Convert 7 and -7.25 to twos complement with 6 bits and add them.So , -7.25 is smth like 1001,01. When I add 000111 to it,the result is 1000,01...and if i convert this to twos complement the result is not -0.25...whats wrong?

You have -7.25 incorrect, I think you have -6.75. Remember to include the fraction in the base 2 7 as well.

You have -7.25 incorrect, I think you have -6.75. Remember to include the fraction in the base 2 7 as well.

 

But if I add another 1,then I won't have 6 bits but 7 bits :S

Convert 7 and -7.25 to twos complement with 6 bits and add them.So , -7.25 is smth like 1001,01. When I add 000111 to it,the result is 1000,01...and if i convert this to twos complement the result is not -0.25...whats wrong?

 

I am not sure why you think it is ok to spam the forum like this. This is the third post on essentially the same subject, the last two posts being identical.

 

Since you are now using a fractional representation we can consider the number as being scaled by x4.

7 x 4 = 28 --> 011100 using 6 bit signed binary

-7.25 x 4 = -29 -> 100011 using 6 bit signed binary

Add to give -1 -> 111111 using 6 bit signed binary

 

It would be much easier to use standard word sizes, 8/16/32 etc, as then you can just use a binary calculator to do the working.

You don't need to add more, you need to add less. Your most negative number possible is -8.