various multiplication fixes

c64/basic/math/mul_FAC2_FAC1_8x8.s

@@ -0,0 +1,67 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+//
+// Helper routine, based on code by djmips from:
+// - https://codebase64.org/doku.php?id=base:8bit_multiplication_16bit_product_fast_no_tables
+//
+// input: .A, INDEX+3
+// output: .X (high byte), .A and INDEX+2 (low byte), carry always clear
+//
+
+mul_FAC2_FAC1_8x8:
+
+	sta INDEX+2
+
+	lda #$00
+
+	dec INDEX+3	// decrement because we will be adding with carry set for speed (an extra one)
+	ror INDEX+2
+	bcc !+
+	adc INDEX+3
+!:
+	ror
+	ror INDEX+2
+	bcc !+
+	adc INDEX+3
+!:
+	ror
+	ror INDEX+2
+	bcc !+
+	adc INDEX+3
+!:
+	ror
+	ror INDEX+2
+	bcc !+
+	adc INDEX+3
+!:
+	ror
+	ror INDEX+2
+	bcc !+
+	adc INDEX+3
+!:
+	ror
+	ror INDEX+2
+	bcc !+
+	adc INDEX+3
+!:
+	ror
+	ror INDEX+2
+	bcc !+
+	adc INDEX+3
+!:
+	ror
+	ror INDEX+2
+	bcc !+
+	adc INDEX+3
+!:
+	ror
+	ror INDEX+2
+	inc INDEX+3
+
+	tax
+	lda INDEX+2
+	clc
+
+	rts