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From: WhiteR@CRPL.Cedar-Rapids.lib.IA.US (Robert S. White)
Subject: Re: Uninterruptible (atomic) operation?
Date: 1996/12/11
Message-ID: <58lb74$e6e@flood.weeg.uiowa.edu>#1/1
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references: <9612091326.AA04263@most> <E25tK3.3G7@world.std.com>
 <EACHUS.96Dec10204319@spectre.mitre.org>
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organization: my ISP is the Cedar Rapids Public Library
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newsgroups: comp.lang.ada,comp.realtime
Date: 1996-12-11T00:00:00+00:00
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In article <EACHUS.96Dec10204319@spectre.mitre.org>, eachus@spectre.mitre.org 

...snip...  using Ada95

>    The trick is to have a lock which
>all the potential modifiers respect, and to use pragma Atomic, and
>Atomic reads and writes, so that the readers can trust the value they
>see.
>
>    (Potential) Writer:
>
>       Acquire(lock);
>       Temp := Atomic_Object;
>       -- operations on Temp...
>       Atomic_Object := Temp;
>       Release(lock);
>
>    Reader:
>
>       Temp := Atomic_Object;
>       -- use value in temp.

  OK. I can live with that.  In the past using Ada83 and a local (company 
generated - DO-178BB) Real Time Executive (RTE) I could do the following 
(assuming a record structure):

  (Potential) Writer:
  with ESR;
    ...
  ESR.Enter_Critical_Section;
  -- various updating operations on Object.
  ESR.Leave_Critical_Section;

  
  Reader:
  
  with ESR;
    ...
  ESR.Enter_Critical_Section;
  Temp := Object.all;
  ESR.Leave_Critical_Section;
  -- use value in temp.

  With the real time constraint that thou use'st a minimum (1/3 of a tick max) 
time within a critical section.  ESR = Executive Service Routine package.

  But what Robert Eachus suggests is semantically equivalent and inclusive in a 
new ISO standardized language rather than a older language and a unique RTE.

_______________________________________________________________________
Robert S. White                    -- an embedded sys software engineer
WhiteR@CRPL.Cedar-Rapids.lib.IA.US --long/cheap alternate I-net address