//
// 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
!:
	lda COLOR
	sta (USER), y
	lda #$20                           // space character screen code
	sta (PNT), y

	dey
	bpl !-

	// Done
	rts