why overflow flag being set?

I've a question about the handling of signed and unsigned numbers by GAS. 
When I
write the following:


    movb $128, %bl
    addb $10, %bl

bl register doesn't overflow and overflow flag is not set. But as soon as I 
use
127 instead of 128 like this:

    movb $127, %bl
    addb $10, %bl

overflow flag is set. Does this mean when i move  a value to a register 
smaller
or equal to the maximum signed value that register can contain, gas treats 
it as
signed number? Thanks.

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