From dbc603322e5a010293a799adb2a6842eab779e0d Mon Sep 17 00:00:00 2001
From: Rikard Nordgren <hrn@posteo.net>
Date: Thu, 29 Jan 2026 20:19:36 +0100
Subject: [PATCH] Update inv_FAC1

---
 STATUS.md                               |  2 +-
 src/basic/math/b947.inv_FAC1.s          | 44 +++++++++++++++++++++++++
 src/basic/math/b947.inv_FAC1_mantissa.s | 36 --------------------
 tools/similarity.c                      |  2 +-
 4 files changed, 46 insertions(+), 38 deletions(-)
 create mode 100644 src/basic/math/b947.inv_FAC1.s
 delete mode 100644 src/basic/math/b947.inv_FAC1_mantissa.s

diff --git a/STATUS.md b/STATUS.md
index ff2a3d89..0e484ed3 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -211,7 +211,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$B867`   | `add_MEM_FAC1`         | DONE     |                                                    |
 | `$B86A`   | `add_FAC2_FAC1`        | DONE     |                                                    |
 | `$B8FE`   | `normal_FAC1`          | DONE     |                                                    |
-| `$B947`   | `inv_FAC1_mantissa`    | DONE     |                                                    |
+| `$B947`   | `inv_FAC1_mantissa`    | DONE     | From the Microsoft BASIC, original name: NEGFAC    |
 | `$B96F`   | `inc_FAC1`             | DONE     | From the Microsoft BASIC, original name: INCFAC    |
 | `$B9EA`   | `log_FAC1`             | PARTIAL  | not fully tested yet                               |
 | `$BA28`   | `mul_MEM_FAC1`         | DONE     |                                                    |
diff --git a/src/basic/math/b947.inv_FAC1.s b/src/basic/math/b947.inv_FAC1.s
new file mode 100644
index 00000000..828a1382
--- /dev/null
+++ b/src/basic/math/b947.inv_FAC1.s
@@ -0,0 +1,44 @@
+;; #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 - neg (2s complement) FAC1
+;
+; This is verified to be identical to the original Microsoft implementation where it was named NEGFAC.
+;
+; The alternative entry point neg_FAC1_mantissa (NEGFCH) will negate only the mantissa
+;
+; See also:
+; - https://www.c64-wiki.com/wiki/BASIC-ROM
+; - https://sta.c64.org/cbm64basconv.html
+;
+
+neg_FAC1:
+	lda FAC1_sign
+	eor #$FF
+	sta FAC1_sign
+
+neg_FAC1_mantissa:
+
+    lda FAC1_mantissa+0
+    eor #$FF
+    sta FAC1_mantissa+0
+    lda FAC1_mantissa+1
+    eor #$FF
+    sta FAC1_mantissa+1
+    lda FAC1_mantissa+2
+    eor #$FF
+    sta FAC1_mantissa+2
+    lda FAC1_mantissa+3
+    eor #$FF
+    sta FAC1_mantissa+3
+    lda FACOV
+    eor #$FF
+    sta FACOV
+    bne inc_FAC1_ret
+    
+    ; Fallthrough to inc_FAC1
diff --git a/src/basic/math/b947.inv_FAC1_mantissa.s b/src/basic/math/b947.inv_FAC1_mantissa.s
deleted file mode 100644
index ce7429f5..00000000
--- a/src/basic/math/b947.inv_FAC1_mantissa.s
+++ /dev/null
@@ -1,36 +0,0 @@
-;; #LAYOUT# STD *       #TAKE
-;; #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
-;; #LAYOUT# *   BASIC_0 #TAKE
-;; #LAYOUT# *   *       #IGNORE
-
-;
-; Math package - invert FAC1 mantissa
-;
-; See also:
-; - [CM64] Computes Mapping the Commodore 64 - page 113
-; - https://www.c64-wiki.com/wiki/BASIC-ROM
-; - https://sta.c64.org/cbm64basconv.html
-;
-
-; 2-complement the FAC1 mantissa
-; The 4 byte mantissa is treated as an integer in big endian form
-
-
-inv_FAC1_mantissa:
-
-    ldx #3
-@1: lda FAC1_mantissa,X
-    eor #$FF
-    sta FAC1_mantissa,X
-    dex
-    bpl @1
-
-    inc FAC1_mantissa+3
-    bne @2
-    inc FAC1_mantissa+2
-    bne @2
-    inc FAC1_mantissa+1
-    bne @2
-    inc FAC1_mantissa
-    
-@2: rts
diff --git a/tools/similarity.c b/tools/similarity.c
index 2fbbb228..66de278b 100644
--- a/tools/similarity.c
+++ b/tools/similarity.c
@@ -24,7 +24,7 @@ int phase_mask[MAX_SIZE+MAX_SIZE];
 // 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,
+    0xB947, 0xB97D,
     0xBBA2, 0xBC1A,
     0xBC58, 0xBC5A
 };