Computing checksum for Ada newbie

["mop" <mop65715@pegasus.cc.ucf.edu>]

I've got two types - we'll call them TYPE_1, TYPE_2.

The checksum (last byte) for TYPE_1 is the byte eight.
The checksum (last byte) for TYPE_2 varies (depends on user parameters)  but
the threshold is byte 517.
The checksum is a simple sum of the the bytes (dont care about overflow)
An example of TYPE_1 is provided.  "BYTE" is defined in my package.

type TYPE_1 is
record
 XX             :  BYTE;  -- first byte
 YY             :  BYTE;  -- second byte
-- etc
 Checksum  :  BYTE;  -- eighth byte
end record;

I want a function - we'll call it Compute_Checksum that'll sum the bytes

function Compute_Checksum return BYTE is  -- dont think this is right.  need
to pass in which type
   INTEGER : Checksum :=0;
begin
-- more help here
end Compute_Checksum;