Hardware Design of an
Arithmetic Logic Unit
(ALU)
Felix Noble
Mirayma V. Rodriguez
Agnes Velez
University of Puerto Rico
Mayagüez Campus
Mayagüez, Puerto Rico
Department of Electrical and Computer Engineering
Outline
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Introduction
ALU Design
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Full Adder
A input device
B input device
Logical Operation Device
Conclusion
Future Work
References
Acknowledgements
Introduction
– An arithmetic-logic unit (ALU) is the part of a
computer processor (CPU) that carries out arithmetic
and logic operations on the operands in computer
instruction words.
– The ALU includes storage places for input operands,
operands that are being added, the accumulated
result (stored in an accumulator), and shifted results.
The flow of bits and the operations performed on
them in the subunits of the ALU is controlled by gated
circuits.
ALU Design
– The ALU is a combinational circuit that
continuously computes its outputs as a
function of its inputs. A change in its inputs
will produce valid outputs by the strobe time
of the current clock cycle.
– This ALU uses only one bus and consist of a
full adder, two inputs, A and B, and a logical
device, which is the part in charge of the
ands, ors, and shifts.
Full Adder
– The 8-bit full adder consists of eight 1-bit
full adders. Each 1-bit adder has three
inputs and two outputs. It receives two bits
to add and a carry-in if needed. The
outputs are the sum and a carry-out if
generated.
Full Adder
Full 8bit
adder
A input device
– The A input part of
the chip consist of
8 AND gates and
three OR gates
connected as
follows to makes
the instruction of
ADD and SUB
corresponding to
the Register A. This
part also
increments the
Program counter by
three.
B input device
– The register B is
also represented
with a group of 8 DFlip-Flops. The
input part of the
chip consists of
eight XOR gates,
and two OR gates
connected to deal
with the NEG, NOT
and SUB
instructions.
Logical Operation Device
– The 8-bit full adder consists of eight 1-bit
full adders. Each 1-bit adder has three
inputs and two outputs. It receives two bits
to add and a carry-in if needed. The
outputs are the sum and a carry-out if
generated.
Sub-ALU
ALU
ALU
Conclusion
The Arithmetic Logic Unit was implemented by doing a
hardware design of the unit. The way the design was
made is very easy to understand since it was made by
doing block diagrams of the different parts of the device.
This abstraction made us understand better each part of
the unit and realize how important these parts are for the
correct operation of the ALU. The bus that was utilized
made it easier to understand the circuit even dough in
the practical area this design will be much slower than
designs using two or three buses. In conclusion this ALU
device will work with the SRC architecture because it
implements every instruction of the SRC.
Future Work
There could be more projects based in the
implementation of the Arithmetic Logic Unit.
First the ALU can be improved in many ways
by changing parts of the design or adding
busses to the circuit. This can lead to create
the entire Control Processing Unit of the SRC.
Having done the CPU the entire processor can
be done too.
References
Computer Systems Design and Architecture
Second Edition
Vincent P. Heuring, Harry F. Jordan
http://ww.google.com
http://www.ece.uprm.edu/~nayda/Courses/arq/Projects/Proj2/VHDL/
Acknowledgements
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Chori’s Café
Sociedad de los Buhos
Prof.Nayda Santiago
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