Implementation Of Page Tables In IA-32
Architecture
The IA-32 architecture utilizes
a two-tier page organization, starting with
the Intel 80386. A table-top level called
directory pages for each task
should be given a separate directory page, a physical address that is stored
in a special control register and where it automatically loads the hardware
when switching context. Table bottom level is called a
page table .
The linear address is divided into three fields:
❖
Directory ( Directory ) - defines the directory entry pages,
indicating
the desired page table
❖
Table ( Table ) -vyznachaye page table element that indicates the
desired frame memory
❖
Offset ( Offset ) - determines the offset within the frame, combined
with the address of the frame forms a physical address
The size of the directory and table fields is 10 bits,
giving page tables
containing 1024 elements, the size of the offset field is 12 bits, giving pages
and frames 4 KB in size. One lower-level page table addresses 4 MB of
memory (1 MB of frames) and the entire directory of pages has 4 GB .
The page table elements at all levels have the same structure. Here are the
following element fields:
❖
Flag presence ( Present ), equal to one of the page is in physical
memory (it corresponds to the frame); equality of this flag means that
the page is not in physical memory, and the operating system may use
other element fields for its purposes
❖
The 20 most significant bits that set the frame start address to be
multiple of 4 KB (1 MB of different start addresses can be specified)
❖
Flag Access ( Accessed ), which is equal to one pin each time the
reference device support paged to the corresponding frame
❖
Flag Changes ( Dirty ), which confer equal to one while recording
each transaction
in the corresponding frame
❖
Flag read-write ( Read / Write ), which specifies the rights to
access the page or page table (for reading and write or read-only)
❖
The privilege ( User / Supervisor )
check box that specifies the
processor mode required to access the page. If this checkbox is zero,
the page can only be addressed from privileged mode, if units are also
available from user mode
Presence, access, and change checkboxes can be used by the OS to organize
virtual memory .
