diff --git a/STATUS.md b/STATUS.md
index b4995691..2e526373 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -235,7 +235,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$BC39`   | `sgn_FAC1`             | DONE     |                                                    |
 | `$BC3C`   | `convert_A_to_FAC1`    | DONE     |                                                    |
 | `$BC44`   | `convert_i16_to_FAC1`  | DONE     |                                                    |
-| `$BC58`   | `abs_FAC1`             | NOT DONE |                                                    |
+| `$BC58`   | `abs_FAC1`             | DONE     | From the Microsoft BASIC, original name: ABS       |
 | `$BC5B`   | `FCOMP`                | DONE     |                                                    |
 | `$BC9B`   | `QINT`                 | NOT DONE | convert FAC1 to 32 bit signed integer              |
 | `$BCCC`   | `int_FAC1`             | DONE     |                                                    |
diff --git a/src/basic/,stubs_math/bc58.abs_FAC1.s b/src/basic/,stubs_math/bc58.abs_FAC1.s
deleted file mode 100644
index caf41ff8..00000000
--- a/src/basic/,stubs_math/bc58.abs_FAC1.s
+++ /dev/null
@@ -1,19 +0,0 @@
-;; #LAYOUT# STD *       #TAKE
-;; #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
-;; #LAYOUT# *   BASIC_0 #TAKE
-;; #LAYOUT# *   *       #IGNORE
-
-;
-; Math package - absolute value
-;
-; See also:
-; - [CM64] Computes Mapping the Commodore 64 - page 115
-; - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
-; - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
-;
-
-; XXX provide implementation
-
-abs_FAC1:
-
-	+STUB_IMPLEMENTATION
diff --git a/src/basic/math/bc58.abs_FAC1.s b/src/basic/math/bc58.abs_FAC1.s
new file mode 100644
index 00000000..7ac6e8dd
--- /dev/null
+++ b/src/basic/math/bc58.abs_FAC1.s
@@ -0,0 +1,17 @@
+;; #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 - absolute value of FAC1
+;
+; This is verified to be identical to the original Microsoft implementation where it was named ABS.
+;
+
+abs_FAC1:
+
+    lsr FAC1_sign
+    rts
diff --git a/src/basic/,stubs_math/bc58.interop b/src/basic/math/bc58.interop
similarity index 100%
rename from src/basic/,stubs_math/bc58.interop
rename to src/basic/math/bc58.interop
diff --git a/tools/similarity.c b/tools/similarity.c
index b7cc704f..9b5573a5 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 1
+#define NUM_CHUNKS 3
 // 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,
-    0xBBA2, 0xBC1A
+    0xBBA2, 0xBC1A,
+    0xBC58, 0xBC5A
 };