On 14 February 2012 04:59, PICdude <picdude3@narwani.org> wrote:

> I goofed on the original question... I have 70 bits, so 9 variables
> should be enough.  But I see what you're doing here and it looks nice.
>

Nope, you need 9 bytes, but as in C you could have 32 bit (4 byte)
variables too, you would need two 32 bit and one 8 bit variable. Or as
suggested by others you could have one 9 byte length array as well.

Tamas




>  Will try it out.
>
> Cheers,
> -Neil
>
>
>
> Quoting Isaac Marino Bavaresco <isaacbavaresco@yahoo.com.br>:
>
> > Did I understand it right and you shift a chain of 70 data registers an=
d
> > 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
> >> 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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> >
>
>
>
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--=20
int main() { char *a,*s,*q; printf(s=3D"int main() { char *a,*s,*q;
printf(s=3D%s%s%s, q=3D%s%s%s%s,s,q,q,a=3D%s%s%s%s,q,q,q,a,a,q); }",
q=3D"\"",s,q,q,a=3D"\\",q,q,q,a,a,q); }
--=20
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