1 ; int gcdAsm(int a, int b)
3 ; computes gcd(a,b) according to:
4 ; 1. a even, b even: gcd(a,b) = 2 * gcd(a/2,b/2),
5 ; and remember how often this happened
6 ; 2. a even, b uneven: gcd(a,b) = gcd(a/2,b)
7 ; 3. a uneven, b even: gcd(a,b) = gcd(a,b/2)
8 ; 4. a uneven, b uneven: a>b ? a -= b : b -= a,
9 ; i.e. gcd(a,b) = gcd(min(a,b),max(a,b) - min(a,b))
10 ; do 1., repeat 2. - 4. until a = 0 or b = 0
11 ; return (a + b) corrected by the remembered value from 1.
24 mov eax,[ebp + 8] ; eax = a (0 <= a <= 2^31 - 1)
25 mov ebx,[ebp + 12] ; ebx = b (0 <= b <= 2^31 - 1)
26 ; by definition: gcd(a,0) = a, gcd(0,b) = b, gcd(0,0) = 1 !
29 bsf ecx,ecx ; greatest common power of 2 of a and b
31 mov eax,1 ; if a = 0 and b = 0, return 1
37 mov eax,ebx ; if a = 0, return b
41 jz done ; if b = 0, return a
42 bsf ecx,eax ; "simplify" a as much as possible
44 bsf ecx,ebx ; "simplify" b as much as possible
49 sbb edx,edx ; edx = 0 if b >= a or -1 if a > b
50 and ecx,edx ; ecx = 0 if b >= a or b - a if a > b
51 add eax,ecx ; a-new = min(a,b)
52 sub ebx,ecx ; b-new = max(a,b)
53 sub ebx,eax ; the difference is >= 0
54 bsf ecx,eax ; "simplify" as much as possible by 2
56 bsf ecx,ebx ; "simplify" as much as possible by 2
58 jnz mainLoop ; keep looping until ebx = 0
59 mov ecx,edi ; shift back with common power of 2