The following message is a courtesy copy of an article that has been posted to alt.folklore.computers,bit.listserv.ibm-main as well.
Anne & Lynn Wheeler <[EMAIL PROTECTED]> writes: > one of the other issues for TLB (hardware that translates virtual page > addresses to real page addresses) ... all the entries were > tagged/associated with specific virtual address spaces > ... i.e. "STO-associative". This generalized mechanism resulted in a > huge number of "duplicated" entries CSA/common-segment. So as a > special case optimization for the whole MVS CSA/common-segment hack > gorp ... a special option was provided that identified virtual > addresses as something belonging to common-segment. These areas then > became associated in the TLB with effectively a system-wide, unique, > artificial "common-segment" virtual address space (effectively > violating the whole generalized virtual address space architecture > ... rather than associated with generalized virtual address space > ... it became associated with a custom operating system specific > construct that was known to have very specific characteristics). re: http://www.garlic.com/~lynn/2007r.html#67 CSA 'above the bar' from z/architecture principles of operation http://publibz.boulder.ibm.com/cgi-bin/bookmgr_OS390/BOOKS/dz9zr003/CCONTENTS segment-table entries http://publibz.boulder.ibm.com/cgi-bin/bookmgr_OS390/BOOKS/dz9zr003/3.11.2.2?DT=20040504121320 defining the (MVS-specific) common-segment bit (in 64bit address segment table) ... aka Common-Segment Bit (C): Bit 59 controls the use of the translation-lookaside-buffer (TLB) copies of the segment-table entry and of the page table which it designates. A zero identifies a private segment; in this case, the segment-table entry and the page table it designates may be used only in association with the segment-table origin that designates the segment table in which the segment-table entry resides. A one identifies a common segment; in this case, the segment-table entry and the page table it designates may continue to be used for translating addresses corresponding to the segment index, even though a different segment table is specified. ... snip ... ... aka segment table (and the corresponding segment table origin address or "STO") is effective equivalent to unique virtual address space. since MVS has the common segment(s) appearing in every virtual address space, rathing than filling up TLB entries with large number of duplicated entries for the same information, effecitvely create a special class of virtual addresses that apply across everything in the system. this ugly common segment gorp then creates all sort of complications (that weren't part of the original virtual memory architecture) ... see the programming notes regarding common segment operation/problems at the above URL describing segment-table entries. ---------------------------------------------------------------------- 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
