Add get_FAC1_via_INDEX

Also change to only use bit 7 as sign in FAC1 and FAC2 (as the original BASIC does) to simplify
implementations.

src/basic/math_mov/bba2.mov_MEM_FAC1.s

@@ -0,0 +1,30 @@
+;; #LAYOUT# STD *       #TAKE
+;; #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
+;; #LAYOUT# *   BASIC_0 #TAKE
+;; #LAYOUT# *   *       #IGNORE
+
+
+; Math package - fetch FAC1 from RAM location
+;
+; Input:
+; - .A - address low byte
+; - .Y - address high byte
+;
+; Output:
+; - .A - FAC1 exponent, affects status flags
+; - .Y - always 0
+;
+; Preserves:
+; - .X - (experimentation with original ROM)
+;
+; See also:
+; - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
+; - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
+
+
+
+mov_MEM_FAC1:
+	sty INDEX+1
+	sta INDEX+0
+
+    ; FALLTHROUGH to get_FAC1_via_INDEX