Here’s what happens when a program, running on a PC under DOS, branches to a function:

1. The address in the instruction pointer is incremented to the next instruction past the function call. That address is then placed on the stack, and it will be the return address when the function returns.

2. Room is made on the stack for the return type you’ve declared. On a system with two-byte integers, if the return type is declared to be int, another two bytes are added to the stack, but no value is placed in these bytes.

3. The address of the called function, which is kept in a special area of memory set aside for that purpose, is loaded into the instruction pointer, so the next instruction executed will be in the called function.

4. The current top of the stack is now noted and is held in a special pointer called the stack frame. Everything added to the stack from now until the function returns will be considered
"local" to the function.

5. All the arguments to the function are placed on the stack.

6. The instruction now in the instruction pointer is executed, thus executing the first instruction in the function.

7. Local variables are pushed onto the stack as they are defined.

When the function is ready to return, the return value is placed in the area of the stack reserved at step 2. The stack is then popped all the way up to the stack frame pointer, which effectively throws away all the local variables and the arguments to the function.

The return value is popped off the stack and assigned as the value of the function call itself, and the address stashed away in step 1 is retrieved and put into the instruction pointer. The program thus resumes immediately after the function call, with the value of the function retrieved.

Some of the details of this process change from compiler to compiler, or between computers, but the essential ideas are consistent across environments. In general, when you call a function, the return address and the parameters are put on the stack. During the life of the function, local variables are added to the stack. When the function returns, these are all removed by popping the stack.

In coming chapters we’ll look at other places in memory that are used to hold data that must persist beyond the life of the function.