// calculates seek on canvas
.vwf_calc_seek:
    rep #$20
    lda vwf_draw_y  // y * 2 + (x >> 3) * 7 * 16
    asl
    sta vwf_seek
    lda vwf_draw_x
    lsr
    lsr
    lsr
    sta vwf_temp_0
    asl
    adc vwf_temp_0
    asl
    adc vwf_temp_0
    asl
    asl
    asl
    asl
    adc vwf_seek
    sta vwf_seek
    rts