Description
The main purpose of this assignment is for you to get familiar with verilog by coding two simple
logic circuits.
Problem 1 : 4-bit ripple carry adder (50 points):
Part A (10 points) :
An ALU (arithmetic and logic unit) is a combinatorial circuit used in the execution stage of
modern pipelined processors. It can do all kinds of arithmetic operations such as addition,
multiplication, division, shift, logical operations and so on. We’d like to implement a simplified
version of a 1-bit ALU, which only performs AND, OR and addition, as shown in figure B.5.6 (on
page B-29) of the textbook. Please implement a behavioral model of this 1-bit ALU using the
given module skeleton. You can read the section “A 1-bit ALU” in Appendix B.5 for a detailed
description of the figure.
Part B (40 points) :
An n-bit adder can be constructed by chaining n 1-bit full-adders together. The carry-out output
of each 1-bit full adder is connected to the carry-in input of the next full adder, except for the last
full adder in the chain.
Please implement a 4-bit ripple carry adder using four 1-bit ALU units. A reference
implementation of the ripple carry adder is available in the class slides
Verilog.Tutorial.v2.6-up.2017.pdf (pg 3 bottom right)
Problem 2 : 4-bit Carry-Lookahead adder (50 points):
A carry lookahead adder is an optimized n-bit full adder implementation which achieves faster
addition than an n-bit ripple carry adder by using multiple ‘levels of abstraction’. The simplest
carry look ahead adder uses a single layer of abstraction, where it computes propagate and
generate values for each bit position and uses these values to compute carry-outs at each bit
position. Even faster addition speeds can be achieved using multiple layers of abstraction.
Please read the the description of the carry-lookahead adder in section B.6 for a more detailed
description. Then, implement a 4-bit carry lookahead adder using a single layer of abstraction in
verilog using the given 6 module skeletons. 4 modules implement the carry-out calculation, one
module implements the propagate and generate bit calculation, and one module is the top level
module.
The equations for propagate, generate and carry-out for each bit position are printed below for
your convenience :
gi = ai.bi
pi = ai + bi
c1 = g0 + (p0.c0)
c2 = g1 + (p1.g0) + (p1.p0.c0)
c3 = g2 + (p2.g1) + (p2.p1.g0) + (p2.p1.p0.c0)
c4 = g3 + (p3.g2) + (p3.p2.g1) + (p3.p2.p1.g0) + (p3.p2.p1.p0.c0)
Submission instructions :
For each of the questions, you are provided several skeleton verilog files which you need to fill
in, as well as a single testbench file. You can unzip the skeleton file using the command →
tar -xvf Lab1_Skeleton.tar.gz
Submit all the verilog files (except for the testbench) and a README in a single compressed file
Lab1_studentName.tar.gz file. You can place all the verilog files in a directory
Lab1_studentName and create a .tar.gz file using the command →
tar -czvf Lab1_studentName.tar.gz Lab1_studentName
Indicate the compilation instructions as well as the name of the top level module of the
submission in a README file. Your solutions will be tested using ISEWebpack on the CSELabs
machines and will be evaluated using the provided testbench as well as additional tests which
are not available to you. Your score will be assigned based on the number of test cases
successfully passed by the submitted code. Partial credit may also be awarded at the discretion
of the instructor, so remember to comment your solutions well. Good luck !
