From eaa517f3022887e368e1a7ee8da6ca55747d2e17 Mon Sep 17 00:00:00 2001
From: Rikard Nordgren <hrn@posteo.net>
Date: Wed, 8 Apr 2026 19:31:39 +0200
Subject: [PATCH] Update toggle_sign_FAC1

---
 STATUS.md                              |  2 +-
 src/basic/math/bfb4.toggle_sign_FAC1.s | 30 +++++++++++---------------
 tools/similarity.c                     |  3 ++-
 3 files changed, 15 insertions(+), 20 deletions(-)

diff --git a/STATUS.md b/STATUS.md
index 55c17c18..38842206 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -255,7 +255,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$BF74`   | `sqr_FAC2`             | NOT DONE |                                                    |
 | `$BF78`   | `pwr_FAC2_MEM`         | PARTIAL  | needs `pwr_FAC2_FAC1`                              |
 | `$BF7B`   | `pwr_FAC2_FAC1`        | NOT DONE |                                                    |
-| `$BFB4`   | `toggle_sign_FAC1`     | DONE     |                                                    |
+| `$BFB4`   | `toggle_sign_FAC1`     | DONE     | From the Microsoft BASIC, original name: NEGOP     |
 | `$BFED`   | `exp_FAC1`             | NOT DONE |                                                    |
 | `$E043`   | `poly1_FAC1`           | PARTIAL  | not fully tested yet                               |
 | `$E059`   | `poly2_FAC1`           | PARTIAL  | not fully tested yet                               |
diff --git a/src/basic/math/bfb4.toggle_sign_FAC1.s b/src/basic/math/bfb4.toggle_sign_FAC1.s
index c3b7b079..5ac7f1ad 100644
--- a/src/basic/math/bfb4.toggle_sign_FAC1.s
+++ b/src/basic/math/bfb4.toggle_sign_FAC1.s
@@ -3,29 +3,23 @@
 ;; #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 - toggle FAC1 sign (if not 0)
 ;
+; This is verified to be identical to the original Microsoft implementation where it was named NEGOP.
+;
 ; See also:
 ; - [CM64] Computes Mapping the Commodore 64 - page 117
 ; - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
 ;
 
 toggle_sign_FAC1:
-
-	lda FAC1_exponent
-	beq toggle_sign_FAC1_end           ; do not toggle the sign if value is 0
-
-	; FALLTROUGH
-
-toggle_sign_FAC1_skipcheck:            ; entry for other routines
-
-	lda FAC1_sign
-	eor #$FF
-	sta FAC1_sign
-
-	; FALLTROUGH
-
-toggle_sign_FAC1_end:
-
-	rts
+        lda FAC1_exponent
+        beq NEGRTS
+        lda FAC1_sign
+        eor #$FF
+        sta FAC1_sign
+NEGRTS:
+        rts
diff --git a/tools/similarity.c b/tools/similarity.c
index f6e4dac8..f199988d 100644
--- a/tools/similarity.c
+++ b/tools/similarity.c
@@ -20,7 +20,7 @@ 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] = {
     0xB391, 0xB39D,
@@ -28,6 +28,7 @@ unsigned short microsoft_basic_chunks[NUM_CHUNKS * 2] = {
     0xB849, 0xB97D,
     0xB983, 0xB9BB,
     0xBA28, 0xBCF2,
+    0xBFB4, 0xBFBE,
 };