[fpc-devel] Freepascal in microcontrollers

[Jonas Maebe]

On 27 Feb 2008, at 22:46, Daniël Mantione wrote:

> There is no reason why bitpacked records should be any different  
> from packed records: They should internally consistent. Therefore,  
> the packing behaviour should be known, documented, and therefore  
> usable for binary use.

They are already very different from regular packed records due to the  
fact that their structure is completely different on little and big  
endian systems (they are not byte swapped, but "bit swapped").

Locking the compiler into a particular implementation by documenting  
and supporting the implementation details mainly lead to trouble (and  
lack of optimization opportunities) later due to the legacy balast it  
causes. And don't worry: even without documenting and supporting it,  
people will still figure it out (well, it's documented by example in  
the source code's comments, so that's not that hard), still depend on  
the layout, and still file bugs if it changes for some reason.

And as Vinzent noted, simply making something "bitpacked" does not  
make it suited for binary interfacing. You need more control over the  
layout if you want to really use it. Bitpacked records are simply not  
fit for defining a particular layout (let alone access patterns:  
writing to a bitfield may involve loading the containing and several  
surrounding bytes, sometimes more than once in the generic  
implementation, and that's certainly not something you want to happen  
when accessing hardware).

> In other words, if there will be ever differences as to target  
> platforms, this has to happen only under $packrecords C, or an  
> equivalent directive for bitpacked records.

There is no equivalent directive for bitpacked records, and supporting  
C-style bitpacking is impossible with regular Pascal syntax ("char c: 
1" and "int c:1" are often laid out differently in C depending on the  
previous fields, but there is no way to express this difference using  
plain Pascal; you need extra attributes -- not to mention that those  
packing rules are often quite hairy, and it would be a mess to  
implement them all).


Jonas