Subtracting two 64bit values?

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Karl20001

Joined: 10 Nov 2005
Posts: 11

Karl20001 05 Feb 2006, 18:34

If i have a 64bit value in edx:eax, and another one in ecx:ebx

Code:

sub eax,ebx
subb edx, ecx

;result in edx:eax

If eax>ebx no problem, if ebx>eax the borrow helps to give correct edx.
But then eax would be a negative number, that cant be right?!

05 Feb 2006, 18:34

Madis731

Joined: 25 Sep 2003
Posts: 2139
Location: Estonia

Madis731 05 Feb 2006, 22:42

You just look at them as unsigned numbers. This is the right way to do 64-bit arithmetic:

Code:

add eax,ebx
adc edx,ecx
;edx:eax holds the result

sub eax,ebx
sbb edx,ecx
;edx:eax holds the result

;if carry occurs the eax would wrap around to 0FFFFFFFFh which is 4294967295
;in unsigned world but if you want you can consider it -1 too

multiplication is a bit harder and division a lot harder

05 Feb 2006, 22:42

Karl20001

Joined: 10 Nov 2005
Posts: 11

Karl20001 06 Feb 2006, 21:07

Hi, thx for your answer Smile

06 Feb 2006, 21:07

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