diff --git a/Makefile b/Makefile
index de38e2f3..f8256f00 100644
--- a/Makefile
+++ b/Makefile
@@ -930,7 +930,7 @@ testremote: build/kernal_custom.rom build/basic_custom.rom $(TARGET_CHR_PXL) bui
 	x64 -kernal build/kernal_custom.rom -basic build/basic_custom.rom -chargen $(TARGET_CHR_PXL) -moncommands build/symbols_custom.vs -remotemonitor
 
 testsimilarity: $(TOOL_SIMILARITY) $(DIR_TST38)/OUTx_x.BIN $(ROM_CBM_KERNAL) $(ROM_CBM_BASIC)
-	$(TOOL_SIMILARITY) $(ROM_CBM_KERNAL) $(DIR_TST38)/OUTx_x.BIN
+	$(TOOL_SIMILARITY) $(ROM_CBM_KERNAL) $(DIR_TST38)/OUTx_x.BIN --basic --high
 	$(TOOL_SIMILARITY) $(ROM_CBM_BASIC)  $(DIR_TST38)/OUTx_x.BIN --basic
 
 build/testresults: testsuite/unit/testresults.cc
diff --git a/STATUS.md b/STATUS.md
index 01fd551c..da4fea08 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -265,7 +265,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$E264`   | `cos_FAC1`             | PARTIAL  | not fully tested yet                               |
 | `$E26B`   | `sin_FAC1`             | PARTIAL  | not fully tested yet                               |
 | `$E2B4`   | `tan_FAC1`             | PARTIAL  | not fully tested yet                               |
-| `$E30E`   | `atn_FAC1`             | PARTIAL  | not fully tested yet                               |
+| `$E30E`   | `atn_FAC1`             | DONE     | From the Microsoft BASIC, original name: ATN       |
 
 <br />
 
diff --git a/src/basic/math_trans/e30e.atn_FAC1.s b/src/basic/math_trans/e30e.atn_FAC1.s
index c58a811c..e620adf8 100644
--- a/src/basic/math_trans/e30e.atn_FAC1.s
+++ b/src/basic/math_trans/e30e.atn_FAC1.s
@@ -2,9 +2,13 @@
 ;; #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 - arc tangent of FAC1
 ;
+; This is verified to be identical to the original Microsoft implementation where it was named ATN
+;
 ; See also:
 ; - [CM64] Computes Mapping the Commodore 64 - page 211
 ; - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
@@ -23,41 +27,34 @@
 
 
 atn_FAC1:
-    lda FAC1_sign               ; Save sign of FAC1 on the stack
-    pha
-    lda #$00                    ; FAC1 = abs(FAC1)
-    sta FAC1_sign
-    sta TEMPF1                  ; AX not scaled
-
-    lda #<const_ONE
-    ldy #>const_ONE
-    jsr FCOMP
-    bmi atn_skipscale
-
-    lda #<const_ONE
-    ldy #>const_ONE
-    jsr div_MEM_FAC1
-    inc TEMPF1                  ; AX scaled
-
-atn_skipscale:
-    lda #<poly_atn
-    ldy #>poly_atn
-    jsr poly1_FAC1
-
-    lda TEMPF1
-    beq atn_skipoffset
-
-    lda #<const_HALF_PI
-    ldy #>const_HALF_PI
-    jsr sub_MEM_FAC1
-
-atn_skipoffset:
-    pla
-    bpl atn_out
-    jmp toggle_sign_FAC1
-    
-atn_out:
-    rts
+        lda FAC1_sign           ; What is sign?
+        pha 			        ; (Meanwhile save it for later)
+        bpl ATN1
+        jsr toggle_sign_FAC1    ; If negative, negate FAC1, use arctan(x) = -arctan(-x)
+ATN1:
+        lda FAC1_exponent
+        pha                     ; Save this too for later
+        cmp #$81                ; See if FAC1 >= 1.0
+        bcc ATN2                ; It is less than 1
+        lda #<const_ONE         ; Get pointer to 1.0
+        ldy #>const_ONE
+        jsr div_MEM_FAC1        ; Compute reciprocal. Use arctan(x) = pi/2 - arctan(1/x)
+ATN2:
+        lda #<poly_atn          ; Pointer to arctan polynomial
+        ldy #>poly_atn          
+        jsr poly1_FAC1
+        pla
+        cmp #$81                ; Was original argument < 1 ?
+        bcc ATN3                ; Yes
+        lda #<const_HALF_PI
+        ldy #>const_HALF_PI
+        jsr sub_MEM_FAC1        ; Subtract arctan from pi/2
+ATN3:
+        pla                     ; Was original argument positive?
+        bpl ATN4                ; Yes
+        jmp toggle_sign_FAC1    ; If negative, negate result
+ATN4:
+        rts
 
 
 } else {
diff --git a/tools/similarity.c b/tools/similarity.c
index 51bd6f0f..69eb4f75 100644
--- a/tools/similarity.c
+++ b/tools/similarity.c
@@ -20,7 +20,7 @@ int matches[MAX_SIZE];
 
 int phase_mask[MAX_SIZE+MAX_SIZE];
 
-#define NUM_CHUNKS 9
+#define NUM_CHUNKS 10
 // 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,
@@ -32,6 +32,7 @@ unsigned short microsoft_basic_chunks[NUM_CHUNKS * 2] = {
     0xBDB3, 0xBDC1,
     0xBDDD, 0xBF51,
     0xBFB4, 0xBFEC,
+    0xE30E, 0xE33D,
 };
 
 
@@ -40,6 +41,7 @@ int main(int argc,char **argv)
 {
   int verbose=0;
   int basic = 0;
+  int high =0;
 
   if (argc<3) {
     fprintf(stderr,"usage: similarity <file1> <file2> [--verbose] [--basic]\n");
@@ -51,6 +53,8 @@ int main(int argc,char **argv)
         verbose = 1;
       } else if (!strcmp(argv[i], "--basic")) {
         basic = 1;
+      } else if (!strcmp(argv[i], "--high")) {
+        high = 1;
       } else {
         fprintf(stderr, "Unrecognised directive.\n");
         exit(-1);
@@ -94,11 +98,21 @@ int main(int argc,char **argv)
   if (basic) {
     for (int i = 0; i < NUM_CHUNKS; i++) {
       for (int addr = microsoft_basic_chunks[i * 2]; addr <= microsoft_basic_chunks[i * 2 + 1]; addr++) {
-        short index = addr - 0xA000;
-        if (f1[index] != f2[index]) {
-            printf("BASIC does not match MS BASIC at %X (%X != %X)\n", addr, f1[index], f2[index]);
+        short index1, index2;
+        if (high && addr < 0xE000 || !high && addr >= 0xE000) {
+          continue;
         }
-        f1[index] = 0xFF;
+        if (!high) {
+          index1 = addr - 0xA000;
+          index2 = index1;
+        } else {
+          index1 = addr - 0xE000;
+          index2 = addr - 0xA000;
+        }
+        if (f1[index1] != f2[index2]) {
+            printf("BASIC does not match MS BASIC at %X (%X != %X)\n", addr, f1[index1], f2[index2]);
+        }
+        f1[index1] = 0xFF;
       }
     }
   }