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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