diff --git a/STATUS.md b/STATUS.md
index 87ae4c88..daa3c350 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -195,7 +195,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$AABC`   | `print_FAC1`           | NOT DONE |                                                    |
 | `$AF7B`   | `compute_TI`           | NOT DONE |                                                    |
 | `$AF7E`   | not named yet          | NOT DONE |                                                    |
-| `$AF87`   | not named yet          | NOT DONE |                                                    |
+| `$AF87`   | `set_FAC1_mantissa`    | DONE     |                                                    |
 | `$AF9A`   | `compute_ST`           | NOT DONE |                                                    |
 | `$B1AA`   | `FACINX`               | NOT DONE | convert FAC1 to 16-bit signed integer              |
 | `$B1BF`   | `convert_FAC1_to_s16`  | NOT DONE |                                                    |
diff --git a/src/basic/,stubs_math/af7e.math_stub.s b/src/basic/,stubs_math/af7e.math_stub.s
index 051a14e5..3a85250e 100644
--- a/src/basic/,stubs_math/af7e.math_stub.s
+++ b/src/basic/,stubs_math/af7e.math_stub.s
@@ -4,7 +4,7 @@
 ;; #LAYOUT# *   *       #IGNORE
 
 ;
-; Math package - sets mantissa to 00yyxxaa
+; Math package - set rest of FAC1 and JMP to $b8d2
 ;
 ; See also:
 ; - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
diff --git a/src/basic/,stubs_math/af87.interop b/src/basic/math/af87.interop
similarity index 100%
rename from src/basic/,stubs_math/af87.interop
rename to src/basic/math/af87.interop
diff --git a/src/basic/,stubs_math/af87.math_stub.s b/src/basic/math/af87.set_FAC1_mantissa.s
similarity index 57%
rename from src/basic/,stubs_math/af87.math_stub.s
rename to src/basic/math/af87.set_FAC1_mantissa.s
index 3a85250e..bfc4367c 100644
--- a/src/basic/,stubs_math/af87.math_stub.s
+++ b/src/basic/math/af87.set_FAC1_mantissa.s
@@ -4,12 +4,17 @@
 ;; #LAYOUT# *   *       #IGNORE
 
 ;
-; Math package - set rest of FAC1 and JMP to $b8d2
+; Math package - sets mantissa to 00yyxxaa
 ;
 ; See also:
 ; - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
 ;
 
-; XXX add name, label, implement, test
 
-	+STUB_IMPLEMENTATION
+set_FAC1_mantissa:
+    sty FAC1_mantissa+1
+    stx FAC1_mantissa+2
+    sta FAC1_mantissa+3
+    lda #0
+    sta FAC1_mantissa
+    rts