CODEWITHSUNDEEP
assemblyx86
samuelbrody1249
samuelbrody1249

Reputation: 4767

Set n least significant bits to 1

I want to set the first N bits of a register to 1. For example, if N is 3 then the register would be: ...00000111. Is there an instruction or short-form way to do this? The way I'm currently doing it is:

   mov $0, %eax
   test %esi, %esi
   jz end
 loop:
   add $0b1, %eax
   dec %esi
   jz end_loop
   shl %eax
   jmp loop
  exit:
    ...

Upvotes: 0

Views: 565

Answers (1)

Nate Eldredge
Nate Eldredge

Reputation: 58750

A useful mathematical fact is that a value with N low-order 1 bits is 1 less than a power of two, which you can get by shifting. So you can simply load the count into %cl and do

mov $1, %eax
shl %cl, %eax
dec %eax

Of course if the count N is a constant known at assembly time, then let the assembler do the math and just load the result:

mov $((1 << N) - 1), %eax

Upvotes: 3

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