implement screen clear routines

c64/kernal/set_pointer_to_current_screen_line.s

@@ -0,0 +1,33 @@
+	;; Put the address of the screen current line
+	;; into the pointer at $D1 (current_screen_line_ptr).
+	;; It must take into account the screen line link table
+	;; (Compute's Mapping the 64 p216)
+set_pointer_to_current_screen_line:
+	;;  Get pointer to start of the screen
+	lda hibase
+	sta current_screen_line_ptr+1
+	lda #$00
+	sta current_screen_line_ptr+0
+
+	;; Now advance for the number of lines
+	ldx current_screen_y
+
+sptcsl_l2:	
+	;; XXX - Should we read the columns per line from
+	;; somewhere?
+	lda #<40
+	;; Double line length if line is linked
+	bit screen_line_link_table,x
+	bpl sptcsl_l1
+	asl
+sptcsl_l1:
+	clc
+	adc current_screen_line_ptr+0
+	lda current_screen_line_ptr+1
+	adc #>40
+	sta current_screen_line_ptr+1
+
+	dex
+	bpl sptcsl_l2
+	
+	rts