Arithmetic Logic Unit

NAME:_________________________________________

Complete the following table (all values in the table MUST be represented in BINARY).

Before submitting your work, verify that the ALU answer [F(3..0)] matches the correct answer. If it does not, ASK FOR HELP and correct your answer.

		Inputs					Outputs
	FUNCTION	A(3..0)	B(3..0)	S(3..0)	CN	M	F(3..0)	Cn+4
example	4+10 (addition)	0100	1010	1001	0	0	1110	0
a)	12+15 (addition)
b)	6-2 (Subtraction) Hint: Carry adds 1				1
c)	9-1 (decrement)
d)	NOT A when A=13
e)	NOT B when B=3
f)	A and B A=11, B=6
g)	A or B A=11, B=4
h)	A+B (addition) A=11, B=14
i)	A=4 B=6
j)	A=15 B=11

For i-j: pick 2 more functions that HAVE NOT BEEN USED and perform those functions. Make sure you type in the function you selected into the first column (A and B are given, but you can select a function that uses only one of the values if you wish).

The ALU also is responsible for comparisons. When placed in SUBTRACT MODE (A MINUS B MINUS 1) and the Cin is low (before being inverted), the A=B output will come on when Operand A = Operand B. Try this in the simulation.

Questions:

1) What does the M input represent (ie what does the ALU do when M=0 and what does it do when M=1)? How does that relate to the Table given in the simulation?

2) If you have result of 0 and Cout is HIGH, what HEX VALUE does this represent?

3) Try generating a negative number (eg 2-5). Did you get the expected answer? If not, speculate why not? If you did, explain how the ALU obtained that value.

4) What is a half adder circuit?

5) Since there is NO multiply function in this ALU, how could a computer that uses this ALU do MULTIPLICATION? (Hint: it must do something a number of times)

6) Since there is NO divide function in this ALU, how could a computer using this ALU do DIVISION? (hint it must do something a number of times).

7) Find a data sheet for the ALU (74LS181) and paste the link and pinout diagram into this document.

8) On what pins would the following information be placed:

a) Operand A

b) The Function

c) Operand B

d) The answer out

e) The M Bit

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