Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!apple!netcomsv!mcmahan
From: mcmahan@netcom.COM (Dave Mc Mahan)
Newsgroups: comp.dsp
Subject: Re: bit reversal of a C array in '30 code
Message-ID: <1991Jun26.052754.5772@netcom.COM>
Date: 26 Jun 91 05:27:54 GMT
References: <1991Jun25.082320.599@otago.ac.nz>
Organization: Dave McMahan @ NetCom Services
Lines: 58


 In a previous article, psyxsgp@otago.ac.nz writes:
>Hi,
>    Could anyone tell us how to zero align an array
>generated in C, but calling an assembler routine in
>tms320c30 code which does bit reversal.

One way to long word-align an array is to put it in a seperate segment,
and then ensure that the segement starts on the proper boundry.  Failing
that, you can do it at run-time.  The code isn't pretty, but it does work.

The trick to this is to declare an array that is 4 bytes larger than you
really need.  You then set a pointer to the first location in the array,
add 4 to the pointer, and then logically AND the pointer with ~(3) to
force it to contain a value that MUST fall on the proper boundry.  This gets
a little messy if you have strict type-checking, but fortunately 'C' doesn't
do that.  The code might be something like this:

#define    MYSIZE    100    /*  Size in bytes you want, not size of
                                longwords you will eventually use.
                            */

static    char  dumb_array[MYSIZE+4];

    long        *ptr_to_use;

main()
{

    /*  Notice the use of casting to prevent the compiler from complaining
        about the fast-and-loose type conversions.
    */

    ptr_to_use = (long *)(&dumb_array[4]);
    ptr_to_use = (long *)((long)ptr_to_use & ~3L);

    ptr_to_use[0]  = 6;
    ptr_to_use[24] = 9943;     /*  This is the last element of the array
                                    that you can safely use.  Anything bigger
                                    may fall outside the declared array.
                               */
}    


Caveat:  I haven't compiled this example, but it should work.  I have used
         this technique before when I had to force alignment within my data
         and couldn't get the compiler to do it for me.


>Stephen Pearce



   -dave

-- 
Dave McMahan                            mcmahan@netcom.com
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