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From: kenner@lab.ultra.nyu.edu (Richard Kenner)
Subject: Re: Not intended for use in medical devices
Date: 1997/05/14
Message-ID: <5lc1rb$jb7$1@news.nyu.edu>#1/1
X-Deja-AN: 241480254
References: <dewar.863006367@merv>
 <mheaney-ya023680000805970900450001@news.ni.net> <dewar.863254614@merv>
Organization: New York University Ultracomputer Research Lab
Newsgroups: comp.lang.ada
Date: 1997-05-14T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <dewar.863254614@merv> dewar@merv.cs.nyu.edu (Robert Dewar) writes:
>   type xt is mod 2**8;
>   x : xt;
>   for x'address use ...
>   pragma Atomic (x);
>   for x'size use 8;
>
>and after that mouthful, we write:
>
>   if (x and 2#0100#) then ..

An even more subtle example with that same declaration is writing

	x := 0;

On a PDP-11, this will normally generate a "CLR" instruction, which
sets a location to zero.  But on most PDP-11 models, CLR first reads
the location, then sets it to zero (because all of the other
one-operand instructions read the location, so the hardware designers
took a shortcut).  If reading and writing to a memory-mapped I/O
location means very different things (relatively common, especially
back in those days), this can have very unexpected results.

(This is not hypothetical; I often made the mistake of using CLR in
assembler code when I needed to use a MOV instruction to zero some
mapped location.)

Of course, an Ada (and C!) compiler for the PDP-11 should probably
avoid using CLR on volatile memory for just this reason.