Representation clauses and side-efects on STM32F411 ravenscar runtime

["Frédéric Praca" <frederic.praca@free.fr>]

Hello guys,
I'm currently playing with the Ravenscar runtime for STM32.
More precisely, I have a Nucleo card which has a STM32F411 processor.
For this, I found this runtime (https://github.com/nickpascucci/ravenscar-
sfp-stm32f411-nucleo ) and decided to experiment few changes by using 
representation clauses instead of playing with bits which I find to be 
error prone.
My problem is that when using simple assignments on a record component, I 
have other bits touched by this assignment.
For example, with the following declaration:

   type Mantissa is range 0 .. 2**12 - 1 with Size => 12;
   type Fraction is range 0 .. 2**4 - 1 with Size => 4;

   type Baud_Rate_Register is
      record
         DIV_Fraction : Fraction;
         DIV_Mantissa : Mantissa;
      end record with Size => 32;
   for Baud_Rate_Register use
      record
         DIV_Fraction at 0 range 0 .. 3;
         DIV_Mantissa at 0 range 4 .. 15;
      end record;

If I write 
	USART2.BRR.DIV_Mantissa := 16#445#;
	USART2.BRR.DIV_Fraction := 16#c#;
A memory dump shows the final content to be 0x00005c5c whereas 
intermediary result was 0x00004450 as expected.
n the other hand, if I write
	USART2.BRR := Baud_Rate_Register'(DIV_Mantissa => 16#445#,
                                          DIV_Fraction => 16#c#);

There is no problem, the final content is 0x0000445c as expected.

Did someone experience such kind of behaviour with STM32 class micro-
controllers ?
Could it be related to the fact that the datasheet specified that 16-bit 
records should be written/accessed by 16-bit (Litteraly, "The peripheral 
registers have to be accessed by half-words (16 bits) or words (32 
bits)") ?
I was expecting the compiler (GNAT GPL 2014 (20140331) from Adacore under 
Linux x64) to handle all and, or and xor operations by itself for such 
kind of representation. Was I wrong ?

Thanks for the clarification.

Fred