From 00c74cdd48df985746e4fd718093fd29bbd72e33 Mon Sep 17 00:00:00 2001
From: Rikard Nordgren <hrn@posteo.net>
Date: Fri, 30 Jan 2026 21:09:13 +0100
Subject: [PATCH] Add clear_FAC1

---
 STATUS.md                        |  1 +
 src/basic/math/bce9.clear_FAC1.s | 28 ++++++++++++++++++++++++++++
 tools/similarity.c               |  5 +++--
 3 files changed, 32 insertions(+), 2 deletions(-)
 create mode 100644 src/basic/math/bce9.clear_FAC1.s

diff --git a/STATUS.md b/STATUS.md
index d670befd..b4fd9be6 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -241,6 +241,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$BC5B`   | `FCOMP`                | DONE     |                                                    |
 | `$BC9B`   | `QINT`                 | NOT DONE | convert FAC1 to 32 bit signed integer              |
 | `$BCCC`   | `int_FAC1`             | DONE     |                                                    |
+| `$BCE9`   | `clear_FAC1`           | DONE     | From the Microsoft BASIC, original name: CLRFAC    |
 | `$BCF3`   | `FIN`                  | PARTIAL  | Only for 38k memory model + not fully tested yet   |
 | `$BD7E`   | `FINLOG`               | DONE     |                                                    |
 | `$BDDD`   | `FOUT`                 | NOT DONE | outputs FAC1 to string at $0100                    |
diff --git a/src/basic/math/bce9.clear_FAC1.s b/src/basic/math/bce9.clear_FAC1.s
new file mode 100644
index 00000000..c7fe81f0
--- /dev/null
+++ b/src/basic/math/bce9.clear_FAC1.s
@@ -0,0 +1,28 @@
+;; #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 - clear FAC1
+;
+; Set each byte of the mantissa of FAC1 to the value of A
+;
+; This is verified to be identical to the original Microsoft implementation where it was named CLRFAC.
+;
+; See also:
+; - https://www.c64-wiki.com/wiki/BASIC-ROM
+; - https://sta.c64.org/cbm64basconv.html
+;
+
+
+clear_FAC1:
+    sta FAC1_mantissa+0
+    sta FAC1_mantissa+1
+    sta FAC1_mantissa+2
+    sta FAC1_mantissa+3
+    tay
+clear_FAC1_rts:             ; Return point needed elsewhere
+    rts
diff --git a/tools/similarity.c b/tools/similarity.c
index bfeabe25..1f156c7d 100644
--- a/tools/similarity.c
+++ b/tools/similarity.c
@@ -20,14 +20,15 @@ int matches[MAX_SIZE];
 
 int phase_mask[MAX_SIZE+MAX_SIZE];
 
-#define NUM_CHUNKS 5
+#define NUM_CHUNKS 6
 // 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,
     0xB947, 0xB97D,
     0xB983, 0xB9BB,
     0xBBA2, 0xBC1A,
-    0xBC58, 0xBC5A
+    0xBC58, 0xBC5A,
+    0xBCE9, 0xBCF2,
 };