Reentrant and recurssive routine.

This explanation is based on a singular app system like DOS and may not apply to multi tasking systems.

Recursive

A recursive program directly or indirectly calls itself. It is typically used for sorting.

the initial sort instruction is broken up into smaller chunks and processed by the same code. Each level have to keep track of its own data (usually start and end counters).

Reentrant

A program is reentrant if it can continue with its original task on return from an external interruption. almost any program can be made reentrant by the interrupting program by saving registers (internal CPU memory) and other status markers.

Typically a TSR (terminate stay resident) program has to ensure compatibility.

A TSR usually watched the keyboard or timer interrupt to activate itself. It then save the registers & status in the stack and install its own requirements. on exit it then has to save his own and restore the previous registers and status.

With multi tasking the opp system can perform this tasks for you.

Your explanation is fine and doesn't need to be qualified on the basis of what operating system is in effect. You done good, pardner!

To our OP: Here is a brief example of a recursive function (programs are typically not recursive; recursion typically applies at the function level). This function computes the factorial of a number:

unsigned int factorial(unsigned int n) {
if (n <= 1) return 1;
return n * factorial(n-1);
}