Add clear_FAC1

src/basic/math/bce9.clear_FAC1.s

@@ -0,0 +1,28 @@
+;; #LAYOUT# STD *       #TAKE
+;; #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
+;; #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 - clear FAC1
+;
+; Set each byte of the mantissa of FAC1 to the value of A
+;
+; This is verified to be identical to the original Microsoft implementation where it was named CLRFAC.
+;
+; See also:
+; - https://www.c64-wiki.com/wiki/BASIC-ROM
+; - https://sta.c64.org/cbm64basconv.html
+;
+
+
+clear_FAC1:
+    sta FAC1_mantissa+0
+    sta FAC1_mantissa+1
+    sta FAC1_mantissa+2
+    sta FAC1_mantissa+3
+    tay
+clear_FAC1_rts:             ; Return point needed elsewhere
+    rts