Page 371 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes into the file system so another process can use it. The next two functions map and unmap files,
respectively. The last one can be used by a process to map in a file currently also mapped in by
a different process. In this way, two or more processes can share regions of their address spaces.
This technique allows them to write in limited regions of each other’s memory.

12.3.3 Implementation of Memory Management

Windows 2000 supports a single linear 4 GB demand-paged address space per process.
Segmentation is not supported in any form. Theoretically, page sizes can be of any power
of two up to 64 KB. On the Pentium they are fixed at 4 KB; on the Itanium they can be 8 KB
or 16 KB. In addition, the operating system itself can use 4 MB pages to reduce page table
space consumed.
Unlike the scheduler, which selects individual threads to run and does not care much about
processes, the memory manager deals entirely with processes and does not care much about
threads. After all, processes, not threads, own the address space and that is what the memory
manager deals with. When a region of virtual address space is allocated, as four of them have
been for process A in Figure 12.10, the memory manager creates a VAD (Virtual Address
Descriptor) for it, listing the range of addresses mapped, the backing store file and offset where
it is mapped, and the protection code. When the first page is touched, the directory of page
tables is created and a pointer to it inserted is in the VAD. In fact, an address space is completely
defined by the list of its VADs. This scheme supports sparse address spaces because unused
areas between the mapped regions use no resources.

12.3.4 Page Fault Handling

Windows 2000 does not use any form of prepaging. When a process starts, none of its pages
are in memory. All of them are brought in dynamically as page faults occurs. On each page
fault, a trap to the kernel occurs. The kernel builds a machine-independent descriptor telling
what happened and passes this to the memory manager part or the executive. The memory
manager then checks it for validity. If the faulted page falls within a committed or reserved
region, it looks up the address in the list of VADs, finds (or creates) the page table, and looks
up the relevant entry.

The page table entries are different for different architectures. For the Pentium, the entry for a
mapped page is shown in Figure 12.12. Unmapped pages also have entries, but their format is
somewhat different. For example, for an unmapped page that must be zeroed before it may be
used, that fact is noted in the page table.

Figure 12.12: A Page Table Entry for a Mapped Page on the Pentium

Bits 20 3 111111111
Not W
Page frame used G L DA C t UWV

G: Page is global to all processes Wt: Write through (no caching)
L: Large (4 MB) page U: Page is accessible in user mode
D: Page is dirty W: Writing to the page permitted
A: Page has been accessed V: Valid page table entry
C: Caching enabled/disabled

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