Update sub_MEM_FAC1

src/basic/math/b850.sub_MEM_FAC1.s

@@ -3,13 +3,17 @@
 ;; #LAYOUT# *   BASIC_0 #TAKE
 ;; #LAYOUT# *   *       #IGNORE
 
-;
+; This file is under the MIT license, it contains code released by Microsoft Corporation.
+; See LICENSE for more information.
+
 ; Math package - subtract FAC1 from memory variable
 ;
 ; Input:
 ; - .A - address low byte
 ; - .Y - address high byte
 ;
+; This is verified to be identical to the original Microsoft implementation where it was named FSUB.
+;
 ; See also:
 ; - [CM64] Computes Mapping the Commodore 64 - page 112
 ; - https://www.c64-wiki.com/wiki/Floating_point_arithmetic