From 37e2ec9d9fc59be8942db5e7f8a6d62f96ebacd2 Mon Sep 17 00:00:00 2001
From: Rikard Nordgren <hrn@posteo.net>
Date: Sun, 1 Feb 2026 11:25:29 +0100
Subject: [PATCH] Update sgn_FAC1_A

---
 STATUS.md                        |  2 +-
 src/basic/math/bc2b.sgn_FAC1_A.s | 30 +++++++++++++++++++-----------
 tools/similarity.c               |  3 ++-
 3 files changed, 22 insertions(+), 13 deletions(-)

diff --git a/STATUS.md b/STATUS.md
index 361d229b..c3e2f405 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -234,7 +234,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$BC0C`   | `mov_r_FAC1_FAC2`      | DONE     | From the Microsoft BASIC, original name: MOVAF     |
 | `$BC0F`   | `mov_FAC1_FAC2`        | DONE     | From the Microsoft BASIC, original name: MOVEF     |
 | `$BC1B`   | `round_FAC1`           | PARTIAL  | not fully tested yet                               |
-| `$BC2B`   | `sgn_FAC1_A`           | DONE     |                                                    |
+| `$BC2B`   | `sgn_FAC1_A`           | DONE     | From the Microsoft BASIC, original name: SIGN      |
 | `$BC39`   | `sgn_FAC1`             | DONE     |                                                    |
 | `$BC3C`   | `convert_A_to_FAC1`    | DONE     |                                                    |
 | `$BC44`   | `convert_i16_to_FAC1`  | DONE     |                                                    |
diff --git a/src/basic/math/bc2b.sgn_FAC1_A.s b/src/basic/math/bc2b.sgn_FAC1_A.s
index 27b10ab6..d34d7bfd 100644
--- a/src/basic/math/bc2b.sgn_FAC1_A.s
+++ b/src/basic/math/bc2b.sgn_FAC1_A.s
@@ -3,12 +3,20 @@
 ;; #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 - evaluate FAC1 sign, to .A
 ;
 ; Output:
 ; - .A - 0 for FAC1 equal 0, 1 for FAC1 > 0, -1 for FAC1 < 0
 ;
+; This is verified to be identical to the original Microsoft implementation where it was named SIGN.
+;
+; Alternative entry points:
+; - sgn_A : Original FCOMPS. Sign of A as 0 or -1 in A
+; - sgn_FAC1_A_nozero : Original FCSIGN. No special treatment for 0.
+;
 ; See also:
 ; - [CM64] Computes Mapping the Commodore 64 - page 115
 ; - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
@@ -17,14 +25,14 @@
 
 sgn_FAC1_A:
 
-	lda FAC1_exponent
-	beq @1
-	lda FAC1_sign
-	bmi @2
-	lda #$01
-@1:
-	rts
-	
-@2:
-    lda #$FF
+    lda FAC1_exponent
+    beq SIGN_ret
+sgn_FAC1_A_nozero:
+    lda FAC1_sign
+sgn_A:
+    rol
+    lda #$FF            ; Assume negative
+    bcs SIGN_ret
+    lda #$01            ; Positive
+SIGN_ret:
     rts
diff --git a/tools/similarity.c b/tools/similarity.c
index fcf29a83..4a5d20db 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 7
+#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] = {
     0xB849, 0xB84F,
@@ -28,6 +28,7 @@ unsigned short microsoft_basic_chunks[NUM_CHUNKS * 2] = {
     0xB947, 0xB97D,
     0xB983, 0xB9BB,
     0xBBA2, 0xBC1A,
+    0xBC2B, 0xBC38,
     0xBC58, 0xBC5A,
     0xBC9B, 0xBCF2,
 };