Update sub_FAC2_FAC1

src/basic/math/b853.sub_FAC2_FAC1.s

@@ -3,23 +3,29 @@
 ;; #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 FAC2
 ;
+; This is verified to be identical to the original Microsoft implementation where it was named FSUBT.
+;
 ; See also:
 ; - [CM64] Computes Mapping the Commodore 64 - page 112
 ; - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
 ; - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
 ;
 
-; XXX test this
 
 sub_FAC2_FAC1:
 
-	; Toggle FAC1 sign
-	lda FAC1_sign
-	eor #$FF
-	sta FAC1_sign
+    ; Toggle FAC1 sign
+    lda FAC1_sign
+    eor #$FF
+    sta FAC1_sign
+    eor FAC2_sign
+    sta ARISGN
+    lda FAC1_exponent       ; Set flags from exponent
 
 	; Perform addition
-	jmp add_FAC2_FAC1
+    jmp add_FAC2_FAC1