virtual memory 4491
in this previous incarnation of the thread-subject from 7aug2004 there was the issue of paging...
cp67 use to support a limited number of shared pages in this "64kbyte" segment option (16 4k pages) in the two level table structures.
a virtual address space was represented by a "segment" table. segment table entries pointed to page tables. page tables had PTE entries that indicated real address. segment table entries in different segment tables (i.e. virtual address spaces) could point at the same page table. such a page table (buttociated with a specific segment or range of virtual addresses) could then be "shared" accross multiple different address spaces. however, real, shared pages would only have a single, unique PTE in a single page table ... even tho multiple different segment tables could point at the same shared page.
as previous mentioned, the original 370 architecture allowed for read-only shared segment protection. basically a spare bit was defined in the segment table entry (which included pointer to a potentially shared page table entry) that precluded instructions executing in that virtual address space from storing-altering locations in that address range. this allowed for storage protection property to be virtual address space specific for the same shared pages i.e. a shared page table is "store" protected for virtual address spaces with the segment protect bit set in their segment table entry ... however the same shared page table may not be "store" protect for virtual address spaces w-o the segment protect bit set.
in the morph from cp67 to vm370, cms was reorganized to take advantage of the 370 shared segment architecture and the segment protection (allowing different address spaces to share the same information but preventing one cms from affecting other cms'es).
Right. All you had to was zero the word, or half word, or bit, that pointed at the index of the page table...
as previously mentioned, when the retrofit of virtual memory hardware to 370-165 fell behind schedule, the segment protect feature was dropped from the architecture (as well as some number of other features in the original 370 virtual memory architecture). this forced vm370-cms to revert to the storage protection mechanism that had been used in cp67.
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as mentioned in the previous post some of the work that i had done in the 60s as...
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