Xilinx MicroBlaze, Reference Manual

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MicroBlaze Processor Reference Guide www.xilinx.com 59
UG081 (v14.7)
Virtual-Memory Management
TLB Entry Format
Figure 2-20 shows the format of a TLB entry. Each TLB entry is 68 bits and is composed of two
portions: TLBLO (also referred to as the data entry), and TLBHI (also referred to as the tag entry).
Figure 2-20: TLB Entry Format
The TLB entry contents are described in Table 2-20, page 38 and Table 2-21, page 40 .
The fields within a TLB entry are categorized as follows:
•
Virtual-page identification (TAG, SIZE, V, TID)—These fields identify the page-translation
entry. They are compared with the virtual-page number during the translation process.
•
Physical-page identification (RPN, SIZE)—These fields identify the translated page in
physical memory.
•
Access control (EX, WR, ZSEL)—These fields specify the type of access allowed in the page
and are used to protect pages from improper accesses.
•
Storage attributes (W, I, M, G, E, U0)—These fields specify the storage-control attributes, such
as caching policy for the data cache (write-back or write-through), whether a page is
cacheable, and how bytes are ordered (endianness).
Table 2-37 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.
TLBLO:
0 22 23 24 28 29 30 31
↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑
RPN EX WR ZSEL W I M G
TLBHI:
0 22 25 26 27 28 35
↑ ↑ ↑ ↑ ↑ ↑
TAG SIZE V E U0
TID
Table 2-37: Page-Translation Bit Ranges by Page Size
Page
Size
SIZE
(TLBHI
Field)
Tag Comparison
Bit Range
Page Offset
Physical
Page
Number
RPN Bits
Clear to 0
1 KB 000 TAG[0:21] - Address[0:21] Address[22:31] RPN[0:21] -
4 KB 001 TAG[0:19] - Address[0:19] Address[20:31] RPN[0:19] 20:21
16 KB 010 TAG[0:17] - Address[0:17] Address[18:31] RPN[0:17] 18:21
64 KB 011 TAG[0:15] - Address[0:15] Address[16:31] RPN[0:15] 16:21
256 KB 100 TAG[0:13] - Address[0:13] Address[14:31] RPN[0:13] 14:21
1 MB 101 TAG[0:11] - Address[0:11] Address[12:31] RPN[0:11] 12:21
4 MB 110 TAG[0:9] - Address[0:9] Address[10:31] RPN[0:9] 10:21
16 MB 111 TAG[0:7] - Address[0:7] Address[8:31] RPN[0:7] 8:21
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