Update sub_FAC2_FAC1

STATUS.md

@@ -206,7 +206,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$B7F7`   | `convert_FAC1_to_ADDR` | NOT DONE |                                                    |
 | `$B849`   | `add_HALF_FAC1`        | DONE     | From the Microsoft BASIC, original name: FADDH     |
 | `$B850`   | `sub_MEM_FAC1`         | DONE     | From the Microsoft BASIC, original name: FSUB      |
-| `$B853`   | `sub_FAC2_FAC1`        | DONE     |                                                    |
+| `$B853`   | `sub_FAC2_FAC1`        | DONE     | From the Microsoft BASIC, original name: FSUBT     |
 | `$B862`   | `add_align_exponents`  | DONE     |                                                    |
 | `$B867`   | `add_MEM_FAC1`         | DONE     |                                                    |
 | `$B86A`   | `add_FAC2_FAC1`        | DONE     |                                                    |

src/basic/math/b853.sub_FAC2_FAC1.s

@@ -3,23 +3,29 @@
 ;; #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 - subtract FAC1 from FAC2
 ;
+; This is verified to be identical to the original Microsoft implementation where it was named FSUBT.
+;
 ; 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
 
 sub_FAC2_FAC1:
 
-	; Toggle FAC1 sign
-	lda FAC1_sign
-	eor #$FF
-	sta FAC1_sign
+    ; Toggle FAC1 sign
+    lda FAC1_sign
+    eor #$FF
+    sta FAC1_sign
+    eor FAC2_sign
+    sta ARISGN
+    lda FAC1_exponent       ; Set flags from exponent
 
 	; Perform addition
-	jmp add_FAC2_FAC1
+    jmp add_FAC2_FAC1

tools/similarity.c

@@ -25,7 +25,7 @@ int phase_mask[MAX_SIZE+MAX_SIZE];
 unsigned short microsoft_basic_chunks[NUM_CHUNKS * 2] = {
     0xB391, 0xB39D,
     0xB3A2, 0xB3A5,
-    0xB849, 0xB853,
+    0xB849, 0xB861,
     0xB8D2, 0xB8D5,
     0xB947, 0xB97D,
     0xB983, 0xB9BB,