Program may be executed from start to finish via a single function call, or step by step.
When executing programs step by step each next instruction execution takes one function call. Such, the application initializes program execution and than for as long as the program execution hasn't completed and application so desires, it executes the program instructions.
Each coded expression translates to an executable instruction and control statements add each one or two extra executable instructions.
No memory allocation is performed by the interpreter itself during program execution, and no other resources are otherwise allocated.