On Wed, 11 Jun 2008 08:03:27 -0400, Bob Shannon wrote: >>In fact "EXCP access method" is a kind of general description for any >>>untypical access methods and their possible untypical shortcomings. >>It can be no support for extfmt-PS, it can be limitation to first 64k >tracks >>on volume (afaik it existed in Adabas), it can be geometry->dependance etc. >>etc. > >EXCP is a perfectly valid access method; it just requires more skill, and more >detailed work, then using something such as QSAM. Regardless of the access >method, CCWs are used to access data on a volume. This indicates that EXCP is >used under the covers by the higher access methods. The only reason that EXCP >cannot be used for extended format datasets or PDSEs is that IBM installed >checks to prevent it. There is nothing inherent in EXCP itself that prevents >it from being used for any data type. > I'm naive here. I suspect many of my misconceptions will be promptly corrected.
It's my understanding that for many decades EXCP has not executed channel programs in place and as provided by the caller. Rather, they are moved to protected storage so the user can not modify them on the fly; they are prefixed to prevent seeks to prohibited tracks; virtual addresses are translated to real; etc. I'd further expect changes to CCW architecture to accommodate XA and later 64-bit addressing and new I/O architecture. So the "checks to prevent it" may be a matter of IBM's resource allotment: rather than continually update EXCP code to all new hardware features, it's easier simply to prohibit use of EXCP for such purposes. It has always struck me as bizarre that the OS supports running channel programs built by problem-state programs. This is secure only if the channel programs are in effect interpreted rather than executed directly. A more rational layering of functions should have channel programs built only by trustworthy supervisor-state code. -- gil ---------------------------------------------------------------------- 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
