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From: kilgallen@eisner.decus.org (Larry Kilgallen)
Subject: Re: Exception Handling
Date: 1996/09/19
Message-ID: <1996Sep19.145824.1@eisner>#1/1
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references: <96091718200832@psavax.pwfl.com>
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organization: LJK Software
newsgroups: comp.lang.ada
Date: 1996-09-19T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <96091718200832@psavax.pwfl.com>, "Marin David Condic, 407.796.8997, M/S 731-93" <condicma@PWFL.COM> writes:
> Larry Kilgallen <kilgallen@EISNER.DECUS.ORG> writes:
>>DEC Pascal triggers exceptions when constraints are violated, and
>>catching them is done in a vendor-specific manner.  I would assume
>>the discussion is not limited to standard-based implementations,
>>because then we would not be talking about who originate the concept
>>but rather whose committee was quickest to adopt research.
>>
>     I've long admired the quality of DEC compilers ever since using
>     their Fortran-10 on the PDP-10. I have two questions: 1) At what
>     point in time did they extend "standard" Pascal with some form of
>     exception processing? 2) Could you show a "simple" example of how
>     a user might raise & handle the exception? Thanks.

>From the start the VMS operating system had exceptions built into
the stack frames, so DEC Pascal V1 (1979 or so ?) users could
force establishment of their own handler by calling the VMS
routine LIB$ESTABLISH.  When DEC Pascal V2 arrived (1982 ?)
it was a total rewrite and required use of a specific language
extension to establish programmer-specified handlers since the
same OS mechanism was also used for language-specified handlers
pertaining to the chaining for uplevel addressing as I recall.

The following code has not been run through a compiler, and
leaves out some of the attributes required for safe programming.
The compiler would surely complain, but the is probably more
clear than the version the compiler would accept:

	PROCEDURE MY_PROCEDURE;

	FUNCTION MY_HANDLER ( SIGVEC : SIGVEC_TYPE;
				 MECHVEC : MECHVEC_TYPE ) : INTEGER;
		BEGIN

		IF SIGVEC [1] = MY_EXCEPTION_VALUE
		THEN
			MY_HANDLER := SS$_CONTINUE
		ELSE
			MY_HANDLER := SS$_RESIGNAL;
		END;
	BEGIN

	ESTABLISH ( MY_HANDLER );

	LIB$SIGNAL ( MY_EXCEPTION_VALUE );

	END;

Larry Kilgallen