Re: Big-endian vs little-endian

[Matthew Heaney <matthew_heaney@acm.org>]

Mike Werner <mwerner@wvu.edu> writes:

> Here's the relevant data structure:
> 
>    type Sys_type is (Zarya, Unity, PMA1, PMA2);
>    type Subsys_type is (CDH, CT, ECLSS, EPS, GNC, SM);
>    subtype Desc_type is String(1..256);
>    subtype Dur_Min_Type is Integer;
>    subtype Dur_Sec_type is Integer;
>    type Apm_Rec is 
>       record
> 	 Description : Desc_Type;      
> 	 System : Sys_Type;
> 	 Subsystem : Subsys_Type;
> 	 Dur_Min : Dur_Min_Type;
> 	 Dur_Sec : Dur_Sec_Type;  
>       end record;
> 
> The problematic parts were the Apm_Rec.Dur_Min and the Apm_Rec.Dur_Sec -
> all the others read in just fine.  If I'm understanding all this, should
> I have changed
> 
>    subtype Dur_Min_Type is Integer;
>    subtype Dur_Sec_type is Integer;
> 
> to
> 
>    subtype Dur_Min_Type is Integer(S'Bit_Order=>Low_Order_First);
>    subtype Dur_Sec_type is Integer(S'Bit_Order=>Low_Order_First);
> 
> or perhaps High_Order_First - haven't got everything handy at the
> moment.  But the main question is do I have the right syntax and usage? 
> Or am I completely off here?


Yes, you are completely off.

Don't bother with the Bit_Order attribute.  No compiler vendors support
it.

That leads us to this:

    type Sys_type is (Zarya, Unity, PMA1, PMA2);

    type Subsys_type is (CDH, CT, ECLSS, EPS, GNC, SM);

    type Apm_Rec is 
       record
 	 Description : Desc_Type (1 .. 256);      
 	 System      : Sys_Type;
 	 Subsystem   : Subsys_Type;
 	 Dur_Min     : Integer range 0 .. 59;
 	 Dur_Sec     : Integer range 0 .. 59;  
       end record;

There are two advantages to this:

1) we can pack the last four fields in one longword

2) the integer types fit in 1 byte, so we don't have to worry about
   byte-swapping

I think you should now be able to write a standard rep clause for this
record ("for Apm_Rec use ..."), that will work for both big- and
little-endian machines.  (Because the latter fields are 1 byte, and the
representation of one-byte data on either machine is the same.)

If you do decide to use 32-bit integers for Dur_Min and Dur_Sec, you
still don't have a rep clause problem.  But you do have a byte-swapping
problem.