Hello World

When a program is executed it is loaded into its virtual memory space. The compiled machine code instructions are loaded into the text segment, which is placed at the bottom of memory, i.e., from 0x0000 0000 upward; and any static data is placed in the data segment, which is located just after it.

In contrast, runtime information, such as the parameters and environment variables are placed at the top of memory, i.e., from 0xFFFF FFFC downward; and the stack is then initialised just below this (and grows downward in memory).

This example does not discuss the dynamic heap which is initialised just after the data segment, and which grows upward in memory toward the heap.

Remember, how a program is stored in memory will differ depending on the machine architecture, operating system, programming language, and compiler used. Also this is a simplified example.

Table 1. shows the contents of the memory after the program below (hello.c) has run. The representation of memory in Table 1 shows the data segment, which is contained within the memory addresses between 0x0000 10000 and 0x0000 1036; and the stack segment, which is shown contained within the memory addresses 0xFFFF FFDC and 0xFFFF FFFFC.

The stack segment contains two stack frames: the first was created when the function 'main' was called and is composed of a return address (for execution to jump back to), the return value, and the two parameters argc and argv that are passed to main. the second was created when the function 'printf' was called and is composed of a return address, the return value, and the two parameters that are passed to printf.

Note, it is likely that printf would have also called functions that would have created stack frames, but, as they are beyond the scope of this example, I will not attempt to show them.

$ ./hello "Hello world"

hello.c

int main( int argc, char** argv )
{
    int length = 99;
    
    length = printf( "%s\n", argv[0] );
    
    return length;
}

References