;; #LAYOUT# STD *        #TAKE
;; #LAYOUT# X16 *        #IGNORE
;; #LAYOUT# *   KERNAL_0 #TAKE
;; #LAYOUT# *   *        #IGNORE

;
; Well-known routine, described in:
;
; - [CM64] Computes Mapping the Commodore 64 - page 219
; - http://sta.c64.org/cbm64scrfunc.html
;
; input: .X - screen line to clear
;
; Checked that original routine preserves X (convenient for loops),
; and leaves space screen code in .A, so duplicate this behaviour
;


screen_clear_line:

	; First, calculate PNT and USER
	+phx_trash_a
	jsr screen_calculate_PNT_USER_from_X
	+plx_trash_a

	; Now clear the line
	ldy #39
@1:
	lda COLOR
	sta (USER), y
	lda #$20                           ; space character screen code
	sta (PNT), y

	dey
	bpl @1

	; Done
	rts