"NULL" is not an architectural concept. I don't even know what a "response" to an instruction is. The correct result for an instruction is whatever the principles of operation says the result is.
Some facility bits are passed through to the guest. Others are provided to the guest only if the host has provided suitable support. Peter Relson z/OS Core Technology Design ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN
