Declaring Arrays of Bits (how)?

Declaring Arrays of Bits (how)?

[agate!spool.mu.edu!sdd.hp.com!cs.utexas.edu!sun-barr!west.West.Sun.COM!cr]

I am trying to declare arrays of bits in Ada.  Here's my attempt:


type Array_Range_Type is range 1..128;

type Bit_Type is (OFF, ON);
for  Bit_Type use (OFF => 0, ON => 1);
for  Bit_Type'size use 1;

type Bit_Array_Type is array (Array_Range_Type) of Bit_Type;

Bit_Array : Bit_Array_Type;


This looks right to me.  I would expect Bit_Array to be represented
as an array of 128 bits (128 contiguous bits).  This is not the case,
however.  My compiler (XDADA) is representing Bit_Array as an array
of 128 bytes.  As an added note, I have been using representation clauses
for records without any problems.  

Am I missing something?  
Is there a special representation clause for arrays that I am overlooking? 
Is my compiler at fault?

Thanks for the INPUT,
Mark Young

Re: Declaring Arrays of Bits (how)?

[jvnc.net!darwin.sura.net!jhunix.hcf.jhu.edu!aplcen.apl.jhu.edu!ddsdx2.jhu]

In <YOUNG.92Sep29101112@kuwait.gdfwc3> young@gdfwc3 (Mark Young) writes:


>I am trying to declare arrays of bits in Ada.  Here's my attempt:

>type Array_Range_Type is range 1..128;

>type Bit_Type is (OFF, ON);
>for  Bit_Type use (OFF => 0, ON => 1);
>for  Bit_Type'size use 1;

>type Bit_Array_Type is array (Array_Range_Type) of Bit_Type;
-- Add this line:
pragma Pack(Bit_Array_Type);
-- and you will find that Bit_Array_Type objects will be stored as 
-- you originally expected.  The compiler chose the most efficient
-- implementation of Bit_Array_Type, as it is allowed to do.  
-- However, see below for a better way to do what you are probably
-- really trying to do.

>Bit_Array : Bit_Array_Type;


-- I believe that you really want a packed array of boolean which
-- is how I often represent bits:

type Array_Range_Type is range 1..128;
type Bit_Array_Type is array(Array_Range_Type) of Boolean;
pragma pack(Bit_Array);

-- This makes it much easier to do logical bitwise operations on
-- the resulting data structure, which is usually why you want
-- arrays of bits in the first place.  Note that the packed boolean
-- array is guaranteed to take up exactly 1 bit per boolean by
-- some AI or other.


--Thor
dlc@ddsdx2.jhuapl.edu
collard@capsrv.jhuapl.edu

Re: Declaring Arrays of Bits (how)?

[Mark A Biggar]

In article <YOUNG.92Sep29101112@kuwait.gdfwc3> young@gdfwc3 (Mark Young) writes
:
>I am trying to declare arrays of bits in Ada.  Here's my attempt:
>type Array_Range_Type is range 1..128;
>type Bit_Type is (OFF, ON);
>for  Bit_Type use (OFF => 0, ON => 1);
>for  Bit_Type'size use 1;
>type Bit_Array_Type is array (Array_Range_Type) of Bit_Type;
>Bit_Array : Bit_Array_Type;
>
>This looks right to me.  I would expect Bit_Array to be represented
>as an array of 128 bits (128 contiguous bits).  This is not the case,
>however.  My compiler (XDADA) is representing Bit_Array as an array
>of 128 bytes.  As an added note, I have been using representation clauses
>for records without any problems.  
>Am I missing something?  
>Is there a special representation clause for arrays that I am overlooking? 
>Is my compiler at fault?

For most compilers you probably need to throw a:

pragma PACKED(Bit_Array_Type);

in the mix somewhere as well.  Otherwise according to the LRM the compiler is
under no obligation to put the array element in anything smaller then a byte
each unless you add the pragma PACKED.

--
Mark Biggar
mab@wdl1.wdl.loral.com

Re: Declaring Arrays of Bits (how)?

[Bob Kitzberger]

>I am trying to declare arrays of bits in Ada.  Here's my attempt:
>
>type Array_Range_Type is range 1..128;
>
>type Bit_Type is (OFF, ON);
>for  Bit_Type use (OFF => 0, ON => 1);
>for  Bit_Type'size use 1;
>
>type Bit_Array_Type is array (Array_Range_Type) of Bit_Type;

Try slapping a rep spec on the array type:

   for Bit_Array_Type'size use 128;


	.Bob.
----------------
Bob Kitzberger          VisiCom Laboratories, Inc.
rlk@visicom.com         10052 Mesa Ridge Court, San Diego CA 92121 USA
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