8.1 The Process's Address Space

The address space of a process consists of all linear addresses that the process is allowed to use. Each process sees a different set of linear addresses; the address used by one process bears no relation to the address used by another. As we shall see later, the kernel may dynamically modify a process address space by adding or removing intervals of linear addresses.

The kernel represents intervals of linear addresses by means of resources called memory regions, which are characterized by an initial linear address, a length, and some access rights. For reasons of efficiency, both the initial address and the length of a memory region must be multiples of 4,096 so that the data identified by each memory region completely fills up the page frames allocated to it. Following are some typical situations in which a process gets new memory regions:

· When the user types a command at the console, the shell process creates a new process to execute the command. As a result, a fresh address space, thus a set of memory regions, is assigned to the new process (see Section 8.5 later in this chapter; also, see Chapter 20).

· A running process may decide to load an entirely different program. In this case, the process ID remains unchanged but the memory regions used before loading the program are released and a new set of memory regions is assigned to the process (see Section 20.4).

· A running process may perform a "memory mapping" on a file (or on a portion of it). In such cases, the kernel assigns a new memory region to the process to map the file (see Chapter 15).

· A process may keep adding data on its User Mode stack until all addresses in the memory region that map the stack have been used. In this case, the kernel may decide to expand the size of that memory region (see Section 8.4 later in this chapter).

· A process may create an IPC-shared memory region to share data with other cooperating processes. In this case, the kernel assigns a new memory region to the process to implement this construct (see Section 19.3.5).

· A process may expand its dynamic area (the heap) through a function such as malloc( ). As a result, the kernel may decide to expand the size of the memory region assigned to the heap (see Section 8.6 later in this chapter).

Table 8-1 illustrates some of the system calls related to the previously mentioned tasks. With the exception of brk( ), which is discussed at the end of this chapter, the system calls are described in other chapters.

Table 8-1. System calls related to memory region creation and deletion
System call	Description
brk( ), sbrk( )	Changes the heap size of the process
execve( )	Loads a new executable file, thus changing the process address space
_exit( )	Terminates the current process and destroys its address space
fork( )	Creates a new process, and thus a new address space
mmap( )	Creates a memory mapping for a file, thus enlarging the process address space
munmap( )	Destroys a memory mapping for a file, thus contracting the process address space
shmat( )	Attaches a shared memory region
shmdt( )	Detaches a shared memory region

As we shall see in the later section Section 8.4, it is essential for the kernel to identify the memory regions currently owned by a process (the address space of a process) since that allows the Page Fault exception handler to efficiently distinguish between two types of invalid linear addresses that cause it to be invoked:

· Those caused by programming errors.

· Those caused by a missing page; even though the linear address belongs to the process's address space, the page frame corresponding to that address has yet to be allocated.

The latter addresses are not invalid from the process's point of view; the kernel handles the Page Fault by providing the page frame and letting the process continue.