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Date: Thu, 13 Feb 2014 17:07:05 -0800 (PST)
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Subject: Best representation for spares
From: "Rego, P." <pvrego@gmail.com>
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Xref: number.nntp.dca.giganews.com comp.lang.ada:184848
Date: 2014-02-13T17:07:05-08:00
List-Id: <comp.lang.ada>

When in a chip mapping, if we have spare (not used) bits, is it better to '=
complete' the spaces, so now we have a contiguous type definition for all b=
its; or is it better to not complete the spaces, and do not consume more st=
ack than necessary? I have seen both approaches, so still did not get a con=
clusion.

For example, using a Raspberry Pi bits definitions for auxiliary registers =
in BCM2835 chip:

   type Auxiliary_Interrupt_Status_Type is
      record
         Mini_Uart_IRQ : Boolean;
         SPI_1_IRQ     : Boolean;
         SPI_2_IRQ     : Boolean;
      end record;
   pragma Pack (Auxiliary_Interrupt_Status_Type);
   for Auxiliary_Interrupt_Status_Type'Size use 3;

   type Auxiliary_Enables_Type is
      record
         Mini_Uart_Enable : Boolean;
         SPI_1_Enable     : Boolean;
         SPI_2_Enable     : Boolean;
      end record;
   pragma Pack (Auxiliary_Enables_Type);
   for Auxiliary_Enables_Type'Size use 3;

   type Auxiliary_Peripherals_Register_Map_Type is
      record
         AUX_IRQ            : Auxiliary_Interrupt_Status_Type;
         AUX_ENABLES        : Auxiliary_Enables_Type;
      ...=20
      end record;
   for Auxiliary_Peripherals_Register_Map_Type use
      record
         AUX_IRQ            at 16#00# range 00 .. 02;
         AUX_ENABLES        at 16#04# range 00 .. 02;
      end record;

using this kind of definitions, the compiler warns (correctly) me=20
   warning: 29-bit gap before component "AUX_ENABLES"=20

Would it be better if the definition were something like=20

   type Auxiliary_Interrupt_Status_Type is
      record
         Mini_Uart_IRQ : Boolean;
         SPI_1_IRQ     : Boolean;
         SPI_2_IRQ     : Boolean;
         Spare         : Some_Type_With_29_Bits;=20
      end record;
   pragma Pack (Auxiliary_Interrupt_Status_Type);
   for Auxiliary_Interrupt_Status_Type'Size use 32;

   ...
   for Auxiliary_Peripherals_Register_Map_Type use
      record
         AUX_IRQ            at 16#00# range 00 .. 32;
         AUX_ENABLES        at 16#04# range 00 .. 02;
      end record;

or just ignore the warning?

Regards,
Rego.