From a5f32a48e80c3bff823240e47be69d4296af8104 Mon Sep 17 00:00:00 2001
From: Rikard Nordgren <hrn@posteo.net>
Date: Wed, 4 Feb 2026 20:22:25 +0100
Subject: [PATCH] Add shift_carry_into_FAC1

---
 STATUS.md                                   |  1 +
 src/basic/math/b936.shift_carry_into_FAC1.s | 34 +++++++++++++++++++++
 tools/similarity.c                          |  5 ++-
 3 files changed, 37 insertions(+), 3 deletions(-)
 create mode 100644 src/basic/math/b936.shift_carry_into_FAC1.s

diff --git a/STATUS.md b/STATUS.md
index 86bee4ca..e83e253b 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -213,6 +213,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$B8D2`   | `abs_and_normal_FAC`   | DONE     | From the Microsoft BASIC, original name: FADFLT    |
 | `$B8D7`   | `normal_FAC1`          | DONE     | From the Microsoft BASIC, original name: NORMAL    |
 | `$B8F7`   | `zero_FAC1`            | DONE     | From the Microsoft BASIC, original name: ZEROFC    |
+| `$B936`   | `shift_carry_into_FAC` | DONE     | From the Microsoft BASIC, original name: SQUEEZ    |
 | `$B947`   | `inv_FAC1_mantissa`    | DONE     | From the Microsoft BASIC, original name: NEGFAC    |
 | `$B96F`   | `inc_FAC1`             | DONE     | From the Microsoft BASIC, original name: INCFAC    |
 | `$B983`   | `shiftr_FAC1`          | DONE     | From the Microsoft BASIC, original name: SHIFTR    |
diff --git a/src/basic/math/b936.shift_carry_into_FAC1.s b/src/basic/math/b936.shift_carry_into_FAC1.s
new file mode 100644
index 00000000..be527d90
--- /dev/null
+++ b/src/basic/math/b936.shift_carry_into_FAC1.s
@@ -0,0 +1,34 @@
+;; #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 - shift carry into FAC1
+;
+; This is verified to be identical to the original Microsoft implementation where it was named SQUEEZ.
+;
+; Alternative entry point: RNDSHF to also shift in a CF=0
+;
+; See also:
+; - [CM64] Computes Mapping the Commodore 64 - page 113
+; - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
+; - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
+;
+
+shift_carry_into_FAC1:
+
+SQUEEZ:
+    bcc RNDRTS
+RNDSHF:
+    inc FAC1_exponent
+    beq overflow_error
+    ror FAC1_mantissa+0
+    ror FAC1_mantissa+1
+    ror FAC1_mantissa+2
+    ror FAC1_mantissa+3
+    ror FACOV
+RNDRTS:
+    rts
diff --git a/tools/similarity.c b/tools/similarity.c
index 4950143f..c985ca27 100644
--- a/tools/similarity.c
+++ b/tools/similarity.c
@@ -20,15 +20,14 @@ int matches[MAX_SIZE];
 
 int phase_mask[MAX_SIZE+MAX_SIZE];
 
-#define NUM_CHUNKS 10
+#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,
     0xB3A2, 0xB3A5,
     0xB849, 0xB869,
     0xB8D2, 0xB8FD,
-    0xB91D, 0xB935,
-    0xB947, 0xB97D,
+    0xB91D, 0xB97D,
     0xB983, 0xB9BB,
     0xBA8C, 0xBAB6,
     0xBBA2, 0xBC1A,