IBM Mainframe Discussion List <IBM-MAIN@BAMA.UA.EDU> wrote on 10/18/2007 07:32:53 AM:
> I have never delved into I/O prevention, so I don't know what it does, how, > or why, but I believe it is used in preventing an I/O's being executed rather > than positioning the request in the queue. I/O Prevention was introduced in SP2.1.6 in support of HARP a.k.a XRF (it was a hot standby thing). It was used for stopping I/O on the dying system which was being switched to the hot standby. It could be considered to be a software ancestor to what we now do via Fencing (a.k.a Channel Subsystem Iosolation) in the machine when XCF/SFM partitions a system out of a sysplex. I think I/O prevention is currently used to implement the IOACTION STOP command. > The purpose of the UCB Capture service is to allow anciently compiled > programs using ancient access methods that require the DCB, DEB, > IOB, etc., to be > in storage below the ancient 16MB line to do I/O with a thoroughly modern UCB > that is above the line. The big problem was that the DEB has room only for a > 3-byte address of the UCB. Rather than redesign the ancient access methods > that were based on EXCP, IBM provided a way for old access methods to get to > new UCBs above the line with the system service IOSCAPU. IOSCAPU creates a > copy (called "capturing" the UCB) of an above-the-line UCB in the user's > private storage and puts that copy below the line. The various > ancient control > blocks that need to point to a UCB will point to the private > storage copy with > a 24-bit address. The captured UCB isn't a copy of the UCB - it is simply another virtual view of the UCB created via the IARVSERV page level sharing services. > Leading edge access methods are continually updated by IBM > to support every new wrinkle that the IOS developers apply to UCBs in order > to support system growth. Many of the original constraints in the software > and hardware architecture of S/360 have been relaxed, with occasional seismic > jolts to the appearance of the UCB; e.g., UCBs can now be above the 16MB > line, devices can have five hexadecimal digits in the device number(formerly > known as "device address"), there can be more than 65536 devices onone LPAR, > UCBs can be dynamically created in order to support dynamically installed new > I/O hardware, and more than one I/O can be simultaneously active > with the same > device. Technically, a device number is still only 4 hex digits, but the same device number can exist in more than one subchannel set. For convenience in things like GTF and System Trace, we concatenate the subchannel set id to the device number, and sometimes loosely think of that as a 5 digit device number. Jim Mulder z/OS System Test IBM Corp. Poughkeepsie, NY ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
