From abd67c40f9ca3389247bfb72feb11701a190e4cf Mon Sep 17 00:00:00 2001
From: Rikard Nordgren <hrn@posteo.net>
Date: Fri, 10 Apr 2026 20:32:55 +0200
Subject: [PATCH] Add floating point constants for FOUT

---
 src/basic/math_consts/bdb3.consts_FOUT.s | 24 ++++++++++++++++++++++++
 tools/generate_constants.cc              |  5 +++++
 tools/similarity.c                       |  3 ++-
 3 files changed, 31 insertions(+), 1 deletion(-)
 create mode 100644 src/basic/math_consts/bdb3.consts_FOUT.s

diff --git a/src/basic/math_consts/bdb3.consts_FOUT.s b/src/basic/math_consts/bdb3.consts_FOUT.s
new file mode 100644
index 00000000..32433be6
--- /dev/null
+++ b/src/basic/math_consts/bdb3.consts_FOUT.s
@@ -0,0 +1,24 @@
+;; #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 - constants used by FOUT
+;
+; This is verified to be identical to the original Microsoft implementation
+;
+
+const_NZ0999:
+
+	+PUT_CONST_NZ0999
+	
+const_NZ9999:
+
+	+PUT_CONST_NZ9999
+
+const_NZMIL:
+
+	+PUT_CONST_NZMIL
diff --git a/tools/generate_constants.cc b/tools/generate_constants.cc
index 2f3c2cd7..a3023eef 100644
--- a/tools/generate_constants.cc
+++ b/tools/generate_constants.cc
@@ -58,6 +58,11 @@ std::vector<ConstEntry> GLOBAL_constants =
 	ConstEntry(     "LOG_2", std::log(2.0)        ),
 	ConstEntry( "INV_LOG_2", 1.0 / std::log(2.0)  ),
 
+    // Constants for FOUT
+    ConstEntry(    "NZ0999", 99999999.9           ),        // Comment in MS-BASIC says 99999999.9499, but that is off by one ulp
+    ConstEntry(    "NZ9999", 999999999.2          ),        // Comment is MS-BASIC says 999999999.499, but that is off by one ulp
+    ConstEntry(     "NZMIL", 1e9                  ),
+
 	// Constants for exp approximation, taken from https://www.c64-wiki.com/wiki/POLY1
 	// original source is probably "Computer Approximations" by John Fraser Hart et al. (ISBN 0-88275-642-7)
 	ConstEntry( "POLY_EXP_1", 2.1498763701e-5 ),
diff --git a/tools/similarity.c b/tools/similarity.c
index fe1a7e05..18c19f33 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 8
+#define NUM_CHUNKS 9
 // 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,
@@ -29,6 +29,7 @@ unsigned short microsoft_basic_chunks[NUM_CHUNKS * 2] = {
     0xB983, 0xB9BB,
     0xBA28, 0xBCF2,
     0xBD7E, 0xBD90,
+    0xBDB3, 0xBDC1,
     0xBF11, 0xBF51,
     0xBFB4, 0xBFBE,
 };