A Process s Virtual Address Space

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Every process is given its very own virtual address space. For 32-bit processes, this address space is 4 GB, since a 32-bit pointer can have any value from 0x00000000 through 0xFFFFFFFF. This allows a pointer to have one of 4,294,967,296 values, which covers a process's 4-GB range. For 64-bit processes, this address space is 16 EB (exabytes), since a 64-bit pointer can have any value from 0x00000000'00000000 through 0xFFFFFFFF'FFFFFFFF. This allows a pointer to have one of 18,446,744,073,709,551,616 values, which covers a process's 16-EB range. This is quite a range!

Since every process receives its very own private address space, when a thread in a process is running, that thread can access memory that belongs only to its process. The memory that belongs to all other processes is hidden and inaccessible to the running thread.

NOTE

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In Windows 2000, the memory belonging to the operating system itself is also hidden from the running thread, which means that the thread cannot accidentally access the operating system's data. In Windows 98, the memory belonging to the operating system is not hidden from the running thread. Therefore, the running thread could accidentally access the operating system's data and corrupt the operating system (potentially causing it to crash). It is not possible in Windows 98 for one process's thread to access memory belonging to another process.

As I said, every process has its own private address space. Process A can have a data structure stored in its address space at address 0x12345678, while Process B can have a totally different data structure stored in its address space—at address 0x12345678. When threads running in Process A access memory at address 0x12345678, these threads are accessing Process A's data structure. When threads running in Process B access memory at address 0x12345678, these threads are accessing Process B's data structure. Threads running in Process A cannot access the data structure in Process B's address space, and vice versa.

Before you get all excited about having so much address space for your application, keep in mind that this is virtual address space—not physical storage. This address space is simply a range of memory addresses. Physical storage needs to be assigned or mapped to portions of the address space before you can successfully access data without raising access violations. We will discuss how this is done later in this chapter.