How do I use ADA to calculate a checksum?

How do I use ADA to calculate a checksum?

[Charles Phillips]

Hello,

I'm relatively new to ADA95, and would like to ask for assistance
from those of you who are more knowledgeable.

I have an embedded application which requires me to perform several
checksum calculations.  The question is, when passed a starting 
address and a length, how do I "increment" the address and then
access the data at the location?

Any help or recommendations with respect to reference books would
be greatly appreciated.

Charles

Re: How do I use ADA to calculate a checksum?

[Tom Moran]

I presume you want to calculate a byte or word-wise checksum on data
which is a record (ie, not an array of bytes or words)?  Yoy can
delcare an array of bytes/words and use a representation clause to
make it overlay, ie, start at the same address as, the record.  Then
just run through the array calculating the checksum, CRC, or whatever.
If you are passed an Access type rather than an Address, use the
appropriate function in System.Address_To_Access_Conversions to get an
Address. 

Re: How do I use ADA to calculate a checksum?

[David C. Hoos, Sr.]

Hi Charles,

For what it's worth, here's how I would go about it, given the
limited "requirements specification.":

-- First, define a type for the checksum.
type Checksum_Type is <implementation-defined>;
-- Perhaps if you're looking for an n-bit checksum, the type would
-- be mod 2 ** N. (See the  LRM about modular types)


-- Next, define the checksum function specification:

function Checksum
   (Starting_Address : System.Address;
    Data_Length : Natural
   ) return Checksum_Type;

-- Now, we can define the function body:

function Checksum
   (Starting_Address : System.Address;
    Data_Length : Natural
   ) return Checksum_Type is
   -- Define a "storage unit" type
   type Storage_Unit_Type is mod 2 ** System.Storage_Unit;
   -- Define the type of an array of these "storage units"
   type Storage_Unit_Array_Type is array (1 .. Data_Length) of
Storage_Unit_Type;
   -- Define the array of data to be checksummed
   Storage_Unit_Array : Storage_Unit_Array_Type;
   -- Now, locate the array where the data is
   for Storage_Unit_Array'Address use Starting_Address;
  -- Define the object to be returned, initialized to zero
  The_Checksum : Checksum_Type := 0;
begin
   -- Now, the storage elements can be accessed individually -- e.g.,
   -- Note that this loop will be executed Data_Length times,
   -- even if Dat_Length is 0
   for i in Storage_Unit_Array'range loop
      --....:=... Storage_Unit_Array (i) .........
   end loop;
   -- ..........
   return The_Checksum;
end Checksum;

There are many good books on Ada95, and I own well over a dozen.  But.. if
I had
to limit myself to one it would be Norman Cohen's "Ada as a Second
Language",
Second Edition.  You can get it at Madison Books and Computers.

David C. Hoos, Sr.,
david.c.hoos.sr@ada95.com
(205) 726-4965

Charles Phillips <charles@scarecrow.gcs.redstone.army.mil> wrote in article
<3427DF0F.4467@scarecrow.gcs.redstone.army.mil>...
> Hello,
> 
> I'm relatively new to ADA95, and would like to ask for assistance
> from those of you who are more knowledgeable.
> 
> I have an embedded application which requires me to perform several
> checksum calculations.  The question is, when passed a starting 
> address and a length, how do I "increment" the address and then
> access the data at the location?
> 
> Any help or recommendations with respect to reference books would
> be greatly appreciated.
> 
> Charles
> 

Re: How do I use ADA to calculate a checksum?

[Charles Phillips]

David C. Hoos, Sr. wrote:

Sorry I didn't provide enough specifications.  Thanks for the
assistance in directing me to storage units.  Seems I was making
it more difficult(in other words, like C) than necessary.

> to limit myself to one it would be Norman Cohen's "Ada as a Second
> Language",
> Second Edition.  You can get it at Madison Books and Computers.
> 

Thanks for the recommendation.

Charles

Re: How do I use ADA to calculate a checksum?

[Charles Phillips]

David C. Hoos, Sr. wrote:

Sorry I didn't provide enough specifications.  Thanks for the
assistance in directing me to storage units.  Seems I was making
it more difficult(in other words, like C) than necessary.

> to limit myself to one it would be Norman Cohen's "Ada as a Second
> Language",
> Second Edition.  You can get it at Madison Books and Computers.
> 

Thanks for the recommendation.

Charles