Re: Record initialisation question

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

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.)


>        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?


> 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? 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.