diff --git a/STATUS.md b/STATUS.md
index f21b68a8..932b3690 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -206,7 +206,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$BF71`   | `SQR`        | NOT DONE | XXX - no stub added yet                            |
 | `$BF78`   | `FPWR`       | NOT DONE | set FAC1 as RAM raised to the power of FAC1        |
 | `$BF7B`   | `FPWRT`      | NOT DONE | set FAC1 as FAC2 raised to the power of FAC1       |
-| `$BFB4`   | `NEGOP`      | NOT DONE | toggle FAC1 sign (if not 0)                        |
+| `$BFB4`   | `NEGOP`      | DONE     |                                                    |
 | `$BFED`   | `EXP`        | NOT DONE | XXX - no stub added yet                            |
 | `$E043`   | `POLY1`      | NOT DONE | evaluate polynomial with odd powers only           |
 | `$E059`   | `POLY2`      | NOT DONE | evaluate polynomial with odd and even powers       |
diff --git a/c64/aliases/,aliases_ram_lowmem.s b/c64/aliases/,aliases_ram_lowmem.s
index efb85ef8..828aef45 100644
--- a/c64/aliases/,aliases_ram_lowmem.s
+++ b/c64/aliases/,aliases_ram_lowmem.s
@@ -31,7 +31,7 @@
 	.label TEMPPT    = $16 //          -- NOT IMPLEMENTED --
 	.label LASTPT    = $17 // $17-$18  -- NOT IMPLEMENTED --
 	.label TEMPST    = $19 // $19-$21  -- NOT IMPLEMENTED --
-	.label INDEX     = $22 // $22-$25  -- NOT IMPLEMENTED -- temporary variables
+	.label INDEX     = $22 // $22-$25  temporary variables, [!] our usage might be different
 	.label RESHO     = $26 // $26-$2A  -- NOT IMPLEMENTED --
 	.label TXTTAB    = $2B // $2B-$2C  start of BASIC code
 	.label VARTAB    = $2D // $2D-$2E  end of BASIC code, start of variables
diff --git a/c64/basic/,stubs_math/bfb4.NEGOP.s b/c64/basic/math/bfb4.NEGOP.s
similarity index 66%
rename from c64/basic/,stubs_math/bfb4.NEGOP.s
rename to c64/basic/math/bfb4.NEGOP.s
index 838f92c5..c231407b 100644
--- a/c64/basic/,stubs_math/bfb4.NEGOP.s
+++ b/c64/basic/math/bfb4.NEGOP.s
@@ -10,7 +10,13 @@
 // - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
 //
 
-// XXX provide implementation
-
 NEGOP:
-	STUB_IMPLEMENTATION()
+
+	lda FAC1_exponent
+	beq !+                             // do not toggle the sign if value is 0
+
+	lda FAC1_sign
+	eor #$FF
+	sta FAC1_sign
+!:
+	rts
diff --git a/c64/basic/,stubs_math/bfb4.interop b/c64/basic/math/bfb4.interop
similarity index 100%
rename from c64/basic/,stubs_math/bfb4.interop
rename to c64/basic/math/bfb4.interop