Sign in to follow this  

2's complement problem?

This topic is 2574 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

Quote:
use only 4 bits and add +9 , -4 using 2's

Would help a bit if you clarified exactly what the heck you're saying.

Depending on if you're using signed or unsigned, 4 bits may be enough. Unsigned gives you between [0,15], signed gives you [-8,7]

Share this post


Link to post
Share on other sites
Adding and subtracting will overflow with 4 bits, but the overflow is...nondestructive? in this case, and you'll get the correct result.

2's complement form is a way to convert a number to/from a bitwise representation.

For example if I give you the following bits without any information about what form they're in, they're just bits:

1001

That could be 9 in unsigned form or -7 in 2's complement form.

Addition/subtraction operations don't care about the form the bits are in.


-4 in 2's complement 4-bit form is 1100.



1001
+ 1100
------
0101 with carry


so the result is that you have 5 as the result, and your carry flag has been set.

Share this post


Link to post
Share on other sites
Using 4 bits and 2's complement you can only represent integers in the range [-8,8), so your question about adding +9 and -4 doesn't make much sense.

In a more mathematically-sound way of looking at things, 4-bit numbers are actually congruency classes of the integers modulo 16. In that sense, +9 and -7 are the same number (1001), and so are -4 and +12 (1100). You can add them together and the result is 9+12 = 21 = 5 (0101).

Share this post


Link to post
Share on other sites

This topic is 2574 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this