From 03db0610a882934355e34a4bd424671fe2b3ab45 Mon Sep 17 00:00:00 2001
From: Rikard Nordgren <hrn@posteo.net>
Date: Thu, 29 Jan 2026 20:12:53 +0100
Subject: [PATCH] Add inc_FAC1

---
 STATUS.md                      |  1 +
 src/basic/math/b96F.inc_FAC1.s | 30 ++++++++++++++++++++++++++++++
 tools/similarity.c             |  3 ++-
 3 files changed, 33 insertions(+), 1 deletion(-)
 create mode 100644 src/basic/math/b96F.inc_FAC1.s

diff --git a/STATUS.md b/STATUS.md
index 2e526373..ff2a3d89 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -212,6 +212,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$B86A`   | `add_FAC2_FAC1`        | DONE     |                                                    |
 | `$B8FE`   | `normal_FAC1`          | DONE     |                                                    |
 | `$B947`   | `inv_FAC1_mantissa`    | DONE     |                                                    |
+| `$B96F`   | `inc_FAC1`             | DONE     | From the Microsoft BASIC, original name: INCFAC    |
 | `$B9EA`   | `log_FAC1`             | PARTIAL  | not fully tested yet                               |
 | `$BA28`   | `mul_MEM_FAC1`         | DONE     |                                                    |
 | `$BA2B`   | `mul_FAC2_FAC1`        | DONE     |                                                    |
diff --git a/src/basic/math/b96F.inc_FAC1.s b/src/basic/math/b96F.inc_FAC1.s
new file mode 100644
index 00000000..80bdba00
--- /dev/null
+++ b/src/basic/math/b96F.inc_FAC1.s
@@ -0,0 +1,30 @@
+;; #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 - increment FAC1
+;
+; This is verified to be identical to the original Microsoft implementation where it was named INCFAC.
+;
+; See also:
+; - https://www.c64-wiki.com/wiki/BASIC-ROM
+; - https://sta.c64.org/cbm64basconv.html
+;
+
+inc_FAC1:
+
+    inc FAC1_mantissa+3
+    bne inc_FAC1_ret
+    inc FAC1_mantissa+2
+    bne inc_FAC1_ret
+    inc FAC1_mantissa+1
+    bne inc_FAC1_ret
+    inc FAC1_mantissa+0
+
+inc_FAC1_ret:
+
+    rts
diff --git a/tools/similarity.c b/tools/similarity.c
index 9b5573a5..2fbbb228 100644
--- a/tools/similarity.c
+++ b/tools/similarity.c
@@ -20,10 +20,11 @@ int matches[MAX_SIZE];
 
 int phase_mask[MAX_SIZE+MAX_SIZE];
 
-#define NUM_CHUNKS 3
+#define NUM_CHUNKS 4
 // Pairs of start, end memory addresses of known exact similarity with the open source Microsoft BASIC
 unsigned short microsoft_basic_chunks[NUM_CHUNKS * 2] = {
     0xB849, 0xB84F,
+    0xB96F, 0xB97D,
     0xBBA2, 0xBC1A,
     0xBC58, 0xBC5A
 };