Virtual memory implementation in S370 3341

370 virtual memory architecture segment & page tables. 360-67 provided for both 24-bit virtual addressing and 32-bit virtual addressing (in its segment-page virtual memory structure).

the introduction of virtual memory on 370 was just 24-bit virtual addressing with segment and page tables, originally with 1mbyte and 64kbyte segment options as well as 4kbyte and 2kbyte page options.

the original 370 virtual memory architecture also had segment protection and some selective invalidate instructions. these additional features were dropped to help get the virtual memory hardware retrofit to 370-165 back on schedule.

in the morph from cp67-cms to vm370-cms, cms had been restructured to take advantage of "shared segments" (single copy of the same virtual memory page in physical storage for multiple different virtual address spaces) with the segment protect feature. when segment protect was dropped from 370 virtual memory architecture, the protection of these shared pages had to be reworked.

more than 24-bit virtual addressing wasn't re-introduced until 370-xa with the 3081.

I had done a page-mapped filesystem for CMS, which included compatibility for existing CMS filesystem API

It also included a number of extended features allowing page-mapped objects (shared & non-shared) to appear at arbitrary virtual address locations. the same shared (segment) object could even appear at different virtual addresses in different virtual address space. this required a lot of fiddling with the prevalent 360-370 software use of address constants. Lots of posts mentioning the fiddling with address constants

Only a small subset of the virtual memory object support was ever released called DisContiguous Shared Segment (DCSS). The generalized virtual memory object support and the page mapped filesystem support was never released.

various posts mentioning the segment protect issue:

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and misc. past posts mentioning dcss:

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