MMU/Cache analysis for OpenRISC

Hesham Moustafa heshamelmatary at
Mon Jul 28 17:20:01 UTC 2014


I have read about MMU and Caches in OpenRISC to start implementing
related managers in the new or1k port. Before beginning to write the
code, I would like to introduce some of the architecture details
regarding MMU and Caches.

There are both; data and instruction MMU, TLBs, Caches.
First, page tables there are divided into two levels; one level can be
used with 32 GB or 16 MB pages, and two-levels for 8 KB pages. 32 GB
and 16 MB would be too big for RTEMS, and that led us to two-level 8
KB pages. Two-level pages would hurt the performance unless we setup
TLBs at startup statically; there are 128 TLB entries. Other than
defined run-time sections, the remaining pool of TLB entries can be
used for dynamic page-table setup.

Protection domains are set into a special purpose registers forming 7
groups, and each PTE can associates its protection domain to one of
them. Protection provides control over pages for read, write, and
execute accesses.

Christian, I would like to know more about or1ksim implementation. I
guess TLB misses are handled in software and accordingly software
look-up operation. Also I would say that PTE reloads are also handled
in software. Please correct me if I am wrong.

Cache is working with PTEs; each of of which has some control bits to
define the Cache attributes for its page.

Given the previous little piece of info, I would appreciate your
opinion regarding implementation decisions for or1k RTEMS port.


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