Update sgn_FAC1_A

src/basic/math/bc2b.sgn_FAC1_A.s

@@ -3,12 +3,20 @@
 ;; #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 - evaluate FAC1 sign, to .A
 ;
 ; Output:
 ; - .A - 0 for FAC1 equal 0, 1 for FAC1 > 0, -1 for FAC1 < 0
 ;
+; This is verified to be identical to the original Microsoft implementation where it was named SIGN.
+;
+; Alternative entry points:
+; - sgn_A : Original FCOMPS. Sign of A as 0 or -1 in A
+; - sgn_FAC1_A_nozero : Original FCSIGN. No special treatment for 0.
+;
 ; See also:
 ; - [CM64] Computes Mapping the Commodore 64 - page 115
 ; - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
@@ -17,14 +25,14 @@
 
 sgn_FAC1_A:
 
-	lda FAC1_exponent
-	beq @1
-	lda FAC1_sign
-	bmi @2
-	lda #$01
-@1:
-	rts
-	
-@2:
-    lda #$FF
+    lda FAC1_exponent
+    beq SIGN_ret
+sgn_FAC1_A_nozero:
+    lda FAC1_sign
+sgn_A:
+    rol
+    lda #$FF            ; Assume negative
+    bcs SIGN_ret
+    lda #$01            ; Positive
+SIGN_ret:
     rts