[fpc-devel] Freepascal in microcontrollers

[Vinzent Hoefler]

On Thursday 28 February 2008 11:28, Michael Schnell wrote:
> > AFAICS, it would be useful for bitpacked records only, so it could
> > appear anywhere where a {PACKRECORDS} directive or similar can
> > appear currently.
>
> IMHO it would only be useful (allowed with, regarded by) bitpacked
> record, as any other data representation is supposed to be optimized
> for speed according to the processor architecture.

Hmm, not necessarily. I frequently use enumeration types to express the 
meaning of a set of hardware bits. So thinking about it, what if I'd 
use enumerations in a bitpacked record? Maybe like this:

-- 8< --
type
   // 2 bits.
   Enable_Set = (Dont_Care := 0,  // 00
                 Disable   := 1,  // 01
                 Enable    := 3); // 11

type
   Control =
   bitpacked record
      Continuous_Mode   : Boolean;    // bit 0/rightmost bit
      Alternate_Compare : Boolean;    // bit 1
      ...
      Enable_Mode       : Enable_Set; // bit 14 .. 15/leftmost bits
   end;
-- 8< --


I don't know if FPC can pack enumerations into a bitpacked record at 
all, but if it does, it might consider the bit order here, too.

Consider something like:

-- 8< --
var
   My_Set    : Enable_Set;
   My_Record : Control;
   

My_Set := My_Record.Enable_Mode;
-- 8< --

How should the assignment be handled if the bit order for that bitpacked 
record is changed?


Vinzent.