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Re: Decomposing a bit field? [message #71786 is a reply to message #71716]

Wed, 21 July 2010 15:41

Chris[6]
Messages: 84
Registered: July 2008

Member

On Jul 21, 1:25 am, Rob <rj...@le.ac.uk> wrote:
> On Jul 21, 11:29 am, Steve <f...@k.e> wrote:
>
>
>
>> Rob wrote:
>>> for i=0, 31 do begin test[i]=(ISHFT((15982561),-i) AND 1B)
>
>>> However, for example bits 1-4 need to be combined (and contain data
>>> from 0-15). As I don't quite understand what the code above is doing
>>> I'm not sure how to (or if it can be) modified to not just operate on
>>> the individual bits but on groups of bits.
>
>> ISHFT - moves bits right or left so that the bits you want are at the
>> end of the value. AND selects the bits that you want.
>
>>> What I need to extract is the information in the folowing form:
>
>>> Bit 0: 0 or 1
>
>> bit0= ishft(15982561,0) and 1B
>
>>> Bits 1-4: 0-15
>
>> bits1to4 = ishft(15982561,-1) and 17B
>
>>> Bit 5: 0 or 1
>
>> bit5 = ishft(15982561,-5) and 1B
>
>>> Bits 6-8: 0-8
>
>> bits6to8 = ishft(15982561,-6) and 7B
>
>>> etc
>
>>> Thanks in advance
>
> Thanks everyone, that makes much more sense now.
>
> I just wanted to confirm where the 7B comes from above.
>
> I assume it's because I'm checking 3 bits so it's 1+2+4. If I had a
> group of 4 bits it's be 1+2+4+8 (15 like in Chris's example below),
> right?

right- 7 is (000000....111) in binary. So if you take "x and 7" it
will set all but the last three bits to zero. So it throws away all
the leading bits, and returns the last 3 bits as some number (0-7)

15 (0000.....1111) does the same thing, but with 4 bits instead of 3.
chris

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[Message index]

		Decomposing a bit field? By: rjp23 on Wed, 21 July 2010 02:40
		Re: Decomposing a bit field? By: Chris[6] on Wed, 21 July 2010 15:41
		Re: Decomposing a bit field? By: penteado on Wed, 21 July 2010 09:21
		Re: Decomposing a bit field? By: David Fanning on Wed, 21 July 2010 09:09
		Re: Decomposing a bit field? By: Steve[5] on Wed, 21 July 2010 09:04
		Re: Decomposing a bit field? By: rjp23 on Wed, 21 July 2010 04:25
		Re: Decomposing a bit field? By: Chris[6] on Wed, 21 July 2010 03:33
		Re: Decomposing a bit field? By: greg.addr on Wed, 21 July 2010 03:31
		Re: Decomposing a bit field? By: Steve[5] on Wed, 21 July 2010 03:29

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