begin implementing chrout

c64/kernal/chrout.s

@@ -1,4 +1,40 @@
 ; Function defined on pp272-273 of C64 Programmers Reference Guide
 chrout:
-	inc $d020
+	;; Crude implementation of character output	
+	sta $0400
+	inc $0401
+	jmp chrout
+	
+	;; Write character on the screen
+	ldy current_screen_x
+	sta (current_screen_line_ptr),y
+
+	;; Advance the column, and scroll screen down if we need
+	;; to insert a 2nd line in this logical line.
+	;; (eg Compute's Mapping the 64 p41)
+	ldx current_screen_y
+	iny current_screen_x
+	sty current_screen_x
+	cpy #40
+	beq screen_grow_logical_line
+	cpy #79
+	bcs screen_advance_to_next_line
+	rts
+
+screen_grow_logical_line:
+	;; XXX - Scroll screen down to make space
+
+	rts
+	
+screen_advance_to_next_line:
+	;;  Go to start of line
+	lda #0
+	sta current_screen_x
+	;;  Advance line number
+	inc current_screen_y
+	;; Work out if we have gone off the bottom of the screen?
+
+	;; XXX -- implement this check and scrolling of the screen
+	;; and updating the screen line links.
+	
 	rts