Re: why overflow flag being set?

>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.

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