There are two cases under which an unwind may be invoked. Those initiated while an exception is active and those initiated while no exception is active.
An unwind that is initiated when no exception is active is usually done to perform a nonlocal GOTO; that is, to transfer control directly to some code location that is not part of the currently executing procedure or is not statically known. Even this type of operation must provide a mechanism to allow cleanup operations of terminated invocations (including restoring a consistent set of register values) to be performed. The unwind mechanism is used to support this type of operation.
The handler, or any descendant procedure called directly or indirectly by the handler, can continue execution of the thread at a different location than that at which the exception was raised by initiating an unwind operation.
An unwind operation specifies a target invocation in the procedure invocation chain and a location in that procedure. The operation terminates all invocations up to the target invocation, and continues thread execution at the specified location in that procedure.
Before control is transferred to the target location, the unwind operation invokes each frame-based handler that was established by any procedure invocations being terminated, plus the handler for the target invocation.