-----Original Message----- From: IBM Mainframe Discussion List [mailto:[EMAIL PROTECTED] On Behalf Of Paul Gilmartin Sent: Wednesday, June 11, 2008 8:26 AM To: [email protected] Subject: Re: EXCP access methos
On Wed, 11 Jun 2008 08:03:27 -0400, Bob Shannon wrote: <SNIP> 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. <SNIP> Ok, here goes from about the 10,000' level. You are basically correct when it comes to VM. If you are not a preferred guest, then expect ALL your CCWs to be "interpreted". If you are a preferred guest, then you get dispatched with SIE (Start Interpretive Execution, or some equivalent in the IEF, Interpretive Execution Facility -- been gone from H/W for too long) where VM sets certain masks.... And so some or all of your CCWs will basically be run as written. NOW for the "MVS" world. The system will start your CCWs AFTER it has run its initial chain (by a TIC to your first CCW). Depending on the H/W (devices, controllers, channels, etc.) will determine which of the control CCWs will be executed to LIMIT what your CCW string is allowed to do (such as setting CYL LIMITS where you can't seek outside of those without getting your hand slapped). I'm sure that others will be able to take you down to the settings of the ORB, SCHIB, etc. should it be needed. Regards, Steve Thompson -- All opinions expressed by me are my own and may not necessarily reflect those of my employer. -- ---------------------------------------------------------------------- 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
