Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uunet!mcsun!ukc!inmos!mph
From: mph@lion.inmos.co.uk (Mike Harrison)
Newsgroups: comp.arch
Subject: Re: RISC Machine Data Structure Word Alignment Problems?
Message-ID: <3919@ganymede.inmos.co.uk>
Date: 1 Feb 90 13:36:14 GMT
References: <1810@sunquest.UUCP>
Sender: news@inmos.co.uk
Reply-To: mph@inmos.co.uk (Mike Harrison)
Organization: INMOS Limited, Bristol, UK.
Lines: 64

In article <1810@sunquest.UUCP> terry@sunquest.UUCP (Terry Friedrichsen) writes:
>The abstruct/memstruct proposal for aligning/not aligning C structure
>members leads me to post what I thought was the obvious idea all
>along :-):
>
>Borrow Pascal's idea of "records" and "packed records".  So in C,

Or even borrow Ada's ideas, which separate the abstaction and the 
representation.

Given a sset of declarations such as:

    type INTEGER32 is range -2147483648 .. 2147483647;

    type INTEGER16 is range -32768 .. 32767;

    type STRUCT is 
      record
        F1 : INTEGER32;
        F2 : INTEGER16;
	F : INTEGER32;
      end record;

objects of type STRUCT will be mapped in any way that the compiler wishes,
with fields (potentially) re-ordered, padding added etc., for efficiency of
access.

If I need a specific mapping I can achieve it by providing a Representation
Clause, eg. for maximum packing:

    WORD : constant := 4; -- assumes storage unit is byte, 4 bytes per word.

    for INTEGER32'SIZE use 32;

    for INTEGER16'SIZE use 16;

    for STRUCT use
      record at mod 16;
        F1 at 0 * WORD range 0 .. 31;
        F2 at 1 * WORD range 0 .. 15;
        F3 at 1 * WORD range 16 .. 47;
      end record;

In this case objects of type STRUCT will be laid out exactly as shown and will
occupy exactly 80 bits, and will be aligned on a half-word boundary.
The compiler will generate appropriate code to access complete objects or 
idividual fields, for a RISC of the kind being discussed this will obviously
be (?much) less efficient, but if the application needs it who cares?

If I wish to declare an array of these objects, such as:

    type STRUCT_ARRAY is array (NATURAL range <>) of STRUCT;

I can inform the compile that I want maximum packing by writing :

    pragma PACK (STRUCT_ARRAY);

Mike,


Michael P. Harrison - Software Group - Inmos Ltd. UK.
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