Dealing with project euler i wrote this program to find the last digit of a big exponentation:



and when it finish running ebx contains 88770249 that actually are the last eight digits of 3^50 (=17897987691852588770249).
My problem is that i need the last 10 digits but 10000000000 is too big for a 32-bit register, is there an easy solution? should i rewrite it using 64 bit registers? and how does mul and div instructions work in long mode?

another question, why (in the loop):

works fine,


cause a division by zero and


cannot even be compiled?

thank in advice for the answers!

Larger size operations are definitely needed - as you know. Can either use 64-bit registers in longmode, or multi-part operations in 32-bit mode*. Longmode use of MUL/DIV support up to RDX:RAX and work similar to 32-bit mode.

DIV does not support an immediate value. As for division by zero - watch in debugger and it will be very clear. EDX:EAX is divided by EDX - in the loop EDX probably gets smaller and smaller until zero.

*Search the board for examples, and there is a tutorial on x86asm.net.

you maybe can to that in floating point.

fp numbers supports up to 19 digits numbers.

here, you cannot use edx as a divisor maybe cause it is the high part of the dividende also, it appears that even if it is overwriten, the edx value can be 0 during the division.


you don't need to reset the edi registerto 100000000 every loop.

what can be interresting is to try to read the edx value in the DIV#0! interupt.
if it is 100000000, it can be a cool hardware bug...