Logical and airthmatic left shift.

Question

I just read that the logical and airthmatical left shift are same while there is a difference in case of right shift.

https://en.wikipedia.org/wiki/Bitwise_operation (under Logical shift)

Can understand how are they same. Like What is arithmetic left shift of 01001001? 

I would think it is 00010010 i.e. it tries to maintain the sign bit as is

On the other hand, the logical left shift by 1 pos would be 10010010

but both are not same. any example on that. 

Answer 1

In left shift(logical+arithmetic) we just remove leftmost bit and add a 0-bit in the rightmost 

leftshift of 01001001 is 10010010

But in right-shift:

In logical : we just remove the rightmost bit and add 0-bit to the leftmost side.

In Arithmetic: We kept the sign bit as it was earlier, i,e. remove the rightmost bit and add earlier MSB to leftmost side.

Comments

What is arithmetic left shift of 01001001? 

Ans: 010010010 = (0 removed) 10010010 (0 added)

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now considering the number. it was positive at first and ater left shift it become negative. How ? in righ shif the sign is conserved and value is halfed

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@target2017 first number was 01001001 = 73

after logical left shift it becomes 10010010 = 146 ok till this point everything is fine.

​​​​​​Question arises now if u take logical left shift of 146 i.e. 10010010 which should again be multiplied by 2 and it should be 392 in binary. ! Ok !

but I can understand this that we have only 8 bits representation in hands so we are not able to represent number above 255. Ok!

now my question is if we take logical left shift of 146 i.e. 10010010 we got 00100100 which is 36 ok!

now what to do next ? How we can correct it or stop it somewhere with some note ?

can anybody help me in this, any kind of help should be appreciated !!!

@Tendua​​​​​

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Answer 2

Both arithmetic and logical left shift are same, and sign wont get changed unless there's an overflow.

This is because in 2's compliment representation the left side of the number will be filled with the sign bit itself (sign bit extension). Therefore left shift wont change the sign of the number, unless too many shifts are done.

 What is arithmetic left shift of 01001001?  

00000000000000000000000001001001 (+73)

After 1 left shift:

00000000000000000000000010010010 (+146)

Comments

""Left shift won't change the sign of the number, unless too many shifts are done.""

@sarannya too many shifts are how many shifts ?

and when it becomes negative then what to do next, I mean whether we can get again logical left shifted number of given binary string or not? explain please.