Re: Record initialisation question

[DrPi <314@drpi.fr>]

Le 09/01/2021 à 16:44, Niklas Holsti a écrit :
> On 2021-01-09 11:30, DrPi wrote:
>> Hi,
>>
>> I'm working on a µP BSP. The boot sequence of this µP requires byte 
>> structures located in FLASH memory.
>> For example :
>>
>>     type t_Dcd_Header is record
>>        Tag     : Unsigned_8  := 16#D2#;
>>        Length  : Unsigned_16 := 4;      -- Length in byte of the DCD 
>> structure (this header included)
> 
> 
> (As the default value (4) does not include the rest of the DCD 
> structure, the comment is rather misleading. Best to remove the default 
> value, I think.)
Maybe, I'm sure sure.
Value of 4 means only the header is present.

> 
> 
>>        Version : Unsigned_8  := 16#41#;
>>     end record
>>       with Object_Size => 32,
>>            Bit_Order => System.Low_Order_First;
>>
>>     for t_Dcd_Header use record
>>        Tag     at 0 range  0 .. 7;
>>        Length  at 0 range  8 .. 23;
>>        Version at 0 range 24 .. 31;
>>     end record;
>>
>> The t_Dcd_Header is part of t_Dcd record.
>> The Length field of t_Dcd_Header must contain the length of t_Dcd.
>>
>>     Dcd : constant t_Dcd :=
>>      ( Dcd_Header => ( Length => ???, -- Length of Dcd
>>                        others => <>),
>>        ...
>>      );
>>
>> Is there a way to automatically set Length ?
> 
> 
> If the Length should apply to the whole t_Dcd object, it is hard to 
> answer without knowing how the rest of the t_Dcd type is defined. Does 
> it have a fixed size, or are there some components of variable size?
> 
The DCD is fully specified.

> 
>> Dcd goes in a dedicated .txt section.
> 
> 
> Do you expect the compiler/linker to generate the full value of the Dcd 
> object into the program image at compilation and link time? 
That's it.

> Have you 
> ensured that the construction of the Dcd object requires no elaboration 
> code? Most Flash memories cannot be written in the same way as RAM, so 
> even if that .txt section is not write-protected, normal RAM-oriented 
> elaboration code would not be able to write into Flash.
I'm aware of this (I'm an electronics guy).
I'll add a "pragma No_Elaboration_Code_All;" when I'm ready.