x86 assembly (MASM) - Square root of a 64-bit integer?

assembly x86 masm masm32
qwertium picture qwertium · Jan 10, 2012 · Viewed 12k times · Source

I'm coding a simple primality tester program for Windows in x86 assembly language (MASM32), which involves calculating a square root of a (64-bit) integer. My question is: Is there any simple way for obtaining the square root? Should I use some combination of ADD/SUB/DIV/MUL instructions?

I found some information on how this could be achieved in C language, but I'm just wondering if I'm missing something here?

Answer

Smi picture Smi · Jan 10, 2012

I think the simplest way is to use the FPU instruction fsqrt like this:

.data?
int64 dq ?
squareRoot dd ?

.code
fild int64        ;load the integer to ST(0)
fsqrt             ;compute square root and store to ST(0)
fistp squareRoot  ;store the result in memory (as a 32-bit integer) and pop ST(0)

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