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478
March 2002 Release
1-800-255-7778
Virtex-II Pro™ Platform FPGA Documentation
Chapter 6:
Virtual-Memory Management
R
Z5 is selected. The selected ZPR field is used to modify the access protection specified
by the TLB entry EX and WR fields. It is also used to prevent access to a page by
overriding the TLB V (valid) field. See
, for more
information.
Storage-Attribute Fields
The storage-attribute portion of a TLB entry contains the following fields:
•
E
(Endian)—TLBHI, bit 26. When this bit is set to 1, the page is accessed as a little-
endian page. When cleared to 0, the page is accessed as a big-endian page. See
, for information on little-endian and big-endian byte accesses.
•
U0
(User defined)—TLBHI, bit 27. When this bit is set to 1, access to the page is
governed by a user-defined storage attribute. When cleared to 0, the user-defined
storage attribute does not govern accesses to the page. See
, for more information.
•
W
(Write Through)—TLBLO, bit 28. When this bit is set to 1, accesses to the page are
cached using a write-through caching policy. When cleared to 0, accesses to the page
are cached using a write-back caching policy. See
, for
more information.
•
I
(Caching inhibited)—TLBLO, bit 29. When this bit is set to 1, accesses to the page are
not cached (caching is inhibited). When cleared to 0, accesses to the page are
cacheable, under the control of the W attribute (write-through caching policy). See
, for more information.
•
M
(Memory coherent)—TLBLO, bit 30. Setting and clearing this bit does not affect
memory accesses in the PPC405. In implementations that support multi-processing,
this bit can be used to improve the performance of hardware that manages memory
coherency.
•
G
(Guarded)—TLBLO, bit 31. When this bit is set to 1, speculative page accesses are
not allowed (memory is guarded). When cleared to 0, speculative page accesses are
allowed. The G attribute is often used to protect memory-mapped I/O devices from
inappropriate accesses. See
, for more information.
In real mode, the storage-attribute control registers are used to define storage attributes.
See
Storage-Attribute Control Registers
for more information.
shows the relationship between the TLB-entry SIZE field and the translated page
size. This table also shows how the page size determines which address bits are involved in
a tag comparison, which address bits are used as a page offset, and which bits in the
physical page number are used in the physical address. The final column, “
n
”, refers to a
When assigning sizes to instruction pages, software must be careful to avoid creating the
opportunity for instruction-cache synonyms. See
,
for more information.
Table 6-2:
Page-Translation Bit Ranges by Page Size
Page
Size
SIZE
(TLB Field)
Tag Comparison
Bit Range
Page
Offset
Physical-Page
Number
RPN Bits
Clear to 0
n
(
1 KB
0b000
TAG
0:21
↔
EA
0:21
EA
22:31
RPN
0:21
—
22
4 KB
0b001
TAG
0:19
↔
EA
0:19
EA
20:31
RPN
0:19
20:21
20
16 KB
0b010
TAG
0:17
↔
EA
0:17
EA
18:31
RPN
0:17
18:21
18
64 KB
0b011
TAG
0:15
↔
EA
0:15
EA
16:31
RPN
0:15
16:21
16
256 KB
0b100
TAG
0:13
↔
EA
0:13
EA
14:31
RPN
0:13
14:21
14
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