From 7eda12aa3401d4c9c92776b046e424369ea2a7db Mon Sep 17 00:00:00 2001
From: Rikard Nordgren <hrn@posteo.net>
Date: Wed, 8 Apr 2026 21:21:23 +0200
Subject: [PATCH] Update FINLOG

---
 STATUS.md                    |  2 +-
 src/basic/math/bd7e.FINLOG.s | 21 ++++++++++++++-------
 tools/similarity.c           |  3 ++-
 3 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/STATUS.md b/STATUS.md
index 38842206..138d7653 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -249,7 +249,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$BCCC`   | `int_FAC1`             | DONE     | From the Microsoft BASIC, original name: INT       |
 | `$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     |                                                    |
+| `$BD7E`   | `FINLOG`               | DONE     | From the Microsoft BASIC, original name: FINLOG    |
 | `$BDDD`   | `FOUT`                 | NOT DONE | outputs FAC1 to string at $0100                    |
 | `$BF71`   | `sqr_FAC1`             | PARTIAL  | needs `sqr_FAC2`                                   |
 | `$BF74`   | `sqr_FAC2`             | NOT DONE |                                                    |
diff --git a/src/basic/math/bd7e.FINLOG.s b/src/basic/math/bd7e.FINLOG.s
index a5980a36..7bd8570f 100644
--- a/src/basic/math/bd7e.FINLOG.s
+++ b/src/basic/math/bd7e.FINLOG.s
@@ -3,17 +3,24 @@
 ;; #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 an ASCII digit in A that has been converted to a signed integer to FAC1
 ;
+; This is verified to be identical to the original Microsoft implementation where it was named FINLOG.
+;
 ; See also:
 ; - [CM64] Computes Mapping the Commodore 64 - page 113
 ;
 
 FINLOG:
-    pha
-    jsr mov_FAC1_FAC2
-    pla
-    jsr convert_A_to_FAC1
-    jsr add_FAC2_FAC1
-    rts
+        pha
+        jsr mov_r_FAC1_FAC2
+        pla
+        jsr convert_A_to_FAC1
+        lda FAC2_sign
+        eor FAC1_sign
+        sta ARISGN                  ; Resultant sign
+        ldx FAC1_exponent           ; Set signs on thing to add
+        jmp add_FAC2_FAC1
diff --git a/tools/similarity.c b/tools/similarity.c
index f199988d..be5e9bc4 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 6
+#define NUM_CHUNKS 7
 // 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,
+    0xBD7E, 0xBD90,
     0xBFB4, 0xBFBE,
 };