Re: why overflow flag being set?

["A D" <a_d_249@xxxxxxxxxxx>]

> Robert Plantz wrote:
> The range of 8-bit, signed number is -128 -> +127. There is no +128. But
> there is (unsigned) 128. Unfortunately, the assembler doesn't know this.
> When you wrote movb $128, %bl, it blindly used the bit pattern 0x80 for
> 128, which is the bit pattern for (signed) -128. The bit pattern for 127
> is 0x7f.
>
> The addb instruction sets both the carry flag and the overflow flag
> according to the results of the addition. In 8-bit addition (in hex):
>       0x80 + 0x0a = 0x8a      CF = 0   OF = 0
>       0x7f + 0x0a = 0x89      CF = 0   OF = 1
>
> In signed decimal this is
>       -128 + 10 = -118   ==> correct arithmetic
>        127 + 10 = -119   ==> incorrect arithmetic
>
> If your program were using unsigned values, the range of 8-bit numbers
> is 0 -> 255.
> In unsigned decimal this is
>        128 + 10 = 138   ==> correct arithmetic
>        127 + 10 = 137   ==> correct arithmetic
>
> The bit patterns (expressed in hex here) are the same. If your program
> is treating them as signed values, you should test the overflow flag. If
> they are being treated as unsigned, look at the carry flag.
>
> This illustrates the importance of using the "unsigned" keyword in C/C++
> where appropriate. Assembly language has no concept of "type." You have
> to take care of that in your code.
>
> Good news: You can do whatever the hardware is capable of in assembly
> language.
>
> Bad news: You have to do it.



Thanks for the comprehensive explanation. That answers my question.

_________________________________________________________________
Find the best places on campus to get take out, study & unwind 
http://www.liveu.ca/explore.aspx

-
To unsubscribe from this list: send the line "unsubscribe linux-assembly" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html