Add add_mantissas_FAC2_FAC1

src/basic/math/b8fe.add_mantissas_FAC2_FAC1.s

@@ -0,0 +1,41 @@
+;; #LAYOUT# STD *       #TAKE
+;; #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
+;; #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 - add FAC2 mantissa to FAC1
+;
+; This is verified to be identical to the original Microsoft implementation where it was named FADD2.
+;
+;
+; 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
+;
+
+add_mantissas_FAC2_FAC1:
+
+    adc OLDOV
+    sta FACOV
+
+    lda FAC1_mantissa+3
+    adc FAC2_mantissa+3
+    sta FAC1_mantissa+3
+
+    lda FAC1_mantissa+2
+    adc FAC2_mantissa+2
+    sta FAC1_mantissa+2
+
+    lda FAC1_mantissa+1
+    adc FAC2_mantissa+1
+    sta FAC1_mantissa+1
+
+    lda FAC1_mantissa+0
+    adc FAC2_mantissa+0
+    sta FAC2_mantissa+0
+
+    jmp shift_carry_into_FAC1