cc65/libsrc/creativision/clrscr.s

;*
;* clrscr
;*
;* NB: All screen functions assume Graphics Mode 1 in a default configuration.
;* Therefore, this is hard coded to use $1000-$12FF as screen VRAM.

    .export     _clrscr
    .include    "creativision.inc"
    
_clrscr:

    sei         ; Disable interrupts. Default INT handler reads VDP_STATUS
                ; and would lose any setup done here.
                
    lda         #$00    ; VRAM offset low
    sta         VDP_CONTROL_W
    
    lda         #$50    ; VRAM offset high ($10 OR $40)
    sta         VDP_CONTROL_W
    
    lda         #$C0    ; Space from ROM setup
    
    ldx         #0
    ldy         #3
    
L1:     sta             VDP_DATA_W
    inx
    bne         L1
    dey
    bne         L1
    
    cli                         ; Let interrupts go again
    
    lda         #0
    sta         CURSOR_X
    sta         CURSOR_Y
    sta         <SCREEN_PTR
    lda         #$10
    sta         >SCREEN_PTR
    
    rts