title: Control-Flow Stack
description:

The ANS Forth standard says that it may not physically exist:
> 3.2.3.2   Control-flow stack
[...]
> ~~~tag[The control-flow stack may, but need not, physically exist]tag~~~
> ~~~tag[in an implementation.]tag~~~  If it does exist, it may be, but
> need not be, implemented using the data stack.  The format of
> the control-flow stack is implementation defined.

This means that the control-flow stack may exist
only logically rather than both logically and physically.

> Since the control-flow stack may be implemented
> using the data stack, items placed on the data stack
> are unavailable to a program after items are placed
> on the control-flow stack and remain unavailable until
> the control-flow stack items are removed.

Equally placing new items onto the data stack blocks
access to the control-flow stack items.

A standard program cannot assume that the items left on the data
stack are not modified, moved or removed while there are
control-flow stack elements above them.

See also: ../colon-sys