[fpc-devel] Freepascal in microcontrollers

[Vinzent Hoefler]

On Wednesday 27 February 2008 14:15, Florian Klaempfl wrote:
> Vinzent Hoefler schrieb:
> > And that's why I'd rather go with Ada's representation clauses. If
> > the hardware platform is unsuitable for the thing I want it to do,
> > it simply doesn't compile. Which is justified, because in 99% of
> > the times such RCs are hardware specific.
>
> I still don't see how ada could handle things like little endian cpus
> accessing big endian peripherial with bit patterns overlapping byte
> borders (no, this is not an academic case, just look at the brain
> dead arm fpa).

Well, I'm not sure what you want here, let me try:

|type Data is ...
|
|for Data'Bit_Order use High_Order_First;
|for Data use
|  record
|     Byte_3 at 0 range 0 .. 7;
|     Byte_2 at 1 range 0 .. 7;
|     Byte_1 at 2 range 0 .. 7;
|     Byte_0 at 3 range 0 .. 7;
|  end record;

Don't get it wrong. An Ada compiler does *not* automatically flip bytes 
for you, so if you really have some type spanning byte boundaries in 
the wrong endianess, you still have to flip it accordingly. IOW:

|type Data is:
|
|for Data'Bit_Order use High_Order_First;
|for Data use
|  record
|     Word_1 at 0 range 0 .. 15;
|     Word_0 at 2 range 0 .. 15;
...

does not work as one might expect. The bytes would still be flipped on a 
little endian machine.

But at least it is possible to define a record representation totally 
endianess independent. It's a bit more complex than necessary, so it's 
probably better to read the whole stuff (although it might require some 
Ada experience to really see it through)

http://www.sigada.org/ada_letters/sept2005/Endian-Independent%20Paper.pdf

instead of letting me give you an example.

After all, for embedded programming the guarantees I get from Ada record 
representation clauses is usually enough to know it'll work.


Vinzent.