add files for BASIC interpretor main loop and message printing

c64/basic/basic_main_loop.s

@@ -0,0 +1,27 @@
+basic_main_loop:
+
+	;; XXX - Check if direct or program mode, and get next line of input
+	;; the appropriate way.
+	;; XXX - For now, it is hard coded to direct mode
+	
+	;; Read a line of input
+	ldx #$00
+read_line_loop:	
+	jsr $ffe4 		; KERNAL CHRIN
+	cmp #$0d
+	beq got_line_of_input
+	;; Not carriage return, so try to append to line so far
+	cpx #80
+	bcc +
+	;; Report STRING TOO LONG error (Compute's Mapping the 64 p93)
+	ldx #23
+	jmp basic_do_error
+*
+	sta basic_input_buffer,x
+	inx
+	jmp read_line_loop
+
+got_line_of_input:	
+	jsr ready_message
+
+	jmp basic_main_loop