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March 2002 Release
Virtex-II Pro™ Platform FPGA Documentation
1-800-255-7778
Virtual Mode
R
contains a page-number field and an offset field. The
page number represents the portion of the address translated by the MMU. The offset
represents the byte offset into a page and is not translated by the MMU. The virtual
address consists of an additional field, called the process ID (PID), which is taken from the
PID register (see
). The combination of PID and effective
page number (EPN) is referred to as the virtual page number (VPN). The value
n
is
determined by the page size, as shown in
System software maintains a page-translation table that contains entries used to translate
each virtual page into a physical page (see
). The page size defined by a page-
translation entry determines the size of the page number and offset fields. For example,
when a 4 KB page size is used, the page-number field is 20 bits and the offset field is 12 bits.
The VPN in this case is 28 bits. See
, for more information on page size.
Then the most frequently used page translations are stored in the translation look-aside
buffer (TLB). When translating a virtual address, the MMU examines the page-translation
entries for a matching VPN (PID and EPN). Rather than examining all entries in the table,
only entries contained in the processor TLB are examined (see
, for information on
the TLB). When a page-translation entry is found with a matching VPN, the corresponding
physical-page number is read from the entry and combined with the offset to form the 32-
bit physical address. This physical address is used by the processor to reference memory.
System software can use the PID to uniquely identify software processes (tasks,
subroutines, threads) running on the processor. Independently compiled processes can
operate in effective-address regions that overlap each other. This overlap must be resolved
by system software if multitasking is supported. Assigning a PID to each process enables
system software to resolve the overlap by relocating each process into a unique region of
virtual-address space. The virtual-address space mappings enable independent translation
of each process into the physical-address space.
shows an example of how the
PID is used in virtual-memory mapping (overlapping areas are shaded gray).
Figure 6-2:
Process-Mapping Example
UG011_39_033101
PID A
Process B
Process A
Process C
PID B
PID C
Process A
Process B
Process C
Process A
Process B
Process C
Virtual
Address Space
Effective
Address Space
Physical
Address Space
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