Linear Activation Chain

A particular subroutine (say "A") may call several other subroutines in succession (say "B", then "X", then "Y", then "Z"). But at any moment, it will have only one subroutine linked to it in the chain of activation. The calls in most modern programming languages follow this stack-based behavior. The currently executing subroutine is at the end of a linear chain of activations that link back to the operating system that first started "main".

The run-time stack has the same form as the chain of activation. The top of the stack is for the currently running subroutine. When that subroutine is finished its stack data is popped. Stacks are "Last In, First Out" data structures. Subroutines are activated in a "Last Called, First Finished" order.

When a subroutine reaches its end, it returns to its caller, and the chain is shortened. Its caller might call another subroutine, and the chain is lengthened again.

While a program that contains many subroutines is executing, the activation chain grows up and down like a yo-yo. Ultimately the chain is of length zero when the main routine returns control to the operating system.