implement screen clear routines

c64/kernal/,aliases.s

@@ -33,10 +33,10 @@
 	.alias cursor_is_visible $CF
 	.alias input_source $D0
 	.alias current_screen_line_ptr $D1
-	.alias current_screen_line_logical_column $D3
+	.alias current_screen_x $D3
 	.alias quote_mode_flag $D4
 	.alias max_logical_line_length $D5
-	.alias cursor_physical_screen_line $D6
+	.alias current_screen_y $D6
 	.alias last_printed_character_ascii $D7
 	.alias insert_mode $D8
 	; Bits 0 -3 = bits 8-11 of screen line address

c64/kernal/clear_screen.s

@@ -56,5 +56,6 @@ clearscreen_l3:
 	sta $db00-24,y    	; so we only erase 1000 bytes
 	iny
 	bne clearscreen_l3
-	
-	rts
+
+	;; (Compute's Mapping the 64 p216)
+	jmp home_cursor

c64/kernal/home_cursor.s

@@ -0,0 +1,10 @@
+	;; Home the cursor
+	;; (Compute's Mapping the 64 p216)
+
+home_cursor:
+
+	lda #$00
+	sta current_screen_x
+	sta current_screen_y
+
+	jmp set_pointer_to_current_screen_line

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