Re^3: Shift Operators And Bitwise Operators

In a 32-bit int,

$core_dumped = ($? >> 15) & 1;

00000000 00000000 CSSSSSSS EEEEEEEE

   >> 15

00000000 00000000 00000000 0000000C

   & 1

00000000 00000000 00000000 00000001 = 1
00000000 00000000 00000000 0000000C
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Ok, everything to the left of "C" should always be zero, so the & 1 doesn't do anything. Me bad. The signal one is a better example:

$signal = ($? >> 8) & 127;

00000000 00000000 CSSSSSSS EEEEEEEE

   >> 8

00000000 00000000 00000000 CSSSSSSS

   & 127

00000000 00000000 00000000 01111111 = 127
00000000 00000000 00000000 0SSSSSSS
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The shift repositioned the bits of interest and removed the lower-precision bits we didn't want.

The "and" removed the higher-precision bits we didn't want.

You could do the operations in the opposite order:

$signal = ($? & 0x7F00) >> 8;

00000000 00000000 CSSSSSSS EEEEEEEE

   & 0x00007F00

00000000 00000000 01111111 00000000 = 0x00007F00
00000000 00000000 0SSSSSSS 00000000

   >> 8

00000000 00000000 00000000 0SSSSSSS
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The "and" removed all the bits we didn't want.

The shift repositioned the bits of interest.

You should be start by understanding how machines store integers.

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Re^4: Shift Operators And Bitwise Operators by biohisham (Priest) on Jun 22, 2009 at 23:50 UTC
Great, I think I am getting it somehow, albeit in a random way... I can summarize this as follows `#working in octets of bits. 01-shift $! bits to the right. #zeroing on target 02-spread &ing carpet under the target #2(No.of bits) #carpet size is (1,4,8,16,32) that covers 1,2,3,4..etc bits #from the second bit the relation is (22, 23, #24...232) i.e. +2(bit position). 03-&ing makes us extract the bits we want #(shifted bits)*(appropriate carpet size)=(C or SSs or EEs)` [download] does this seem all there is to it??? p.s. I don't know anything about carpets by the way except that they are spread or pinned on to a wall	[reply] [d/l]

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Re^4: Shift Operators And Bitwise Operators
by biohisham (Priest) on Jun 22, 2009 at 23:50 UTC

#working in octets of bits.

01-shift $! bits to the right. #zeroing on target
02-spread &ing carpet under the target #2**(No.of bits)

#carpet size is (1,4,8,16,32) that covers 1,2,3,4..etc bits

#from the second bit the relation is (2**2, 2**3, #2**4...2**32) i.e. 
+2**(bit position).
 
03-&ing makes us extract the bits we want
#(shifted bits)*(appropriate carpet size)=(C or SSs or EEs)
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[reply]
[d/l]