IS 139 Assignment 1 - UDSM - 2014

1. IS 139 ­ Introduction of Computer Architecture
Assignment 1 ­ ALU Design
Design and implement a simple 4­bit Arithmetic and Logic Unit (ALU) than can perform the
following arithmetic and logical operations: ADDITION, SUBTRACTION, AND & OR
Figure below shows the input & output specification of such an ALU:
● A and B represent your two 4­bit input operands e.g. 0010 & 1100
● Op (Operation) represents the control input to the ALU that specifies the operation to be
performed by the ALU i.e. addition, subtraction, and & or
● Result is the 4­bit output of the ALU depending on the two input values (A & B) and the
control signals (Op)
Deliverables
A functioning logic circuit of the specified ALU implemented in Deeds () that is able to perform
the specified operations on its two 4 bit input values.
For example, given inputs values 0010 and 0011 with the operation set as addition (e.g. 00) your
ALU circuit should correctly output 0101 which is the result of adding the two inputs
Note: For some values of the input in the arithmetic operations there is a possibility of having an
overflow condition (where the result does not fit in 4 bits) leading to incorrect results. For the
purposes of this assignment you can ignore those cases and just output those 4 bits

2. Recommended Steps
● Choose and assign each operation (i.e. addition, subtraction etc) a unique operation code
(i.e. 00, 01) to signify the function the ALU should perform
● Design & implement a 1 bit ALU which can perform all the operations on 1 bit inputs
● Combine together 4 of such 1 bit ALUs for a final 4 bit ALU
Bonus
Implement and add to the ALU the ability to detect an overflow condition. The ALU should have
an additional one bit output (OF) which is set when there is an overflow as a result of the
performed arithmetic operation.
NOTE: An overflow occurs when the most significant bit is changed by adding two numbers with
the same sign (or subtracting two numbers with opposite signs) ­ or in other words when adding
two positive numbers result in a negative number & addinf two negative numbers result in a
positive number.