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March 2002 Release
475
Virtex-II Pro™ Platform FPGA Documentation
1-800-255-7778
Translation Look-Aside Buffer
R
responsible for reading entries from the page-translation table in system memory and
storing them in the TLB. The following section describes the unified TLB in more detail.
Internally, the MMU also contains a 4-entry shadow TLB for instructions and an 8-entry
shadow TLB for data. These shadow TLBs are managed entirely by the processor
(transparent to software) and are used to minimize access conflicts with the unified TLB.
shows the relationship of the page-translation tables and the TLBs.
Translation Look-Aside Buffer
The translation look-aside buffer (TLB) is used by the PPC405 MMU for address
translation, memory protection, and storage control when the processor is running in
virtual mode. Each entry within the TLB contains the information necessary to identify a
virtual page (PID and effective page number), specify its translation into a physical page,
determine the protection characteristics of the page, and specify the storage attributes
associated with the page.
The PPC405 TLB is physically implemented as three separate TLBs:
•
Unified TLB
—The UTLB contains 64 entries and is fully associative. Instruction-page
and data-page translation can be stored in any UTLB entry. The initialization and
management of the UTLB is controlled completely by software.
•
Instruction Shadow TLB
—The ITLB contains four instruction page-translation entries
and is fully associative. The page-translation entries stored in the ITLB represent the
four most-frequently accessed instruction-page translations from the UTLB. The ITLB
is used to minimize contention between instruction translation and UTLB-update
operations. The initialization and management of the ITLB is controlled completely by
hardware and is transparent to software.
Figure 6-4:
Page-Translation Table and TLB Organization
UG011_40_033101
Software-Managed
Interface
System Memory
Processor
Page
Translation
Table
64-Entry
Unified
TLB
4-Entry
Instruction
Shadow
TLB
8-Entry
Data
Shadow
TLB
Processor-Managed
Interface
Processor-Managed
Interface
Содержание Virtex-II Pro PPC405
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