From 5b8dc457ab45b8a2f3300d436678a5da987d053c Mon Sep 17 00:00:00 2001
From: Rikard Nordgren <hrn@posteo.net>
Date: Thu, 29 Jan 2026 20:23:59 +0100
Subject: [PATCH] Add shiftr_FAC

---
 STATUS.md                        |  1 +
 src/basic/math/b983.shiftr_FAC.s | 65 ++++++++++++++++++++++++++++++++
 tools/similarity.c               |  3 +-
 3 files changed, 68 insertions(+), 1 deletion(-)
 create mode 100644 src/basic/math/b983.shiftr_FAC.s

diff --git a/STATUS.md b/STATUS.md
index 0e484ed3..d670befd 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -213,6 +213,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$B8FE`   | `normal_FAC1`          | DONE     |                                                    |
 | `$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    |
 | `$B9EA`   | `log_FAC1`             | PARTIAL  | not fully tested yet                               |
 | `$BA28`   | `mul_MEM_FAC1`         | DONE     |                                                    |
 | `$BA2B`   | `mul_FAC2_FAC1`        | DONE     |                                                    |
diff --git a/src/basic/math/b983.shiftr_FAC.s b/src/basic/math/b983.shiftr_FAC.s
new file mode 100644
index 00000000..6cf67738
--- /dev/null
+++ b/src/basic/math/b983.shiftr_FAC.s
@@ -0,0 +1,65 @@
+;; #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 a floating point register right
+;
+; The main entry point is shiftr_FAC which shifts any FAC to the right with sign extension
+; Inputs:
+; - A is the negative of the number of bits to shift
+; - X points to to the bytes to be shifted
+; Returns:
+; - Y = 0
+; - CF = 0
+; - A = overflowed bits
+;
+; Alternative entry point shift_for_mul_RES will shift the RES register for multiplication.
+; Alternative entry point short_shift_FAC will makes a short shift without sign extension
+;
+; This is verified to be identical to the original Microsoft implementation where it was named SHIFTR.
+;
+
+shift_for_mul_RES:
+    ldx #RESHO-1
+
+.bytes:
+    ldy 4,X             ; Shift entire bytes
+    sty FACOV
+    ldy 3,X
+    sty 4,X
+    ldy 2,X
+    sty 3,X
+    ldy 1,X
+    sty 2,X
+    ldy BITS
+    sty 1,X
+
+shiftr_FAC:
+    adc #8
+    bmi .bytes          ; More than or equal to 8 bits remaining
+    beq .bytes
+    sbc #8              ; Go back to previous value
+    tay
+    lda FACOV
+    bcs shiftr_FAC
+.sext:
+    asl 1,X
+    bcc @2
+    inc 1,X
+@2:
+    ror 1,X             ; Undo previous shift
+    ror 1,X
+short_shift_FAC:
+    ror 2,X
+    ror 3,X
+    ror 4,X
+    ror
+    iny
+    bne .sext
+shftrt:
+    clc
+    rts
diff --git a/tools/similarity.c b/tools/similarity.c
index 66de278b..bfeabe25 100644
--- a/tools/similarity.c
+++ b/tools/similarity.c
@@ -20,11 +20,12 @@ int matches[MAX_SIZE];
 
 int phase_mask[MAX_SIZE+MAX_SIZE];
 
-#define NUM_CHUNKS 4
+#define NUM_CHUNKS 5
 // 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,
     0xB947, 0xB97D,
+    0xB983, 0xB9BB,
     0xBBA2, 0xBC1A,
     0xBC58, 0xBC5A
 };