Xilinx MicroBlaze, Справочное руководство

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64 www.xilinx.com MicroBlaze Processor Reference Guide
UG081 (v14.7)
Chapter 2: MicroBlaze Architecture
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
is set to 1, the UTLB block RAM is protected by parity. In case of a
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 the missing
translation can be loaded. It is desirable for system software to replace infrequently used
translations rather than frequently used translations.
Solving the above problems in an efficient manner requires keeping track of page accesses and page
modifications. MicroBlaze does not track page access and page modification in hardware. Instead,
system software can use the TLB-miss exceptions and the data-storage exception to collect this
information. As the information is collected, it can be stored in a data structure associated with the
page-translation table.
Page-access information is used to determine which pages should be kept in physical memory and
which are replaced when physical-memory space is required. System software can use the valid bit
in the TLB entry (TLBHI[V]) to monitor page accesses. This requires page translations be
initialized as not valid (TLBHI[V]=0) to indicate they have not been accessed. The first attempt to
access a page causes a TLB-miss exception, either because the UTLB entry is marked not valid or
because the page translation is not present in the UTLB. The TLB-miss handler updates the UTLB
with a valid translation (TLBHI[V]=1). The set valid bit serves as a record that the page and its
translation have been accessed. The TLB-miss handler can also record the information in a separate
data structure associated with the page-translation entry.
Page-modification information is used to indicate whether an old page can be overwritten with a
new page or the old page must first be stored to a hard disk. System software can use the write-
protection bit in the TLB entry (TLBLO[WR]) to monitor page modification. This requires page
translations be initialized as read-only (TLBLO[WR]=0) to indicate they have not been modified.
The first attempt to write data into a page causes a data-storage exception, assuming the page has
already been accessed and marked valid as described above. If software has permission to write into
the page, the data-storage handler marks the page as writable (TLBLO[WR]=1) and returns. The set
write-protection bit serves as a record that a page has been modified. The data-storage handler can
also record this information in a separate data structure associated with the page-translation entry.
Tracking page modification is useful when virtual mode is first entered and when a new process is
started.
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