Re: [SDLTF-Members] SDL-News: SAVE: Request for additional feedba ck

Subject: Re: [SDLTF-Members] SDL-News: SAVE: Request for additional feedba ck
From: Rick Reed TSE (
Date: Fri Jan 09 2004 - 16:22:21 GMT

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Frend Patrick-BPF005 at wrote on 01/08/04 17:43:

> It would be sensible semantics that the queue was flushed up to the signal
> that
> instigated the reset (E2). It's also very simple, easy to implement/understand
> and it doesn't preclude the inclusion of "save" or "priority input" etc.

Dear All,

This would be an additional feature for SDL - which is the point the William
was making.

However, every feature - even a small one - makes the language more complex
- the opposite of considering a well defined subset. I doubt that this is
needed frequently enough to to justify a language change.

In any case this particular feature CAN be implemented by buffering the
information internally. This is not really surprising, because it seems that
any manipulations of the input queue CAN be changed to copying the message
information into an internal buffer, and then doing the manipulations on
that internal buffer. Indeed this approach allows the queue to be
manipulated in ways NOT currently permitted the current SDL mechanisms for
consuming (not consuming) signals from the input queue (input, priority
input, enabling condition, spontaneous input, continuous signal and save).
To use this approach requires additional data and code in the application to
handle the internal buffering written using other SDL features (data types
such as SEQUENCE OF the message type (possibly a union CHOICE of messages),
variables of this type, manipulation of the variables ...).

The whole discussion about SAVE is concerned with interpretation of the
terms of reference for the subset.

The argument given by William is that only "essential" features should be
included in the subset. Initially this seemed to be generally agreed, but it
now seems that different people have different opinions on what "essential"

It seems to me (and I hope William will confirm or deny my assertion and/or
elaborate) that William takes "essential" to mean:
   cannot be implemented using other features of the language.

Clearly such items as process, state, input, decision, data types, channels,
variables, output, assignment are "essential" in this sense.

As far as SAVE is concerned, it is not "essential" according to this
definition because it is possible to replace save by coding as outlined

The actual terms of reference for the task force are to propose:

"a small, useful, enhanced SDL subset.
It is intended to be the simplest, useful subset of SDL with Œenhancements
considered to be essentialı, prioritising:
* graphical representation, ensuring auto-layout is possible
* test capabilities, such as SDL based test scripts
* ASN.1 (1994) support, including encoding/decoding of PDUs
* associated methodology issues, such as maximum integration of tool chain."

The "essential" criteria in this case applies to enhancements.

The key words therefore for deciding what should and should not be in the
subset are "simplest" and "useful". The debate about whether SAVE should or
should not be in the subset then is an opinion whether it is simpler and/or
more useful to include/exclude SAVE from the subset.

It does appear to me the majority of SDL users are of the opinion that
including SAVE in the subset better meets the "simplest, useful" criteria,
but in the other hand the Solinet tool SAFIRE does not include SAVE (as far
as I am aware) and therefore illustrates that it is possible to produce
useful systems with finite state machines without SAVE.

The position being taken by William is "unless there is a technical
justification presented as an essential need for a specific situation, it
will not be part of the subset" being defined by the task force.

Rick Reed -
Tel:+44 15394 88462 Mob.:+44 7970 50 96 50

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