MESS

//Instructions: Replace the vga_vblank function, with this one here.

static UINT8 vga_vblank(running_machine &machine)
{
    UINT8 res;
    UINT16 vblank_start,vblank_end,vpos;

    /* calculate vblank start / end positions */
    res = 0;
    vblank_start = vga.crtc.vert_blank_start;
    vblank_end = vga.crtc.vert_blank_start + vga.crtc.vert_blank_end - 1;
    vpos = machine.primary_screen->vpos();

    /* check if we are under vblank period */
    if(vblank_end > vga.crtc.vert_total)
    {
        vblank_end -= vga.crtc.vert_total;
        if(vpos >= vblank_start || vpos < vblank_end)
            res = 1;
    }
    else
    {
        if(vblank_end < vblank_start) //Needed for Fractint's 600 scanline tweaked modes. TODO: turn this into general logic
        {
            if(vpos >= vblank_start && vpos < vga.crtc.vert_total)
                res = 1;
            else if(vpos < vblank_end)
                res = 1;
        }
        else
        {
            if(vpos >= vblank_start && vpos < vblank_end)
                res = 1;
        }
    }

    //popmessage("%d %d %d",vblank_start,vblank_end,vga.crtc.vert_total);

    return res;
}