Factorial for large numbers: product of 32-bit x 128-bit numbers in x86 asm

Question

I want to write a x86 masm program that calculates a big factorials (results up to 128 bits), but I am not sure how to do that. I've written a version that calculates up to 32bits, how could I change it up so that it works for larger numbers?

Answer 1

128 bits is too big for mul instruction, which saves only 64-bits result (EDX (32bits) + EAX (32bits)).

So first of all you need 4 DWORDS of memory to store 128bits result. Then you should replace your mul ebx; on some procedure call, that can handle 128bits multiplication. The code can be something like this:

.data
;...
bignum dword 1, 0, 0, 0; initial answer = 0000 0000 0000 0001h

....
call largemul; replace for mul ebx
....
largemul proc; 
 mov ebx, 10            ;EBX = multiplier
 mov eax, [bignum]  
 mul ebx                ;EDX:EAX = EAX*EBX
 mov [bignum], eax  ;save result
 mov ecx, edx           ;save carried part in ECX

 mov eax, [bignum+4]
 mul ebx
 add eax, ecx           ;add carried part from previous multiplication
 mov [bignum+4], eax
 mov ecx, edx

 mov eax, [bignum+8]
 mul ebx
 add eax, ecx
 mov [bignum+8], eax

 mov eax, [bignum+8]
 mul ebx
 add eax, ecx
 mov [bignum+12], eax
largemul endp;