Oops! Little mistake, forgot the "( 1 << ( BitNumber & 7 ))".


if( Data[ BitNumber >> 3 ] & ( 1 << ( BitNumber & 7 )))
    SendHiBit();
else
    SendLoBit();
if( ++BitNumber >=3D sizeof Data * 8 )
    {
    BitNumber =3D 0;
    LoadNewData();
    }


Isaac


Em 12/2/2012 16:43, Isaac Marino Bavaresco escreveu:
> Did I understand it right and you shift a chain of 70 data registers and
> test the lowest bit every round?
>
> We discussed many months ago a more efficient way of testing arbitrary
> bits in an array:
>
>
> char Data[70];
> int BitNumber =3D 0;
>
> ...
>
> if( Data[ BitNumber >> 3 ] & ( BitNumber & 7 ))
>     SendHiBit();
> else
>     SendLoBit();
> if( ++BitNumber >=3D sizeof Data * 8 )
>     {
>     BitNumber =3D 0;
>     LoadNewData();
>     }
>
> ...
>
>
> No real shifting of the data is done, so the size of the array doesn't
> matter for the efficiency of the process.
> This routine should be much faster than shifting a chain of 70 bytes.
>
>
> Best regards,
>
> Isaac
>
>
>
>
>
> Em 12/2/2012 12:03, PICdude escreveu:
>> Ah yes, I knew this time would come, where I'd need an efficient way =20
>> to do efficient bit operations in C, and in this case, shift bits held =
=20
>> in multiple registers.
>>
>> Specifically, I have a stream of ~70 bytes and I need to sequentially =20
>> spit them out an I/O pin, on each timer interrupt (to generate a =20
>> specific output data stream).
>>
>> In assembly, I'd hold these in several registers and do something like..=
..
>>
>>        rrf Data9,F
>>        rrf Data8,F
>>        ...
>>        rrf Data0,F
>>        btfsc STATUS,C      ; C hold current bit value
>>        goto SendHiBit
>> SendLoBit:
>>        ...
>>
>>
>> In C though, it's nice to have single variables that can hold 4 bytes, =
=20
>> but I can't find a shift operator that let's me extract the last bit, =20
>> so I'm thinking I'd have to so something like ...
>>
>> OutBit =3D Data0 & 0b1;      // Extract lowest bit and hold
>> Data0 >> 1;                  // Assume long (4 bytes)
>> if (Data1 & 0b1)
>>        Data0 &=3D 0b1;      // Uppermost bit =3D 1
>> Data1 >> 1;
>> if (Data1 & 0b1)
>>        Data1 &=3D 0b1;
>> Data1 >> 2;
>> // Send output bit here...
>>
>>
>> As compact as it looks, it seems so inefficient compared to the =20
>> assembly version.  (I haven't actually implemented this yet, btw).  Is =
=20
>> there a better way to do this in C?  I don't actually need to shift if =
=20
>> there's a more efficient way to read the bit values using some type of =
=20
>> index variable.
>>
>> An incomplete thought running through my head currently is holding "1" =
=20
>> in some long var, shifting that on each iteration, then using that as =20
>> a mask to "AND" the Data.  I'd also hold another index var (ranging =20
>> from 0 to 2) which would tell me which data variable I was currently =20
>> at.  But this method may be even worse.
>>
>> Or is this where I do inline assembly?  With inline assembly I'd =20
>> probably expend several cycles moving the data to know register =20
>> locations, unless there's some simple way to figure out in RAM where =20
>> the Data2,1,0 variables are held.
>>
>> How would you C pros do it?
>>
>> Cheers,
>> -Neil.
>

--=20
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