Intel ITANIUM ARCHITECTURE - SOFTWARE DEVELOPERS VOLUME 3 REV 2.3, Manual

Page: 820 / 1898

2:572 Volume 2, Part 2: Memory Management
tables, or as a primary page table with collision chains. The long format VHPT is a
much better representation for address spaces that are sparsely populated, since
the short format VHPT has a linear layout and would consume a large amount of
memory. Single address space operating systems may prefer the long format VHPT
for this reason.
5.3.1 Short Format
The short format VHPT is a per-region linear table that contains translation entries for
every page in the region’s virtual address space. This makes the VHPT very large, but
since the VHPT itself lives in virtual address space only those parts of the VHPT that
actually contain valid translation entries have to be present in physical memory. If the
operating system’s page table is a hierarchical data structure and the last level of the
hierarchy is a linear list of translations, the VHPT can be mapped directly onto the page
table as shown in
Figure 5-1 .
If the VHPT walker tries to access a location in the VHPT for which no translation is
present in the TLB, a VHPT Translation fault is raised. The original address for which the
VHPT walker was trying to find an entry in the VHPT is supplied to the fault handler in
the IFA register. The fault handler can use this address to traverse the page table and
insert a translation into the TLB that maps the address the VHPT walker tried to access
(in IHA) to the page that contains the corresponding leaf page table.
Table 5-1. Comparison of VHPT Formats
Attribute Short Format Long Format
Entry Size 8 Byte 32 Byte
Lookup Linear Hashed
Protection Keys No Yes
Page Size per region per entry
Figure 5-1. Self-mapped Page Table
PTA
. . .
. . . . . .
. . . . . . . . .
Page Table
VHPT
P
T
E
P
T
E
P
T
E
P
T
E
P
T
E
P
T
E
P
T
E
P
T
E
P
T
E
P
T
E
P
T
E
P
T
E
P
T
E
P
T
E
P
T
E