Access Control

  The page tables are also used by the CPU and the Operating System to control access to memory. Here the access control information in the page tables is used to check, again on each virtual memory access, that the action is allowable. Traditionally the code in an image is separated out from the data and made read only. In this way the code is protected from corruption by not allowing anything to write to it. Process specific data structures, for example those holding the environment variables, are also protected in this way. A process that attempted to access a page in a way that was not allowed would be terminated with an access violation.

The access control information is held in the PTE and is CPU specific; figure  shows the PTE for Alpha AXPThe bit fields have the following meanings:

V

Valid, if set this PTE is valid,

FOE

``Fault on Execute'', Whenever an access of this type occurs, the CPU reports a page fault and passes control to the operating system,

FOW

``Fault on Write'',

FOR

``Fault on Read'',

ASM

Address Space Match. This is used when the operating system wishes to clear only some of the entries from the Translation Buffer,

KRE

Code running in kernel mode can read this page,

URE

Code running in user mode can read this page,

GH

Granularity hint used when mapping an entire block with a single Translation Buffer entry rather than many,

KWE

Code running in kernel mode can write to this page,

UWE

Code running in kernel mode can read this page,

PFN

For PTEs with the V bit set, this field contains the physical Page Frame Number (PFN) for this PTE. For invalid PTEs, if this field is not zero, it contains information about where the page is in the swap file.  

The following two bits are defined and used by Linux:

__PAGE__DIRTY

if set, the page needs to be written out to the swap file,  

__PAGE__ACCESSED

Used by Linux to mark a page as having been accessed.