Ack!!  I meant 70 *BITS* (hence my use of 9 bytes in assembly to hold =20
the data).  I see replies, so will get to them in a couple hours when =20
I get home.

Cheers,
-Neil


Quoting PICdude <picdude3@narwani.org>:

> Ah yes, I knew this time would come, where I'd need an efficient way
> to do efficient bit operations in C, and in this case, shift bits held
> in multiple registers.
>
> Specifically, I have a stream of ~70 bytes and I need to sequentially
> spit them out an I/O pin, on each timer interrupt (to generate a
> 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,
> but I can't find a shift operator that let's me extract the last bit,
> 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
> assembly version.  (I haven't actually implemented this yet, btw).  Is
> there a better way to do this in C?  I don't actually need to shift if
> there's a more efficient way to read the bit values using some type of
> index variable.
>
> An incomplete thought running through my head currently is holding "1"
> in some long var, shifting that on each iteration, then using that as
> a mask to "AND" the Data.  I'd also hold another index var (ranging
> from 0 to 2) which would tell me which data variable I was currently
> at.  But this method may be even worse.
>
> Or is this where I do inline assembly?  With inline assembly I'd
> probably expend several cycles moving the data to know register
> locations, unless there's some simple way to figure out in RAM where
> the Data2,1,0 variables are held.
>
> How would you C pros do it?
>
> Cheers,
> -Neil.
>
>
>
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