# Designing simple 4 BIT ALU ( Need Help ! )

Discussion in 'Homework Help' started by Sparda90, May 29, 2013.

1. ### Sparda90 Thread Starter New Member

May 29, 2013
1
0
Hallo guys , I am first year electrical Engineer Major & we are asked to design and simulate using Multisim a small ALU ,

The ALU is a 4-bit digital circuit that performs addition, subtraction, ANDing, ORing, complementing, XORing, XNORing and comparison.
The ALU also outputs if the answer is negative, zero, or overflow.
Use 3 bits to select the function of the ALU.

I am kinda clueless where should i start , any hints on Multisim or how should i start this ? it would be really really appreciated if anyone Can contribute helping me or pointing me in the right direction .

2. ### MrChips Moderator

Oct 2, 2009
12,648
3,458
Create a design table showing inputs, function selection and outputs.

Look up the data sheet of 74181 as a guide.

3. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
Let's consider the three output status bits. These bits detect various conditions that are true about the 4-bit output.

If the bits of the result are named R0, R2, R2, R3 then a boolean equation which ANDs together the complement of each bit would look like:

Z = R3* & R2* & R1* & R0*

Z will be equal to a 1, if and only if all four bits of the result are 0

Negative is easy it is just the most significant bit. That would be R3 in this case.

Overflow is more complicated but it basically comes down to getting a negative result from adding two positive numbers or a positive result from adding two negative numbers.