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Posted on 1999-01-07
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Last Modified: 2010-03-05
Looking to do some arithmetic operation on a number to clear all the hex-bits of a number except the lower 4. For example:

The number '96237' is equal to the hex value '0001 77ed'. I want to just keep the lower 4 bits which in this case is '77ed'. Hex value '77ed' is 30701, the number I want. Sooo... How can I take the number '96237' (or any number) do some arithemetic to clear all the bits except the last 4, to end up with the value '96237';

$num = 96237;

HELP!  Thanks so much!!
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Question by:rw263
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Expert Comment

by:scrapdog
ID: 1207447
Logically 'and' the number with 65535.
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Expert Comment

by:ozo
ID: 1207448
$num &= 0xffff;
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Author Comment

by:rw263
ID: 1207449
Ozo your idea worked! One more question, sheesh you good at this!

How do I turn on all the high bits?  That is leave the lower four hex bits alone and turn on the upper four hex bits?  for example: $num_second = 46071

46071 = 0000 B3F7
            FFFF B357 = 4294947831

so I want to go from 46071 to 4294947831, via a similar same math operation.
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Expert Comment

by:scrapdog
ID: 1207450
Ozo's answer was the same as mine...oh well.

>How do I turn on all the high bits?  That is leave the lower >four hex bits alone and turn on the upper four hex bits?  for >example: $num_second = 46071

$num |= 0xffff0000;

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Expert Comment

by:scrapdog
ID: 1207451
>Ozo's answer was the same as mine...oh well.

This was not meant to criticize ozo...we must have posted at the exact same time, so neither of us had seen each other's posts.  What I meant was that "65535" is the same as "0xffff".
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Author Comment

by:rw263
ID: 1207452
whoopsie in that case ya get the points scrappy :) reanswer!
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scrapdog earned 150 total points
ID: 1207453
Thanks.  Did you get $num |= 0xffff0000 to work?
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Expert Comment

by:scrapdog
ID: 1207454
ozo probably gave you the better answer, so if you want to reject and give him/her the points, go ahead...
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Author Comment

by:rw263
ID: 1207455
whoopsie in that case ya get the points scrappy :) reanswer!
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Author Comment

by:rw263
ID: 1207456
yes
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