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March 2002 Release
477
Virtex-II Pro™ Platform FPGA Documentation
1-800-255-7778
Translation Look-Aside Buffer
R
•
Storage attributes
—These fields specify the storage-control attributes, such as whether
a page is cacheable and how bytes are ordered (endianness).
The following sections describe the fields within each category.
Virtual-Page Identification Fields
The virtual-page identification portion of a TLB entry contains the following fields:
•
TAG
(TLB-entry tag)—TLBHI, bits 0:21. This field is compared with the EPN portion
of the EA (EA[EPN]) under the control of the SIZE field.
, shows
the bit ranges used in comparing the TAG with EA[EPN]. In this table, TAG
x:y
represents the bit range from the TAG field in TLBHI and EA
x:y
represents the bit
range from EA[EPN].
•
SIZE
(Page size)—TLBHI, bits 22:24. This field specifies the page size as shown in
. The SIZE field controls the bit range used in comparing the TAG
field with EA[EPN].
•
V
(Valid)—TLBHI, bit 25. When this bit is set to 1, the TLB entry is valid and contains
a page-translation entry. When cleared to 0, the TLB entry is invalid.
•
TID
(Process Tag)—TLBHI, bits 28:35. This 8-bit field is compared with the PID field
in the process-ID register. When TID is clear (0x00), the field is ignored and not
compared with the PID field. A clear TID indicates the TLB entry is used by all
processes.
Physical-Page Identification Fields
The physical-page identification portion of a TLB entry contains the following field:
•
RPN
(Physical-page number, or real-page number)—TLBLO, bits 0:21. When a TLB
hit occurs, this field is read from the TLB entry and is used to form the physical
address. Depending on the value of the SIZE field, some of the RPN bits are not used
in the physical address.
Software must clear unused bits in this field to 0.
, for information on which bits must be cleared.
Access-Control Fields
The access-control portion of a TLB entry contains the following fields:
•
EX
(Executable)—TLBLO, bit 22. When this bit is set to 1, the page contains executable
code and instructions can be fetched from the page. When this bit is cleared to 0,
instructions cannot be fetched from the page. Attempts to fetch instructions from a
page with a clear EX bit cause an instruction-storage exception.
•
WR
(Writable)—TLBLO, bit 23. When this bit is set to 1, the page is writable and store
instructions can be used to store data at addresses within the page. When this bit is
cleared to 0, the page is read only (not writable). Attempts to store data into a page
with a clear WR bit cause a data-storage exception.
•
ZSEL
(Zone select)—TLBLO, bits 24:27. This field selects one of 16 zone fields (Z0–
Z15) from the zone-protection register (ZPR). For example, if ZSEL
=
0b0101, zone field
0
21 22
24 25 26 27 28
35
TAG
SIZE
V
E U0
TID
TLBHI (Tag Entry)
0
21 22
23 24
27 28 29 30 31
RPN
EX WR
ZSEL
W
I
M
G
TLBLO (Data Entry)
Figure 6-6:
TLB-Entry Format
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