Question

I have a 32 Bit number and want to count know how many bits are 1.

I’m thinking of this pseudocode:

mov eax, [number]
while(eax != 0)
{
  div eax, 2
  if(edx == 1)
  {
   ecx++;
  } 
  shr eax, 1
}

Is there a more efficient way?

I’m using NASM on a x86 processor.

(I’m just beginning with assembler, so please do not tell me to use code from extern libraries, because I do not even know how to include them 😉 )

(I just found How to count the number of set bits in a 32-bit integer? which also contains my solution. There are other solutions posted, but unfortunatly I can’t seem to figure out, how I would write them in assembler)

Answer 1

The most efficient way (in terms of execution time, anyway) is to have a lookup table. Obviously you’re not going to have a 4-billion entry table, but you could break the 32 bits down into 8-bit chunks and only need a 256-entry table, or further down into 4-bit chunks and only need 16 entries. Good luck!