Help with Adder/ALU cirucit design

[The Axe Dude]

Hi, I have a task I am busy doing for uni, but it has really stumped me!

Exercise 2 (Advanced)
The adder presented in class adds two binary numbers. Without changing the 1/2 adders in the
adder, perform the appropriate modifications so that it can perform the following functions:
a. Receive as input a “subtract bit'' and a “shift bit'', such that
* when the subtract bit is 1, it subtracts the second binary number from the first.
* when the shift bit is 1, it shifts left the first number (and ignores the second).
b. The shift and subtract bits cannot be 1 at the same time, but you don't have to check for
that in your design.
Note: You can do each modification separately or you can do both together.

I'm not really bothered about the shifting of the digits, just the subtractions. I understand that I would need a control signal, which when has a value of '1', it swaps all of the digits in the second number.

I just can't think of a way to do this using gates...

The best idea I can come up with is an 'if-statement' of sorts, except I don't know how to present it graphically...

BTW: Here is the "adder presented in class"
http://mybroadband.co.za/photos/showphoto.php?photo=3150&size=big&cat=500

Thanks,



Edit: Can a mod change cirucit to circuit please?

 

[bison234]

your homework and the internet

have you actually read ALL of the relevant work in your textbook? and understood it? if you plan to become an engineer, looking for help with your homework on the internet is not a good sign.

 

[The_Techie]

have you actually read ALL of the relevant work in your textbook? and understood it? if you plan to become an engineer, looking for help with your homework on the internet is not a good sign.

Might be the best place to seek help, there are countless experts on the internet

 

[The Axe Dude]

I don't want to be an engineer, Im doing my degree more for the programming aspect, and this is just one aspect which I cannot think of how to implement. I understand what I need to do, I just cant think of how to do it. Only half of the lecture has been delivered so far, so I am trying to do these exercises to get ahead a little. I've sat drawing designs for this circuit, but it just doesnt seem to be working. I think there is a reason it is marked with 'Advanced'. I'm still going to figure it out though

 

[The Axe Dude]

Nevermind guys, got it figured out