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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