Re: newbie :adding larger value

[Frederic Marmond <fmarmond@eprocess.fr>]

! yes !
Hum, I feel like I have to play more often with x86 asm, I'm forgetting 
all but the basic... (my work makes me let the asm in the side and focus 
on C/C++ progs, ...)
Of course you are right!
Thanks to refresh my (so volatile?) memory! ;)

Fred

Philip Jacob Smith wrote:

>> jnc no_lowcarry      : if no carry, just add the upper 32bits
>>     inc ecx            ;if carry, increments the upper 32bits
>> no_lowcarry:
>
>
> Oh my god...
>
> Ok, this is real easy now.  Say we have two 64-bit numbers, as such:
>
> big_one dd 1, 2
> big_two dd 3, 4
>
> In normal intel backwardness (the right way, btw) 1 and 3 are the 
> least signifigant dwords.
>
> It's as easy as this:
>
> add [big_one+0], [big_two+0]
> adc [big_one+4], [big_two+4]
>
> or, if you prefer code that compiles:
>
> mov eax, [big_two]
> add [big_one], eax
> mov eax, [big_two+4]
> adc [big_one+4], eax
>
> jc overflow_handler
>
> You just go from least signifigant to most signifigant adding them 
> together, and use 'add' for the first and 'adc' for each one after 
> that, and when you're done, the carry flag is the same as if you had 
> added just one number, the 'jc overflow_handler' will jump only if the 
> answer in this case would not fit into 64 bits.  To subtract you do 
> exactly the same thing, just using sub and sbc.
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