Add get_FAC2_via_INDEX

src/basic/math_mov/ba90.get_FAC2_via_INDEX.s

@@ -0,0 +1,59 @@
+;; #LAYOUT# STD *       #TAKE
+;; #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
+;; #LAYOUT# *   BASIC_0 #TAKE
+;; #LAYOUT# *   *       #IGNORE
+
+;
+; Math package - Mov MEM to FAC2 with address in INDEX zero page register
+;
+; See also:
+; - https://sourceforge.net/p/acme-crossass/code-0/38/tree/trunk/ACME_Lib/cbm/c64/float.a?force=True
+;
+
+; XXX implement, test
+
+get_FAC2_via_INDEX:
+
+	; Now copy the data from RAM to FAC2, for the format description see:
+	; - https://www.c64-wiki.com/wiki/Floating_point_arithmetic#Representation_in_the_C-64
+
+	; Copy the mantissa
+
+	ldy #$04
+	lda (INDEX), y
+	sta FAC2_mantissa+3
+
+	dey
+	lda (INDEX), y
+	sta FAC2_mantissa+2
+
+	dey
+	lda (INDEX), y
+	sta FAC2_mantissa+1
+
+	dey
+	lda (INDEX), y
+	ora #$80                           ; clear the sign bit
+	sta FAC2_mantissa+0
+
+	; Copy the sign
+
+	lda (INDEX), y
+	bpl @1                             ; assumption: non-negative numbers are more frequent
+	lda #$FF
+	+skip_2_bytes_trash_nvz
+@1:
+	lda #$00
+
+	sta FAC2_sign
+
+	; Copy the exponent
+
+	dey
+	lda (INDEX), y
+	sta FAC2_exponent
+
+	; Return the FAC1 exponent
+
+	lda FAC1_exponent
+	rts