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474
March 2002 Release
1-800-255-7778
Virtex-II Pro™ Platform FPGA Documentation
Chapter 6:
Virtual-Memory Management
R
Process-ID Register
The process-ID register (PID) is a 32-bit register used in virtual-address translation.
shows the format of the PID register. The fields in the PID are defined as shown
The PID is a privileged SPR with an address of 945 (0x3B1) and is read and written using
the
mfspr
and
mtspr
instructions.
Page-Translation Table
The page-translation table is a software-defined and software-managed data structure
containing page translations. The requirement for software-managed page translation
represents an architectural trade-off targeted at embedded-system applications.
Embedded systems tend to have a tightly controlled operating environment and a well-
defined set of application software. That environment enables virtual-memory
management to be optimized for each embedded system in the following ways:
•
The
page-translation table
can be organized to maximize page-table search performance
(also called
table walking
) so that a given page-translation entry is located quickly.
Most general-purpose processors implement either an indexed page table (simple
search method, large page-table size) or a hashed page table (complex search method,
small page-table size). With software table walking, any hybrid organization can be
employed that suits the particular embedded system. Both the page-table size and
access time can be optimized.
•
Independent
page sizes
can be used for application modules, device drivers, system-
service routines, and data. Independent page-size selection enables system software
to more efficiently use memory by reducing fragmentation (unused memory). For
example, a large data structure can be allocated to a 16 MB page and a small I/O
device-driver can be allocated to a 1 KB page.
•
Page replacement
can be tuned to minimize the occurrence of missing page-
translations. As described in the following section, the most-frequently used page
translations are stored in the translation look-aside buffer (TLB). Software is
responsible for deciding which translations are stored in the TLB and which
translations are replaced when a new translation is required. The replacement
strategy can be tuned to avoid
thrashing
, whereby page-translation entries are
constantly being moved in and out of the TLB. The replacement strategy can also be
tuned to prevent replacement of critical-page translations, a process sometimes
referred to as
page locking
.
The unified 64-entry TLB, managed by software, caches a subset of instruction and data
page-translation entries accessible by the MMU. Software uses the unified TLB to cache a
subset of instruction and data page-translation entries for use by the MMU. Software is
0
23 24
31
PID
Figure 6-3:
Process-ID Register (PID)
Table 6-1:
Process-ID Register (PID) Field Definitions
Bit
Name
Function
Description
0:23
Reserved
24:31
PID
Process Identifier
Used to uniquely identify a software process during address
translation.
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