more work on creating arrays

src/basic/engine_variables/helper_array_create_mul.s

@@ -0,0 +1,76 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+// 16-bit multiplication, INDEX+0/+1 = __FAC1+0/+1 * __FAC1+2/+3; __FAC1+0/+1 = INDEX+0/+1
+// uses INDEX+2/+3 (for helper routine) as work area
+
+
+helper_array_create_mul: // preserves .X and .Y
+
+	phx_trash_a
+
+	// We will reuse 8x8 multiplication routine used by floating point operations
+
+	// __FAC1+0 (lo byte) * __FAC1+2 (lo byte)
+
+	lda __FAC1+0
+	sta INDEX+2
+	lda __FAC1+2
+	jsr mul_FAC2_FAC1_8x8                        // this preserves .Y
+
+	sta INDEX+0                                  // result - lo byte
+	stx INDEX+1                                  // result - hi byte
+
+	// __FAC1+0 (lo byte) * __FAC1+3 (hi byte)
+
+	lda __FAC1+0
+	sta INDEX+2
+	lda __FAC1+3
+	jsr helper_array_create_mul_8x8_add          // multiply and add to high byte of the result
+
+	// __FAC1+1 (hi byte) * __FAC1+2 (lo byte)
+
+	lda __FAC1+1
+	sta INDEX+2
+	lda __FAC1+2
+	jsr helper_array_create_mul_8x8_add          // multiply and add to high byte of the result
+
+	// __FAC1+1 (hi byte) * __FAC1+3 (hi byte) has to be 0,
+	// otherwise the result would be out of range
+	// do not calculate
+
+	lda __FAC1+0
+	beq helper_array_create_mul_success
+	lda __FAC1+3
+	beq helper_array_create_mul_success
+
+	// FALLTROUGH
+
+helper_array_create_mul_error:
+
+	jmp do_OUT_OF_MEMORY_error
+
+helper_array_create_mul_success:
+
+	lda INDEX+0
+	sta __FAC+0
+	lda INDEX+1
+	sta __FAC+1
+
+	plx_trash_a
+	rts
+
+helper_array_create_mul_add:
+
+	jsr mul_FAC2_FAC1_8x8
+
+	cpx #$00
+	bne helper_array_create_mul_error
+
+	clc
+	adc INDEX+1 
+	bcs helper_array_create_mul_error
+	sta INDEX+1
+
+	rts