diff --git a/STATUS.md b/STATUS.md
index ddd0274d..49b5c4c7 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -193,7 +193,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$B853`   | `sub_FAC2_FAC1`        | PARTIAL  | needs `add_FAC2_FAC1`                              |
 | `$B862`   | `add_align_exponents`  | PARTIAL  | not tested yet                                     |
 | `$B867`   | `add_MEM_FAC1`         | PARTIAL  | needs `add_FAC2_FAC1`                              |
-| `$B86A`   | `add_FAC2_FAC1`        | NOT DONE |                                                    |
+| `$B86A`   | `add_FAC2_FAC1`        | PARTIAL  | not tested yet                                     |
 | `$B8FE`   | `normal_FAC1`          | PARTIAL  | not tested yet                                     |
 | `$B947`   | `inv_FAC1_mantissa`    | NOT DONE |                                                    |
 | `$B983`   | `MULSHF`               | NOT DONE |                                                    |
diff --git a/c64/basic/,stubs_math/b86a.add_FAC2_FAC1.s b/c64/basic/,stubs_math/b86a.add_FAC2_FAC1.s
deleted file mode 100644
index bf18b367..00000000
--- a/c64/basic/,stubs_math/b86a.add_FAC2_FAC1.s
+++ /dev/null
@@ -1,57 +0,0 @@
-// #LAYOUT# STD *       #TAKE
-// #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
-// #LAYOUT# *   BASIC_0 #TAKE
-// #LAYOUT# *   *       #IGNORE
-
-//
-// Math package - add FAC2 to FAC1
-//
-// Input:
-// - .A - must load FAC1 exponent ($61) beforehand to set the zero flag
-//
-// Note:
-// - load FAC2 after FAC1, or mimic the Kernals sign comparison (XXX do we need it?)
-//
-// See also:
-// - [CM64] Computes Mapping the Commodore 64 - page 112
-// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
-// - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
-//
-
-// XXX provide implementation
-
-add_FAC2_FAC1:
-
-	// Check if FAC2 is 0; if so, leave FAC1 without changes
-
-	lda FAC2_exponent
-	beq add_FAC2_FAC1_done
-
-	// If FAC1 is 0, just copy FAC2 to FAC1
-
-	lda FAC1_exponent
-	beq_16 mov_FAC2_FAC1
-
-	// Make sure the exponents are aligned
-
-	jsr add_align_exponents
-
-	// Check whether signs are the same
-
-	lda FAC1_sign
-	eor FAC2_sign
-
-	bmi add_FAC2_FAC1_sub              // branch if signs are opposite
-
-
-
-
-
-add_FAC2_FAC1_sub:
-
-	STUB_IMPLEMENTATION()
-
-
-add_FAC2_FAC1_done:
-
-	rts
\ No newline at end of file
diff --git a/c64/basic/math/b86a.add_FAC2_FAC1.s b/c64/basic/math/b86a.add_FAC2_FAC1.s
new file mode 100644
index 00000000..4565af0b
--- /dev/null
+++ b/c64/basic/math/b86a.add_FAC2_FAC1.s
@@ -0,0 +1,114 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+//
+// Math package - add FAC2 to FAC1
+//
+// Input:
+// - .A - must load FAC1 exponent ($61) beforehand to set the zero flag
+//
+// Note:
+// - load FAC2 after FAC1, or mimic the Kernals sign comparison (XXX do we need it?)
+//
+// See also:
+// - [CM64] Computes Mapping the Commodore 64 - page 112
+// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
+// - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
+//
+
+// XXX test this
+
+add_FAC2_FAC1:
+
+	// Check if FAC2 is 0; if so leave FAC1 without changes
+
+	lda FAC2_exponent
+	beq add_FAC2_FAC1_done
+
+	// If FAC1 is 0, if so just copy FAC2 to FAC1
+
+	lda FAC1_exponent
+	beq_16 mov_FAC2_FAC1
+
+	// Make sure the exponents are aligned
+
+	jsr add_align_exponents
+
+	// Check whether signs are the same
+
+	lda FAC1_sign
+	eor FAC2_sign
+
+	bmi add_FAC2_FAC1_sub              // branch if signs are opposite
+
+	// Perform the addition
+	
+	clc
+	lda FAC2_mantissa+3
+	adc FAC1_mantissa+3
+	sta FAC1_mantissa+3
+
+	lda FAC2_mantissa+2
+	adc FAC1_mantissa+2
+	sta FAC1_mantissa+2
+	
+	lda FAC2_mantissa+1
+	adc FAC1_mantissa+1
+	sta FAC1_mantissa+1	
+
+	lda FAC2_mantissa+0
+	adc FAC1_mantissa+0
+	sta FAC1_mantissa+0	
+	
+	bcc add_FAC2_FAC1_normalize        // end if no need to adapt the exponent
+
+	inc FAC1_exponent
+	bne add_FAC2_FAC1_normalize        // end if no overflow
+
+	// Overflow - XXX shouldnt we mark it somewhere in memory?
+	
+	lda #$FF
+	sta FAC1_exponent
+	sta FAC1_mantissa+3
+	sta FAC1_mantissa+2
+	sta FAC1_mantissa+1
+	sta FAC1_mantissa+0	
+
+	// FALLTROUGH
+
+add_FAC2_FAC1_done:
+
+	rts
+
+add_FAC2_FAC1_sub:
+
+	// Perform the subtration
+
+	sec
+	lda FAC1_mantissa+3
+	sbc FAC2_mantissa+3
+	sta FAC1_mantissa+3
+
+	lda FAC1_mantissa+2
+	sbc FAC2_mantissa+2
+	sta FAC1_mantissa+2
+	
+	lda FAC1_mantissa+1
+	sbc FAC2_mantissa+1
+	sta FAC1_mantissa+1	
+
+	lda FAC1_mantissa+0
+	sbc FAC2_mantissa+0
+	sta FAC1_mantissa+0	
+	
+	bcs add_FAC2_FAC1_normalize          // end if no need to adapt the exponent
+
+	dec FAC1_exponent
+	
+	// FALLTROUGH
+	
+add_FAC2_FAC1_normalize:
+	
+	jmp normal_FAC1
diff --git a/c64/basic/,stubs_math/b86a.interop b/c64/basic/math/b86a.interop
similarity index 100%
rename from c64/basic/,stubs_math/b86a.interop
rename to c64/basic/math/b86a.interop