cc65/libsrc/atmos/clrscr.s

;
; 2003-04-13, Ullrich von Bassewitz
; 2013-07-16, Greg King
;

        .export         _clrscr
        .importzp       ptr2

        .include        "atmos.inc"

; ------------------------------------------------------------------------
; void clrscr (void);

.proc   _clrscr

; Set the cursor to top left cursor position

        ldy     #$00
        sty     CURS_X
        sty     CURS_Y

; Set ptr2 to the screen position (left upper border)

        lda     #<SCREEN
        sta     ptr2
        lda     #>SCREEN
        sta     ptr2+1

; Clear full pages. Y is still zero

        ldx     #>(SCREEN_YSIZE * SCREEN_XSIZE)
        lda     #' '
@L1:    sta     (ptr2),y
        iny                     ; Bump low byte of address
        bne     @L1
        inc     ptr2+1          ; Bump high byte of address
        dex
        bne     @L1

; Clear the remaining page

@L2:    sta     (ptr2),y
        iny
        cpy     #<(SCREEN_YSIZE * SCREEN_XSIZE)
        bne     @L2
        rts

.endproc