From 113b3cf7c28904f7cbfa46c4e2424e0835d31e49 Mon Sep 17 00:00:00 2001
From: Rikard Nordgren <hrn@posteo.net>
Date: Tue, 14 Apr 2026 19:28:50 +0200
Subject: [PATCH] Update cos_FAC1

---
 STATUS.md                            |  2 +-
 src/basic/math_trans/e264.cos_FAC1.s | 11 ++++++++---
 tools/similarity.c                   |  3 ++-
 3 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/STATUS.md b/STATUS.md
index da4fea08..421b706d 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -262,7 +262,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$E097`   | `rnd_FAC1`             | DONE     |                                                    |
 | `$E09A`   | `rnd_A`                | DONE     |                                                    |
 | `$E0BE`   | `rnd_generate`         | DONE     |                                                    |
-| `$E264`   | `cos_FAC1`             | PARTIAL  | not fully tested yet                               |
+| `$E264`   | `cos_FAC1`             | DONE     | From the Microsoft BASIC, original name: COS       |
 | `$E26B`   | `sin_FAC1`             | PARTIAL  | not fully tested yet                               |
 | `$E2B4`   | `tan_FAC1`             | PARTIAL  | not fully tested yet                               |
 | `$E30E`   | `atn_FAC1`             | DONE     | From the Microsoft BASIC, original name: ATN       |
diff --git a/src/basic/math_trans/e264.cos_FAC1.s b/src/basic/math_trans/e264.cos_FAC1.s
index 9376777b..d6f940ee 100644
--- a/src/basic/math_trans/e264.cos_FAC1.s
+++ b/src/basic/math_trans/e264.cos_FAC1.s
@@ -2,9 +2,13 @@
 ;; #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 - cosine of FAC1 in radians
 ;
+; This is verified to be identical to the original Microsoft implementation where it was named COS
+;
 ; Adds pi/2 to FAC1 and falls through to the sine function
 ;
 ; See also:
@@ -18,10 +22,11 @@
 
 
 COS:
-    ldy #>const_HALF_PI
+
     lda #<const_HALF_PI
+    ldy #>const_HALF_PI
     jsr add_MEM_FAC1
-    
+ 
     ; FALLTHROUGH to SIN
 
 
diff --git a/tools/similarity.c b/tools/similarity.c
index e304bede..b32a2c70 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 10
+#define NUM_CHUNKS 11
 // 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,
@@ -32,6 +32,7 @@ unsigned short microsoft_basic_chunks[NUM_CHUNKS * 2] = {
     0xBDB3, 0xBDC1,
     0xBDDD, 0xBF51,
     0xBFB4, 0xBFEC,
+    0xE264, 0xE26A,
     0xE30E, 0xE37A,
 };