How to use large numbers?

Question

How to use large numbers in? like 3441881739,30000000 etc

mov eax,3441881739

In this case eax value is a negative number. How to fix it? split it? how? I need to do also add/sub/mul/div etc the cmp operation.

Can someone explain and give an example how to do it?

I marked fasm and nasm tags but anothers assembly are welcome too. I'm on 32-bit machine.

Answer 1

If you want to deal with numbers larger than 2³²-1, you'll need to do multiple-precision arithmetic. For example, to add a pair of 64-bit numbers you could do something like this:

mov eax, A_1    ; essentially C = A + B
mov edx, B_1
add eax, edx    ; note that for the low word, we just add
mov C_1, eax
mov eax, A_2
mov edx, B_2
adc eax, edx    ; but for the high word, we add with carry
mov C_2, eax

Having access to the carry bit means that this is considerably simpler than with most higher level languages. There's also a subtract with borrow to make multi-word subtraction easy as well. Multiplication and division depend a little. A normal 32x32 bit multiplication produces a 64-bit result (low word in eax, high word in edx). If you want something like 64x64 bit multiplication (giving a 128-bit result) you'll need to implement that yourself.

For example, multiplication (without using the built-in mul/imul instructions) can be done something like this:

mult proc
; multiplies eax by ebx and places result in edx:ecx
    xor ecx, ecx
    xor edx, edx
mul1:
    test ebx, 1
    jz  mul2
    add ecx, eax
    adc edx, 0
mul2:
    shr ebx, 1
    shl eax, 1
    test ebx, ebx
    jnz  mul1
done:
    ret
mult endp