6.3. Memory Layout of a Process

The memory allocated to each process is composed of a number of parts, usually referred to as segments. These segments are as follows:

• The text segment contains the machine-language instructions of the program run by the process. The text segment is made read-only so that a process doesn’t accidentally modify its own instructions via a bad pointer value. Since many processes may be running the same program, the text segment is made sharable so that a single copy of the program code can be mapped into the virtual address space of all of the processes.

• The initialized data segment contains global and static variables that are explicitly initialized. The values of these variables are read from the executable file when the program is loaded into memory.

• The uninitialized data segment contains global and static variables that are not explicitly initialized. Before starting the program, the system initializes all memory in this segment to 0. For historical reasons, this is often called the bss segment, a name derived from an old assembler mnemonic for “block started by symbol.” The main reason for placing global and static variables that are initialized into a separate segment from those that are uninitialized is that, when a program is stored on disk, it is not necessary to allocate space for the uninitialized data. Instead, the executable merely needs to record the location and size required for the uninitialized data segment, and this space is allocated by the program loader at run time.

• The stack is a dynamically growing and shrinking segment containing stack frames. One stack frame is allocated for each currently called function. A frame stores the function’s local variables (so-called automatic variables), arguments, and return value. Stack frames are discussed in more detail in Section 6.5.

• The heap is an area from which memory (for variables) can be dynamically allocated at run time. The top end of the heap is called the program break.