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Index > Examples and Tutorials > Newbie - Overflow Flag

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SC0U7



Joined: 20 Feb 2018
Posts: 21
SC0U7
Hello guys i am learning assembly language im currently on Flags.
I thinking about Overflow Flag

when i have somethink like this:

Code:
mov al,7fh
mov cl,1h
add al,cl
 
;al = 0x80
;Overflow flag is set    


But my question is how cpu know how is number negative or positive?
It may be both i am right?
But why is Overflow flag set after this?
Image
Thanks for any reply


Embarassed
Post 02 Nov 2019, 18:26
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7404
Location: Kraków, Poland
Tomasz Grysztar
As a fortunate coincidence, you can find this topic discussed in two of my recent videos.

I explained the basic ideas of the signed encoding and SF/OF in the part 6 of my ongoing x86 tutorial.

And then I also started a new semi-advanced mini-series about using 2-adic numbers to explain these things in detail. There I derive some actual formulas for the value of OF after addition/subtraction operation.
Post 02 Nov 2019, 18:51
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SC0U7



Joined: 20 Feb 2018
Posts: 21
SC0U7
Thank you Tomasz but if i am right then the reason why is Overflow flag set in 8bit register is fact the 128 in complement method not exist ? the last bit make a error for computer 8bit register then set OF. because for make OF on Zero it must be in range −128 and 127 right? Cuz my AL register cannot hold 128 then it only change it on EAX last byte 000DFF80 like 80 iam right? Thanks
Post 02 Nov 2019, 19:50
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7404
Location: Kraków, Poland
Tomasz Grysztar
In signed interpretation the highest bit is the sign bit. In this case you add two numbers that have the sign bit 0, so they are positive numbers, but the result has the highest bit 1 and therefore would be interpreted as negative number, even though the result of adding two positive numbers should also be positive. This discrepancy is an indication that an overflow occurred - the result of addition was too large to be correctly encoded as a positive number in 8 bits with sign bit.
Post 02 Nov 2019, 19:58
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SC0U7



Joined: 20 Feb 2018
Posts: 21
SC0U7
so when i try this

mov al,7fh
mov cl,-1h
add al,cl

The Overflow Flag will be not set
Post 02 Nov 2019, 20:10
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7404
Location: Kraków, Poland
Tomasz Grysztar
Yes. You can also see a similar example (-1 + 3) analyzed near the end of my aforementioned 2-adic video.
Post 02 Nov 2019, 20:14
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