I never really thought of the fact that NAMING the bit fields would make
the code more readable. I'm not a veteran "C" programmer, so I'm not
used to such constructs.

Thanks .
--=20
Larry Bradley =20
Orleans (Ottawa) Canada=20

On Thu, 2014-05-01 at 21:36 -0400, John J. McDonough wrote:

> On Thu, 2014-05-01 at 17:11 -0400, Larry Bradley wrote:
> > Thanks. Somewhere I read that the bitfield stuff was not efficient.
> >=20
> > I tried it. The code using bitfields was the same size as using a mask.
> > And the mask code is a bit (no pun intended) clearer!
>=20
> I'm kind of surprised nobody compared the generated code.  The code
> produced by XC16 is identical:
>=20
>     if ( myBits & 0x0002 )
>         myTestWord =3D 87;
>=20
>     if (myBitfield.b1)
>         myTestWord =3D 83;
>=20
> 53:                    if ( myBits & 0x0002 )
> 00029A  804000     MOV myBits, W0
> 00029C  600062     AND W0, #0x2, W0
> 00029E  500FE0     SUB W0, #0x0, [W15]
> 0002A0  320002     BRA Z, 0x2A6
> 54:                        myTestWord =3D 87;
> 0002A2  200570     MOV #0x57, W0
> 0002A4  884020     MOV W0, myTestWord
> 55:               =20
> 56:                    if (myBitfield.b1)
> 0002A6  BFC802     MOV.B myBitfield, WREG
> 0002A8  604062     AND.B W0, #0x2, W0
> 0002AA  504FE0     SUB.B W0, #0x0, [W15]
> 0002AC  320002     BRA Z, 0x2B2
> 57:                        myTestWord =3D 83;
> 0002AE  200530     MOV #0x53, W0
> 0002B0  884020     MOV W0, myTestWord
>=20
> The real question is, does the optimizer have better luck with one over
> the other.  Problem with that is that it is real hard to come up with
> bogus code the optimizer won't simply optimize out of existence.
>=20
> I would argue that the mask code is only clearer if
> 1) You are doing some bogus example like this or
> 2) You are an assembler programmer
>=20
> With the bitfield you can give the bits names, so for example you could
> say something like:
>=20
>   if ( LCDstatus.LCDisBusy )
>=20
>   rather than
>=20
>   if ( LCDstatus & 0b0000000000100000 )
>=20
>   Granted, you could say
>=20
>   #define LCDBUSY 0x0020
>=20
>   then
>=20
>   if ( LCDstatus & LCDBUSY )
>=20
> but I still don't find that as clear as the bitfield example, which is
> probably why Microchip goes to the trouble of naming all the bits in the
> status words.  You can even give the bit a macro name, making the code
> even more clear.  Something Microchip does with many of the SFR bits.
> So with a define I could simply say
>=20
>   if ( BUSYBIT )
>=20
> far more readable than
>=20
>   if ( LCDstatus & 0b0000000000100000 )
>=20
>=20
> --McD
>=20
>=20
--=20
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