ADD_MASK
ADD_MASK(scpu->scpu_imask[IPL_NET], scpu->scpu_imask[IPL_BIO]);
ADD_MASK(scpu->scpu_imask[IPL_TTY], scpu->scpu_imask[IPL_NET]);
ADD_MASK(scpu->scpu_imask[IPL_VM], scpu->scpu_imask[IPL_TTY]);
ADD_MASK(scpu->scpu_imask[IPL_CLOCK], scpu->scpu_imask[IPL_VM]);
ADD_MASK(scpu->scpu_imask[IPL_HIGH], scpu->scpu_imask[IPL_CLOCK]);
ADD_MASK(scpu->scpu_imask[IPL_IPI], scpu->scpu_imask[IPL_HIGH]);