From 40653732fa3f708fff2ae2f35cf4f777bda8ff60 Mon Sep 17 00:00:00 2001
From: Rikard Nordgren <hrn@posteo.net>
Date: Wed, 4 Feb 2026 20:27:01 +0100
Subject: [PATCH] Add add_mantissas_FAC2_FAC1

---
 STATUS.md                                     |  1 +
 src/basic/math/b8fe.add_mantissas_FAC2_FAC1.s | 41 +++++++++++++++++++
 tools/similarity.c                            |  5 +--
 3 files changed, 44 insertions(+), 3 deletions(-)
 create mode 100644 src/basic/math/b8fe.add_mantissas_FAC2_FAC1.s

diff --git a/STATUS.md b/STATUS.md
index e83e253b..ceff2392 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -213,6 +213,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$B8D2`   | `abs_and_normal_FAC`   | DONE     | From the Microsoft BASIC, original name: FADFLT    |
 | `$B8D7`   | `normal_FAC1`          | DONE     | From the Microsoft BASIC, original name: NORMAL    |
 | `$B8F7`   | `zero_FAC1`            | DONE     | From the Microsoft BASIC, original name: ZEROFC    |
+| `$B8FE`   | `add_mantissas_FAC1..` | DONE     | From the Microsoft BASIC, original name: FADD2     |
 | `$B936`   | `shift_carry_into_FAC` | DONE     | From the Microsoft BASIC, original name: SQUEEZ    |
 | `$B947`   | `inv_FAC1_mantissa`    | DONE     | From the Microsoft BASIC, original name: NEGFAC    |
 | `$B96F`   | `inc_FAC1`             | DONE     | From the Microsoft BASIC, original name: INCFAC    |
diff --git a/src/basic/math/b8fe.add_mantissas_FAC2_FAC1.s b/src/basic/math/b8fe.add_mantissas_FAC2_FAC1.s
new file mode 100644
index 00000000..5daed3a4
--- /dev/null
+++ b/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
diff --git a/tools/similarity.c b/tools/similarity.c
index c985ca27..0ff05235 100644
--- a/tools/similarity.c
+++ b/tools/similarity.c
@@ -20,14 +20,13 @@ int matches[MAX_SIZE];
 
 int phase_mask[MAX_SIZE+MAX_SIZE];
 
-#define NUM_CHUNKS 9
+#define NUM_CHUNKS 8
 // Pairs of start, end memory addresses of known exact similarity with the open source Microsoft BASIC
 unsigned short microsoft_basic_chunks[NUM_CHUNKS * 2] = {
     0xB391, 0xB39D,
     0xB3A2, 0xB3A5,
     0xB849, 0xB869,
-    0xB8D2, 0xB8FD,
-    0xB91D, 0xB97D,
+    0xB8D2, 0xB97D,
     0xB983, 0xB9BB,
     0xBA8C, 0xBAB6,
     0xBBA2, 0xBC1A,