IBM Mainframe Discussion List <[email protected]> wrote on 11/21/2007 12:44:50 PM:
> > > If I'm not mistaken, the nucleus is fixed and non-swappable. > > > Some of it is > > > even designed to run with DAT off. > > > > I wonder why? Is there something in fixed memory which would otherwise > > be unavailable because the frames are not in any page mapping? If not, > > they why not just have such frames be globally mapped V=R in all > address > > spaces? Most curious. A simple example would be when RSM backs a virtual page with a new frame, it needs to zero that frame before validating the virtual page. So the zeroing is done using the real address of the frame. Until z/Architecture, RSM invokes a DAT-off routine (in the DAT-off nucleus) to do the zeroing. In z/Architecture, we have much less use of DAT-off nucleus, because we can access storage via real address while running DAT-on in access register mode, using a pseudo data space (via an ASCE which the Real-space control bit (R bit) is turned on). > The nucleus is intended to contain only those portions of code that have > the most demanding requirements. Being page-fixed and straddling the > 16MB (virtual) line addresses most of that although there is still a > chunk of code (a.k.a. "the DAT-off nucleus") that arrives early and runs > V=R during NIP and is responsible for building the control structures > that enable the rest of the system to run V=V. > 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
