From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00,FREEMAIL_FROM
	autolearn=unavailable autolearn_force=no version=3.4.4
X-Received: by 10.182.226.166 with SMTP id rt6mr7891986obc.47.1403472043464;
 Sun, 22 Jun 2014 14:20:43 -0700 (PDT)
X-Received: by 10.50.43.194 with SMTP id y2mr52127igl.3.1403472043310; Sun, 22
 Jun 2014 14:20:43 -0700 (PDT)
Path: 
 border1.nntp.dca.giganews.com!nntp.giganews.com!hn18no5935550igb.0!news-out.google.com!qf4ni15igc.0!nntp.google.com!uq10no4605085igb.0!postnews.google.com!glegroupsg2000goo.googlegroups.com!not-for-mail
Newsgroups: comp.lang.ada
Date: Sun, 22 Jun 2014 14:20:42 -0700 (PDT)
In-Reply-To: <lo79hk$bss$1@dont-email.me>
Complaints-To: groups-abuse@google.com
Injection-Info: glegroupsg2000goo.googlegroups.com;
 posting-host=108.69.213.73;
 posting-account=Rkpt4AoAAAB8fHBAe9UIH5TYP1BQ2uA2
NNTP-Posting-Host: 108.69.213.73
References: <7eaee5fc-2045-4bb3-8b16-d757b54760da@googlegroups.com>
 <lo79hk$bss$1@dont-email.me>
User-Agent: G2/1.0
MIME-Version: 1.0
Message-ID: <de640d15-a0ae-45f9-b3da-f5ba414f81f1@googlegroups.com>
Subject: Re: Cleanest Ada way to do this?
From: Mike Silva <hungupharps@gmail.com>
Injection-Date: Sun, 22 Jun 2014 21:20:43 +0000
Content-Type: text/plain; charset=ISO-8859-1
Xref: number.nntp.dca.giganews.com comp.lang.ada:187176
Date: 2014-06-22T14:20:42-07:00
List-Id: <comp.lang.ada>

On Sunday, June 22, 2014 12:07:32 PM UTC-7, Simon Clubley wrote:
> On 2014-06-22, Mike Silva  wrote:
> 
> > I'd like some help on the cleanest, or most elegant, way to do this in Ada. 
> 
> > I'm writing to an LCD character display, in 4-bit mode, which requires writing
> 
> > an 8 bit character or command value as two 4 bit pieces.  The port (a memory
> 
> > address) being written to is 32 bits wide.  What I would like to be able to do
> 
> > is some form of this (not valid Ada, I know):
> 
> >
> 
> >  Port32(4..7) := Some_Char(4..7);
> 
> >  ..twiddle some other control bits..
> 
> >  Port32(4..7) := Some_Char(0..3);
> 
> >  ..twiddle some other control bits..
> 
> >
> 
> 
> 
> Do the other bits in Port32 need to be preserved when (4..7) are written ?
> 
> 
> 
> Is it _required_ that Port32 is written in units of 32 bits ?
> 
> (The answer to that is probably yes, but I thought I would check).
> 
> 
> 
> There was some discussion of this class of problem a few weeks ago when
> 
> I was proposing syntax to allow the atomic update of multiple bitfields
> 
> at the same time without having to use an intermediate variables.
> 
> 
> 
> The conclusions from that were that if you need to preserve the other
> 
> bits in Port32(4..7) your only real option is to use a temporary
> 
> variable with a 32-bit record structure to match your bitfield
> 
> requirements (unless you want to get into using C style bitmasks.)

Yes, the other bits need to be preserved.  In C the code would look something like this

P32 = (P32 & ~0x00F0) | (c & 0xF0);
...twiddle some control bits
P32 = (P32 & ~0x00F0) | ((c << 4) & 0xF0);