# The Square-Root Formula

## Recommended Posts

I was wondering, what exactly is the forumla for finding the square root of a number?

##### Share on other sites
Derive the Taylor power series of the function, take the partial sum of however many terms you want (more terms yield better accuracy), and evaluate it at x, the argument of sqrt(x).

##### Share on other sites
here's something nice.

##### Share on other sites
try newtons method

float SQRT( float Number )
{

for ( num_iterations )
{
}
}

i hope this is right its from my head

##### Share on other sites
Newton approximation implemented in C:

float	IterSqrtf(float	r, int numtries){	float	m(0.0f);	float	b(0.0f);	float	currx=r;	float	curry(0.0f);	float	root(0.0f); //this is what gets returned 	while(numtries--)	{		curry = (currx*currx) - r; //y1		//Check to see if curry is < 0		m = 2 * currx;		b = (-m*currx)  + curry;		root = -b / m;		currx = root;	}	return root;}

Newton approximation implemented in x87 (FPU) assembly):
Number DD ?TestResult DW?Two DD 2.0MaxRelErr  DD 0.5E-6Sqrt:   fld1REPEAT01:   fld ST   fld Number   fdiv ST,ST(1)   fadd ST,ST(1)   fDiv Two   fst ST(2)   fsub   fabs   fld MaxRelErr   fmul ST,ST(2)   fcompp   fstsw TestResult   fwait   mov ax, TestResult   sahf   jna REPEAT01   ret

Square root hack approximation that doesn't even rely on the FPU along with the explanation for how it works

float	Faster_Sqrtf(float f){	float	result;		_asm	{		mov eax, f		sub eax, 0x3f800000		sar eax, 1		add eax, 0x3f800000		mov result, eax	}		return result;}

1. Although the magic number 0x3f800000 just happens to be the
floating point representation of +1.0, here it is represents the bias
in a 32-bit float. (127)

2. The sar instruction is "shift arithmetic right" which replicates
the top bit but shifts all other bits one to the right. This has the
effect of dividing by two. It could just as well be "shr". The use of
sar means that it returns the square root of a negative number as a
negative number.

The sub instruction effectively converts the exponent to a unsigned
integer from a bias-127 integer.
The SAR instruction then divides both the mantissa and the unsigned
integer by 2. Where we had n= 1.x * 2^y we now have either 1.1x *
2^(y\2) or 1.0x * 2(y\2), depending on whether the low order bit of
the exponent was set. You can work out the algebra to see what
happens when you square these things. (Note that in base 10 these
represent 2.5+x and 2.0+x respectively) You get a very crude
approximation of a square root. It undoubtedly has more accuracy in a
certain limited range.

The final add then converts the exponent back to a bias-127 exponent.

consider
45000000h = 2048

sub eax, 3f800000h ->
0100 0101 0000 0000 0000 0000 0000 0000
-0011 1111 1000 0000 0000 0000 0000 0000

= 0000 0101 1000 0000 0000 0000 0000 0000

sar eax,1 _>
= 0000 0010 1100 0000 0000 0000 0000 0000

0000 0010 1100 0000 0000 0000 0000 0000
+0011 1111 1000 0000 0000 0000 0000 0000
=0100 0010 0100 0000 0000 0000 0000 0000 = 42400000 = 48

The square root of 2048 is 42.something, and 48 * 48 = 2304.

##### Share on other sites

PS. Raymond, you have a very good memory. :)

##### Share on other sites
thanks this is good

## Create an account

Register a new account

• ### Forum Statistics

• Total Topics
627752
• Total Posts
2978920

• 10
• 10
• 22
• 14
• 14