Page 370 - DCAP103_Principle of operating system

Unit 12: Processes and Threads in Windows

thought they would have enough address space forever. They were wrong. The problem is back. Notes
Large programs often need more than the 2 GB or 3 GB of user address space Windows 2000
allocates to them, so bank switching is back, now called address windowing extensions. This
facility allows programs to map into shuffle chunks of memory in and out of the user portion
of the address space (and especially above the dreaded 4 GB boundary). Since it is only used
on servers with more than 2 GB of physical memory, we will defer the discussion until the
next edition of this site (by which time even entry-level desktop machines will be feeling the
32-bit pinch).

12.3.2 Memory Management System Calls
The Win32 API contains a number of functions that allow a process to manage its virtual
memory explicitly. The most important of these functions are listed in Figure 12.11. All of them
operate on a region consisting either of a single page or a sequence of two or more pages that
are consecutive in the virtual address space.

Figure 12.11: The Principal Win32 API Functions for Managing
Virtual Memory in Windows 2000

Win32 API function Description
VirtualAlloc Reserve or commit a region
VirtualFree Release or decommit a region

VirtualProtect Change the read/write/execute protection on a region
VirtualQuery Inquire about the status of a region
VirtualLock Make a region memory resident (i.e., disable paging for it)

VirtualUnlock Make a region pageable in the usual way
CreateFileMapping Create a file mapping object and (optionally) assign it a name
MapViewOfFile Map (part of) a file into the address space
UnmapViewOfFile Remove a mapped file from the address space
OpenFileMapping Open a previously created file mapping object

The first four API functions are used to allocate, free, protect and query regions of virtual address
space. Allocated regions always begin on 64 KB boundaries to minimize porting problems to
future architectures with pages larger than current ones (up to 64 KB). The actual amount of
address space allocated can be less than 64 KB, but must be a multiple of the page size. The next
two give a process the ability to hardware pages in memory so they will not be paged out and to
undo this property. A real-time program might need this ability, for example. A limit is enforced
by the operating system to prevent processes from getting too greedy. Actually, the pages can
be removed from memory, but only if the entire process is swapped out. When it is brought
back, all the locked pages are reloaded before any thread can start running again. Although
not shown in Figure 12.11, Windows 2000 also has API functions to allow a process to access
the virtual memory of a different process over which it has been given control (i.e., for which
it has a handle). The last four API functions listed are for managing memory-mapped files. To
map a file, a file mapping object (see Figure 12.4) must first be created, with CreateFileMapping.
This function returns a handle to the file mapping object and optionally enters a name for it

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