Committing Physical Storage Within a Region

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To use a reserved region of address space, you must allocate physical storage and then map this storage to the reserved region. This process is called committing physical storage. Physical storage is always committed in pages. To commit physical storage to a reserved region, you again call the VirtualAlloc function.

When you commit physical storage to regions, you do not have to commit physical storage to the entire region. For example, you can reserve a region that is 64 KB and then commit physical storage to the second and fourth pages within the region. Figure 13-1 shows what a process's address space might look like. Notice that the address space is different depending on which CPU platform you're running on. The address space on the left shows what happens on an x86 machine (which has a 4-KB page), and the address space on the right shows what happens on an Alpha machine (which has 8-KB pages).

When your program's algorithms no longer need to access committed physical storage in the reserved region, the physical storage should be freed. This process is called decommitting the physical storage and is accomplished by calling the VirtualFree function.