CODEWITHSUNDEEP
c++bit-manipulation
Ashutosh Tiwari
Ashutosh Tiwari

Reputation: 307

Extracting middle 16 bits of a 32 bit long

I am reading TCPPPL by Stroustrup. It gives an example of a function that extracts the middle 16 bits of a 32 bit long like this:

unsigned short middle(long a){ return (a>>8)&0xffff;}.

My question is: isn't it extracting the last 16 bits? Tell me how am I wrong.

Upvotes: 3

Views: 809

Answers (4)

Angew is no longer proud of SO
Angew is no longer proud of SO

Reputation: 171167

a >> 8 will right-shift the value in a by 8 bits. The low 8 bits are forgotten, and bits previously numbered 31–8 now get moved (renumbered) to 23–0. Finally, masking out the higher 16 bits leaves you with bits 15–0, which were originally (before the shift) at positions 23–8. Voila.

Upvotes: 3

Patrick
Patrick

Reputation: 77

a is going to right shift 8-bit (a>>8) before bitwise and operation.

Upvotes: 1

YSC
YSC

Reputation: 40120

It does indeed extract the middle 16 bits:

//    a := 0b xxxx xxxx 1111 1111 1111 1111 xxxx xxxx 
a>>8;   // 0b 0000 0000 xxxx xxxx 1111 1111  1111 1111
&0xffff // 0b 0000 0000 0000 0000 1111 1111  1111 1111

Upvotes: 7

bipll
bipll

Reputation: 11940

Have you noticed the >>8 part? It shifts the argument right by eight bits, first.

Upvotes: 0

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