/ rof /
Restrictions on Functionality
Restriction on Functionality:
anything that prevents an ANS Forth program to run as
expected on a Standard System (Subset) that provides
all words used by the program.
RoF = Restriction on Functionality
An ANS Forth System may have extensions beyond the Standard.
These extensions may impose additional Restrictions on Functionality.
These Restrictions on Functionality may prevent ANS Forth code
from running on ANS Forth systems.
It is recommended for system implementors to document all
Restricions on Functionality and the circumstances when they
apply, for example:
- No access to the user device (only for background tasks)
- Examples of Restrictions on Functionality:
The code that executes in background tasks usually
cannot output to the user device.
That is, some ANS programs must be modified if we
want them to run in background tasks.
That is, the execution environment within background tasks
has restrictions on functionality.
Many systems support a variant of the word SAVE
(or TURNKEY ) that saves the Forth dictionary
in a file for subsequent running.
On some systems, address relocation is not done
during this process. In consequence of this,
all ANS Forth programs that store absolute
addresses of data structures into variables
(and other data structures) must be rewritten.
Note that we have an ANS Forth Program and
an ANS Forth System, but the reason why the
ANS Forth Program cannot run on the ANS Forth System
is beyond the scope of the current standard:
the word SAVE (whose use is inevitable for
some kinds of applications) imposes significant
Restrictions on Functionality.
Implementations of standard words may also
impose Restrictions on Functionality.
Consider an embedded processor whose return stack is
8 cells deep, and the following definition of PICK
: PICK ( xn .. x0 n -- xn .. x0 xn )
IF 1- SWAP >R RECURSE R> SWAP
ELSE DROP DUP
Note that n PICK requires 2*n+1 free cells on the
return stack. That is, 2 PICK requires 5 cells
on the return stack and therefore cannot be called
when there are more than 3 return addresses on the
return stack. And 3 PICK may be called only from
the main word.
The standard does not include any requiremens about
the depth of stacks.
It is the subject of an argument whether a system
whose Restrictions on Functionality inevitably apply
to all programs run on that system is standard-compliant.
Why inevitably? For example, creation of Windows applications
implies using SAVE or TURNKEY (else you would distribute the
compiler, and this would require a different license),
and therefore any code is useful only if it is SAVE-compatible.
The Standard says in 3. Usage requirements:
> A system may contain non-standard extensions, provided that they
> are consistent with the requirements of this Standard.
but the word "consistent" still may be read in two ways.
The participants of the discussion tend to agree that
extensions whose use requires rewriting standard code
are inconsistent, but in fact, there are degrees of
consistency. In the first example above, IMO we should
label the system as an "ANS Forth System", but
immediately mention that in background tasks there is
only "ANS Forth System Subset".
Another argument has been about whether RoF's are Environmental
Restrictions (ER's). RoF's are restrictions, and are imposed by the
execution environment, but theree also is an opinion that
the term Environmental Restrictions must be reserved for
restrictions on values and ranges only. There is no formal
definition of ER's in the Standard: one may conclude that
restrictions on ranges and values are ER's, but it is not
stated that other kinds of restrictions imposed by the environment
do not count as ER's.
- page written by:
I thank Elizabeth Rather and Andrew Haley
generated Wed Jul 23 02:53:41 2003mlg