal to or greater than zero. */
+            r->used = 1;
         }
     #endif
     }