Xilinx MicroBlaze Reference Manual page 63

Changing a zone field in the ZPR applies a protection override across all pages in that zone.
Without the ZPR, protection changes require individual alterations to each page translation
entry within the zone.
Unimplemented zones (when
UTLB Management
The UTLB serves as the interface between the processor MMU and memory-management
software. System software manages the UTLB to tell the MMU how to translate virtual
addresses into physical addresses. When a problem occurs due to a missing translation or
an access violation, the MMU communicates the problem to system software using the
exception mechanism. System software is responsible for providing interrupt handlers to
correct these problems so that the MMU can proceed with memory translation.
Software reads and writes UTLB entries using the MFS and MTS instructions, respectively.
These instructions use the TLBX register index (numbered 0 to 63) corresponding to one of
the 64 entries in the UTLB. The tag and data portions are read and written separately, so
software must execute two MFS or MTS instructions to completely access an entry. The
UTLB is searched for a specific translation using the TLBSX register. TLBSX locates a
translation using an effective address and loads the corresponding UTLB index into the
TLBX register.
Individual UTLB entries are invalidated using the MTS instruction to clear the valid bit in the
tag portion of a TLB entry (TLBHI[V]).
When
C_FAULT_TOLERANT
parity error, a TLB miss exception occurs. To avoid accumulating errors in this case, each
entry in the UTLB should be periodically invalidated.
Recording Page Access and Page Modification
Software management of virtual-memory poses several challenges:
• In a virtual-memory environment, software and data often consume more memory than
is physically available. Some of the software and data pages must be stored outside
physical memory, such as on a hard drive, when they are not used. Ideally, the most-
frequently used pages stay in physical memory and infrequently used pages are stored
elsewhere.
• When pages in physical-memory are replaced to make room for new pages, it is
important to know whether the replaced (old) pages were modified. If they were
modified, they must be saved prior to loading the replacement (new) pages. If the old
pages were not modified, the new pages can be loaded without saving the old pages.
• A limited number of page translations are kept in the UTLB. The remaining translations
must be stored in the page-translation table. When a translation is not found in the
UTLB (due to a miss), system software must decide which UTLB entry to discard so that
MicroBlaze Processor Reference Guide
UG984 (v2016.2) June 8, 2016
UG984 (v2016.1) April 6, 2016
< 16) are treated as if they contained 11.
C_MMU_ZONES
is set to 1, the UTLB block RAM is protected by parity. In case of a
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Chapter 2: MicroBlaze Architecture
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