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Posted by Pedro Graca on 09/27/22 11:37
Jerry Stuckle wrote:
> Pedro Graca wrote:
>>
>> Let's say -8 in binary is (ignore the spaces)
>> MSB == 10000000 00000000 00000000 00001000 == LSB
>>
>> if you shift this right without concern for the sign, you get
>> MSB == 00100000 00000000 00000000 00000010 == LSB
>>
>> which is 536870914
>>
>>
>> Do you think -8 >> 2 (-8 divided by 4) should equal 536870914? :)
>>
>
> But -8 binary is:
>
> 11111111 11111111 11111111 11111000
Indeed it is (on my machines).
However some other machine running PHP could use sign-and-magnitude
representation of negative numbers instead of two's complement.
For that machine my reasoning above stands :-)
My book on C (The C Programming Language, Kernighan & Ritchie) says this
about >>
<quote>
Right shifting a signed quantity will fill with sign bits
("arithmetic shift") on some machines and with 0-bits
("logical shift") on others.
</quote>
So, taking into account that PHP is written in C, I guess the effect of
0x80000000 >> 8
is implementation defined (can be 0xff800000 on some (most) machines
and 0x00800000 on other machines)
And as PHP does not have C's `unsigned long int` it's best to avoid
right shifting negative values.
--
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