GPX_VDAC_OFFSET
ioaccess(virtual_avail, vax_trunc_page(GPXADDR + GPX_VDAC_OFFSET), 1);
ss->ss_vdac = (void *)(virtual_avail + (GPX_VDAC_OFFSET & VAX_PGOFSET));
GPX_VDAC_OFFSET, 1);