diff --git a/STATUS.md b/STATUS.md
index f74abcf2..34e90738 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -199,7 +199,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$AF9A`   | `compute_ST`           | DONE     |                                                    |
 | `$B1AA`   | `FACINX`               | NOT DONE | convert FAC1 to 16-bit signed integer              |
 | `$B1BF`   | `convert_FAC1_to_s16`  | NOT DONE |                                                    |
-| `$B391`   | `GIVAYF`               | PARTIAL  | not fully tested yet                               |
+| `$B391`   | `GIVAYF`               | DONE     | From the Microsoft BASIC, original name: GIVAYF    |
 | `$B3A2`   | `convert_Y_to_FAC1`    | DONE     | From the Microsoft BASIC, original name: SNGFLT    |
 | `$B794`   | `convert_u8_A_to_FAC1` | DONE     |                                                    |
 | `$B7B5`   | `STRVAL`               | PARTIAL  | Only for 38k memory model                          |
diff --git a/src/basic/math/b391.GIVAYF.s b/src/basic/math/b391.GIVAYF.s
index 92ccbb9c..ddd974ab 100644
--- a/src/basic/math/b391.GIVAYF.s
+++ b/src/basic/math/b391.GIVAYF.s
@@ -3,7 +3,9 @@
 ;; #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 - 16-bit signed integer to FAC1
 ;
 ; Output:
@@ -13,6 +15,8 @@
 ; Sets the data type flag at $0D to "numeric", stores the number in the FAC1 mantissa
 ; then calls convert_i16_to_FAC1
 ;
+; This is verified to be identical to the original Microsoft implementation where it was named GIVAYF.
+;
 ; See also:
 ; - [CM64] Computes Mapping the Commodore 64 - page 107
 ; - https://www.c64-wiki.com/wiki/GIVAYF
@@ -20,8 +24,9 @@
 
 
 GIVAYF:
-    ldx #$00
-    stx $0D
-    sty FAC1_mantissa
-    sta FAC1_mantissa+1
+    ldx #0
+    stx VALTYP
+    sta FAC1_mantissa+0
+	sty FAC1_mantissa+1
+    ldx #$90
     jmp convert_i16_to_FAC1
diff --git a/tools/similarity.c b/tools/similarity.c
index 6dfd16ac..713deee0 100644
--- a/tools/similarity.c
+++ b/tools/similarity.c
@@ -20,9 +20,10 @@ int matches[MAX_SIZE];
 
 int phase_mask[MAX_SIZE+MAX_SIZE];
 
-#define NUM_CHUNKS 7
+#define NUM_CHUNKS 8
 // 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, 0xB84F,
     0xB8D2, 0xB8D5,