Memory Arrangement

Figure 1: Typical Memory Arrangement

1. Text segment: This is the machine instructions that are executed by the CPU. Usually the segment is sharable so that only a single copy needs to be in memory for frequently executed programs. Also, the text segment is often read-only, to prevent a program from accidentally modifying its instructions.
   • Initialized data segment: This is usually just called the data segment and it contains variables that are specifically initialized in the program. For example, the C declaration

     int maxcount=99;
     

     appearing outside any function causes this variable to be stored in the initialized data segment with its initial value.
   • Uninitialized data segment: This segment is often called the "bss" segment, named after an ancient assembler operator that stood for "block started by symbol." Data in this segment is initialized by the kernel to arithmetic 0 or null pointers before the program starts executing. The C declaration

     long sum[1000];
     

     appearing outside any function causes this variable to be stored in the uninitialized data segment.
2. Stack: This is where automatic variables are stored, along with information that is saved each time a function is called. Each time a function is called, the address of where to return to, and certain information about the caller's environment (such as some of the machine registers) is saved on the stack. The newly called function then allocates room on the stack for its automatic and temporary variables. By utilizing a stack in this fashion, C functions can be recursive.
3. Heap: Dynamic memory allocation usually takes place on the heap. Historically the heap has been located between the top of the uninitialized data and the bottom of the stack.

Summary;

• Initialized Read Only Data; This contains the data elements that are initialized by the program and they are read only during the execution of the process.
• Initialized Read Write Data; This contains the data elements that are initialized by the program and will be modified in the course of process execution.
• Uninitialized Data; This contains the elements are not initialized by the program and are set 0 before the processes executes. These can also be modified and referred as BSS(Block Started Symbol).
• Stack; This portion is used for local variables, stack frames.
• Heap; This portion contains the dynamically allocated memory.

  int abc = 1;         ----> Initialized Read-Write Data
  char *str;           ----> BSS
  const int i = 10;    ----> Initialized Read-Only Data
  
  main()
  {
     int ii,a=1,b=2,c; ----> Local Variables on Stack
     char *ptr;
     ptr = malloc(4);  ----> Allocated Memory in Heap
     c= a+b;           ----> Text
  }