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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: aliased & packed?
Date: 1997/05/13
Message-ID: <dewar.863545022@merv>#1/1
X-Deja-AN: 241443026
References: <5l5llv$4e@lotho.delphi.com> <dewar.863401335@merv>
 <3376B2F0.7CE4@bix.com> <dewar.863449539@merv> <EA33z0.2ts@world.std.com>
 <1997May13.065907.1@eisner>
Organization: New York University
Newsgroups: comp.lang.ada
Date: 1997-05-13T00:00:00+00:00
List-Id: <comp.lang.ada>


Larry said

<<The original machines uses the 21064 chip, with Loads and Stores being
at a minimum of 32 bit granularity.  Having loaded the desired 32 bits
it takes a couple of instructions to shift and mask the desired byte
into having a register all to itself.  Storing requires reading the
old value first, combining, and writing.>>

One further comment on this issue. If you use 8-bit characters, it is not
so bad to load and store 8-bits. What is truly horrible is guaranteeing
independence. That is very expensive. 

BUT, the pragma Pack, even if it does not make any difference from a storage
representation point of view, does mean that you no longer have to provide
independence -- so this is the very valuable function of pragma Pack on 
things like char_array and string.