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3.3. BASIC CONCEPTS CHAPTER 3. VM, PROTECTION AND PAGING
Field Bits Description
present 1 If set, the linear address range associated with
this Entry is valid.
writeable 1 If set, writing to the the linear address range
associated with this entry is possible. Note that
unless the WP flag in CR0 is set, supervisor
level code can still write to such pages.
user_access 1 If set, this page is user accessible, otherwise it
is only supervisor accessible (which means only
with privilege level 3)
write_through 1 Switches between write-through and writeback
caching on a per page basis.
cache_disabled 1 Controls caching on an per page basis.
accessed 1 Set by the CPU if a linear address associated
with this page has been accessed. Should be
cleared by the operating system after every read
of this bit to continuously track page access.
dirty 1 Set by the CPU if a page has been written to
since the bit was last cleared by the operating
system.
use_4_m_pages 1 This bit only exists in the Page-Directory-Entry.
If set, the last 10 bits of this Page-Directory-
Entry are interpreted as the page number of a
physical 4MiB Page. Unset, the last 20 bits are
interpreted as the 4KiB physical page number
of a Page-Table
global_page 1 If set by the operating system the CPU assumes
this to be a shared page and handles entries
in the TLB (Translation Look-aside Buffer) accordingly.
Note that this bit has effect only in
a Page-Table-Entry or Page-Directory-Entries
that point to 4MiB Pages
page_table_base_address 20 the 20 most significant bits of the physical address
of a Page-Table
page_base_address 10 or 20 the 20 most significant bits of the start address
of a physical 4KiB-Page or the 10 most significant
bits of the start address of a 4MiB-Page
Table 3.3: Explanation of Fields in Page-Directory- and Page-Table-Entries
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