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2005-08-24

A look at several memory management units...


An interesting paper (via Muli Ben-Yehuda) that describes, in particular, VM organization of several platforms. A bit I didn't knew: ULTRIX uses two level page tables that are traversed bottom-up: leaf level of the tree lives in the virtual space and is contiguous in it. As a result, to handle TLB miss at the virtual address A it's enough to do:

ULTRIX page-fault handler. V0
        phys_address = LEAF_TABLE[A >> PTABLE_SHIFT_LEAF];

Obviously it is somewhat more involved in reality, because appropriate portion of LEAF_TABLE[] can be not in the TLB itself. In the latter case, root node of the tree is consulted:

ULTRIX page-fault handler. V1
        if (tlb_miss(LEAF_TABLE + (A >> PTABLE_SHIFT_LEAF))
                tlb_load(ROOT_NODE[A >> PTABLE_SHIFT_ROOT],
                         LEAF_TABLE + (A >> PTABLE_SHIFT_LEAF));
        phys_address = LEAF_TABLE[A >> PTABLE_SHIFT_LEAF];

Root node is wired down into unmapped (physical) memory.

This design provides following advantages:

  • TLB miss handling requires one memory access in the best case, and two in the worst. In top-to-bottom page tables a la Intel, two (or three) accesses are necessary for every TLB refill;
  • This integrates nicely with virtually indexed processor caches;
  • This allows parts of page tables to be paged out easily.
Unfortunately Digital screwed this design by using slow software filled TLB.
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