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From: ncohen@watson.ibm.com (Norman H. Cohen)
Newsgroups: comp.lang.ada
Subject: Re: Importing C Structures
Date: 27 Mar 1995 16:00:48 GMT
Organization: IBM T.J. Watson Research Center
Distribution: world
Message-ID: <3l6nfg$12kj@watnews1.watson.ibm.com>
References: <3kr4q3$jd9@newsflash.concordia.ca>
  <VLADIMIR.95Mar22235527@speedy.intrepid.com>
  <3ksg66$h1c@newssvr.cacd.rockwell.com>
 <VLADIMIR.95Mar24005132@speedy.intrepid.com>
Reply-To: ncohen@watson.ibm.com
NNTP-Posting-Host: rios8.watson.ibm.com
Date: 1995-03-27T16:00:48+00:00
List-Id: <comp.lang.ada>

In article <VLADIMIR.95Mar24005132@speedy.intrepid.com>,
vladimir@speedy.intrepid.com (Vladimir Vukicevic) writes: 

|> In article <3ksg66$h1c@newssvr.cacd.rockwell.com> rswhite@cacd.rockwell.com (Robert S. White) writes: 
|>
|> > Do you have a clean way to handle hardware registers that have one
|> > meaning when you read them and another when you write to them with
|> > nice seperate names for each type of usage?
...
|> > Recommendations?
|>
|> Something like this (in Ada 95): 
|>
|> Input_Register : Integer;
|> for Input_Register'Address use
|>      System.Storage_Elements.To_Address (16#0020);
|>
|> Output_Register : Integer;
|> for Output_Register'Address use
|>      System.Storage_Elements.To_Address (16#0020);
|>
|> This is perfectly valid and within the spirit of the 95 LRM.

Yes, it is.  Personally, I think memory-mapped I/O is a pernicious
abstraction.  Just because the hardware lies and claims that we are
reading from or writing to a storage location when we are reading from
or writing to a device, I see no need to perpetuate this lie in the
source program and make it appear as though we are reading from or
writing to a variable.  Therefore, I would prefer a procedural interface: 

  with System;
  package Low_Level_IO is  -- Loosely inspired by RM83 14.6
     type Device_Address is new System.Address;
     type Word is mod 2**32; -- or whatever
     procedure Send_Control (Device: in Device_Address; Data: in Word);
     procedure Receive_Control
        (Device: in Device_Address; Data: out Word);
  end Low_Level_IO;

  with System.Address_To_Access_Conversions;
  package body Low_Level_IO is

     use System;

     package Word_Pointers is new Address_To_Access_Conversions (Word);

     procedure Send_Control (Device: in Device_Address; Data: in Word) is
     begin
        Word_Pointers.To_Pointer(Address(Device)).all := Data;
     end Send_Control;

     procedure Receive_Control
        (Device: in Device_Address; Data: out Word) is
     begin
        Data := Word_Pointers.To_Pointer(Address(Device)).all;
     end Receive_Control;

  end Low_Level_IO;

Code procedures would provide even less of a suggestion that some
variable is being updated or referenced.

--
Norman H. Cohen    ncohen@watson.ibm.com