[M3devel] packing problem… how exactly does modula-3 pack data into records?? … "solved"

[Jay]

As in C, if you need portability and predictability, just don't use bitfields. Use integer types of size 8, 16, 32, or possibly 64 bits, and do the appropriate shifting and masking, possibly endian-dependent.


Actually, to avoid endian and packing/alignment concerns, use only 8 bit integers and optionally pack/unpack into more convenient types.


 - Jay (phone)

On Jan 20, 2012, at 4:02 AM, Dragiša Durić <dragisha at m3w.org> wrote:

> TYPE
>  (* This is how it should be written... If only cm3 packed left-to-right-until-spent. todo for me.
> 
>  TSPacketHeader = RECORD
>    sync: BITS 8 FOR [16_0..16_ff];   (* Always 0x47 *)
>    tErrInd,                      (* Transport Error Indicator *)
>    pusi: BITS 1 FOR BOOLEAN;     (* Payload Unit Start Indicator *)
>    transPrio: BITS 1 FOR [0..1];
>    pid: BITS 13 FOR PID;
>    transScramControl: BITS 2 FOR [0..3]; (* 00 means no scrambling *)
>    afc: BITS 2 FOR [0..3]; (* 01 - no adaptation field, payload only *)
>    cc: BITS 4 FOR Nibble;
>  END;
>  *)
> 
>  TSPacketHeader = RECORD
>    sync: BITS 8 FOR [16_0..16_ff];   (* Always 0x47 *)
> 
>    pidHi: BITS 5 FOR [16_00..16_1f];
>    transPrio: BITS 1 FOR [0..1];
>    pusi,
>    tErrInd: BITS 1 FOR BOOLEAN;
> 
>    pidLo: BITS 8 FOR [16_00..16_ff];
> 
>    cc: BITS 4 FOR Nibble;
>    afc: BITS 2 FOR [0..3]; (* 01 - no adaptation field, payload only *)
>    transScramControl: BITS 2 FOR [0..3]; (* 00 means no scrambling *)
>  END;
> 
> With additional methods for getPid() and setPid(), this works as expected.
> 
> But I _really_ think we should have continuous bit fields in cases where it spans byte boundary and combined with how other pieces of this work now - it means we need left to right packing of bit strings into bytes, not right to left as it is done now on my x86_64 box. This is probably some endianess relict, but I also think endiannes has nothing to do with us once we decide to go bit strings.
> 
> I don't see a big problem to implement this in such a way right now. Especially since at this moment nobody is having any expectations from implementation (except me O:) ) - we won't break anything.
> 
> TIA,
> dd
> 
> On Jan 20, 2012, at 8:29 AM, Dragiša Durić wrote:
> 
>> I am not using packing to interface with C or to be compact.
>> 
>> My code is directly producing packets for some network protocol. No C in sight :). 
>> 
>> And, as probably all those pesky :) network protocols around… Order of bytes and (expected) order of bits is - left to right.
>> 
>> I really do not understand why this is something hard to accept as standard (or at least cm3) way for  bit packing?
>