Tlb Structure - ARM ARM926EJ-S Technical Reference Manual

3.7

TLB structure

ARM DDI0198D
The MMU contains a single unified TLB used for both data accesses and instruction
fetches. The TLB is divided into two parts:
• an eight-entry fully-associative part used exclusively for holding locked down
TLB entries
• a set-associative part for all other entries, 2 way x 32 entry.
Whether an entry is placed in the set-associative, or lockdown part of the TLB is
dependent on the state of the TLB lockdown register, when the entry is written into the
TLB (see TLB Lockdown Register c10 on page 2-32).
When an entry has been written into the lockdown part of the TLB, it can only be
removed by being overwritten explicitly, or by an MVA-based TLB invalidate
operation, where the MVA matches the locked down entry.
The structure of the set-associative part of the TLB does not form part of the
programmer's model for the ARM926EJ-S processor. No assumptions must be made
about the structure, replacement algorithm, or persistence of entries in the
set-associative part. Specifically:
• Any entry written into the set-associative part of the TLB can be removed at any
time. The set-associative part of the TLB must be considered as a temporary cache
of translation/page table information. No reliance must be placed on an entry
either residing or not residing in the set-associative TLB, unless that entry already
exists in the lockdown TLB. The set-associative part of the TLB can contain
entries that are defined in the page tables but do not correspond to address values
that have been accessed since the TLB was invalidated.
• The set-associative part of the TLB must be considered as a cache of the
underlying page table, where memory coherency must be maintained at all times.
If a level one descriptor is modified in main memory, then to guarantee coherency
either an invalidate TLB or invalidate TLB by entry operation must be used to
remove any cached copies of the level one descriptor. This is required regardless
of the type of level one descriptor (section, level two page table reference, or
fault).
• If any of the subpage permissions for a given page are different, then each of the
subpages are treated separately. To invalidate all the entries associated with a page
with subpage permissions then four MVA-based invalidate operations are
required, one for each subpage.
Copyright © 2001-2003 ARM Limited. All rights reserved.
Memory Management Unit
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