14. The Process Address Space

Chapter 11, “Memory Management,” looked at how the kernel manages physical memory. In addition to managing its own memory, the kernel also has to manage the process address space—the representation of memory given to each user-space process on the system. Linux is a virtual memory operating system, and thus the resource of memory is virtualized among the processes on the system. To an individual process, the view is as if it alone has full access to the system's physical memory. More importantly, the address space of even a single process can be much larger than physical memory. This chapter discusses how the kernel manages the process address space.

The process address space consists of the linear address range presented to each process and, more importantly, the addresses within this space that the process is allowed to use. Each process is given a flat 32- or 64-bit address space, with the size depending on the architecture. The term “flat” describes the fact that the address space exists in a single range. (As an example, a 32-bit address space extends from the address 0 to 429496729.) Some operating systems provide a segmented address space, with addresses existing not in a single linear range, but instead in multiple segments. Modern virtual memory operating systems generally have a flat memory model and not a segmented one. Normally, this flat address space is unique to each process. A memory address in one process's address space tells nothing of that memory address in another process's address space. Both processes can have different data at the same address in their respective address spaces. Alternatively, processes can elect to share their address space with other processes. We know these processes as threads.