Getting most significant bits...
I could only think of this:

#define GetLSBs(x,n) (x&((1<<n)-1))

unsigned int GetMSBs(unsigned int x,unsigned n)  // n number of bits to get from i = 0
{  // n = numbers of bits to get
unsigned int LSB = GetLSBs(x,32-n);
unsigned int MSB = x-LSB;
return MSB;

}

I've thinked of getting MSB with "and" instruction; anyway I've failed to get bits mask:
first bit, first two bits, first 3 bits ...:
1000  // 8
1100  // 12
1110  // 14
1111  // 15

 

Edited September 15, 20186 yr by CodeExplorer

I guess you have to right shift the result, if I understood correctly what you need

Something like this should work (C# code btw)

         uint GetMSBs(uint x, int n, int totalBits)  // n number of bits to get from i = 0
        {  
            // n = numbers of bits to get
            uint msb = (x >> (totalBits - n));
                 msb = (uint) ( msb & ((1<<n)-1) );
            
            return msb;
        } 

 

with totalBits = 32 for dwords.

[EDIT]

Or, if you need the value for the bit mask, just shift-left the lsb mask (i.e. "((1<<n)-1 )")  of (totalBits - n) places;

Regards,
Tony

 

 

Edited September 15, 20186 yr by tonyweb

@tonyweb: Thank you, that was exactly what I needed!

Here is the Pelles C version:

unsigned int GetMSBs(unsigned int x, unsigned int n)
// n number of bits to get from i = 0
{
// n = numbers of bits to get
unsigned int msb = (x >> (32 - n));  // 32 = number of bits
msb = (unsigned int) (msb & ((1<<n)-1));

return msb;
}

Your solution will get MSB bits in low part (since they are shifted) which is 100% OK.
Thank you once again.

Another question: I don't understand why the last part is needed: msb = (uint) ( msb & ((1<<n)-1) );
seems to works ok even without it!
 

Last part takes into account the shifting of "negative" numbers ...
If the MSB bit is set (i.e. x & 0x80000000 != 0), when shifting the sign might be preserved, so it would result in an incorrect value without the proper masking.

Regards,
Tony

[EDIT] But, given you're dealing with unsigned vars, maybe it's an overkill

 

Edited September 15, 20186 yr by tonyweb

If you are just getting the MSB that is set, you can use the 'BSR' instruction. https://c9x.me/x86/html/file_module_x86_id_20.html

If using Visual Studio there is an intrinsic version of this: https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanreverse-bitscanreverse64?view=vs-2017

For other raw format versions, there's a bunch of examples suggested on StackOverflow for similar here:

https://stackoverflow.com/questions/671815/what-is-the-fastest-most-efficient-way-to-find-the-highest-set-bit-msb-in-an-i