implement main screen clear routine

c64/kernal/cint.s

@@ -35,8 +35,7 @@ cint:
 	sta key_buffer_size
 	
 	;; Fallthrough/jump to screen clear routine (Compute's Mapping the 64 p215)
-	;; 	jmp clear_screen
-
+	jmp clear_screen
 
 	rts
 

c64/kernal/clear_screen.s

@@ -0,0 +1,60 @@
+	;; Clear screen and initialise line link table
+	;; (Compute's Mapping the 64 p215-216)
+
+clear_screen:	
+	
+	;; Clear line link table 
+	;; (Compute's Mapping the 64 p215)
+
+	lda #$00
+	ldy #24
+clearscreen_l1:	
+	sta screen_line_link_table,y
+	dey
+	bpl clearscreen_l1
+
+	;; Y now = #$FF
+
+	
+	;; Clear screen RAM.
+	;; We should do this at HIBASE, which annoyingly
+	;; is no ZP, so we need to make a vector
+	;; (Compute's Mapping the 64 p216)
+	;; Get pointer to the screen into current_screen_line_ptr
+	;; as it is the first appropriate place for it found when
+	;; searching through the ZP allocations listed in
+	;; Compute's Mapping the 64
+	sta current_screen_line_ptr+0
+	lda hibase
+	sta current_screen_line_ptr+1
+	ldx #$03		; countdown for pages to update
+	iny 			; Y now = #$00
+	lda #$20		; space character
+clearscreen_l2:
+	sta (current_screen_line_ptr),y
+	iny
+	bne clearscreen_l2
+	;; To draw only 1000 bytes, add 250 to address each time
+	lda current_screen_line_ptr
+	clc
+	adc #<250
+	sta current_screen_line_ptr
+	lda current_screen_line_ptr+1
+	adc #>250
+	sta current_screen_line_ptr+1
+	lda #$20		; get space character again
+	dex
+	bpl clearscreen_l2
+
+	;; Clear colour RAM
+	;; (Compute's Mapping the 64 p216)
+	lda text_colour
+clearscreen_l3:	
+	sta $d800,y
+	sta $d900,y
+	sta $da00,y
+	sta $db00-24,y    	; so we only erase 1000 bytes
+	iny
+	bne clearscreen_l3
+	
+	rts