BIT ARITHMETIC LOGIC UNIT (ALU) USING VHDL - ijesr.orgijesr.org/admin/upload_journal/journal_IJESR Saif abbas 12ap18sr.pdf · An Arithmetic logic Unit (ALU) is used for arithmetic,

ISSN 2277-2685 IJESR/April 2018/ Vol-8/Issue-4/58-68 Saif Abbas et. al., / International Journal of Engineering & Science Research

*Corresponding Author www.ijesr.org 58

DESIGN OF 32-BIT ARITHMETIC LOGIC UNIT (ALU) USING VHDL

Saif Abbas*1, Bhavya Chaturvedi1, Rituraj Akhauri1, Rupali Singh2

1Student, Dept. of Electronics & Communication Engineering, SRM Institute of Science & Technology, Delhi-NCR Campus, Modinagar (U.P), India.

2Asst. Prof., Dept. of Electronics & Communication Engineering, SRM Institute of Science & Technology, Delhi-NCR Campus, Modinagar (U.P), India.

ABSTRACT

An Arithmetic logic Unit (ALU) is used for arithmetic, logic and shift operations. It is most important component of a system and is used in many appliances like calculators, cell phones, and computers. A 32-bit ALU will be designed using VHDL. The logical gates which will be used are AND and OR and NOT and XOR for each 32- bit ALU circuit and using them other gates such as NOR, NAND, XNOR will be designed. The design will be implemented in Xilinx. It works faster than the ALU designed using less power. In this project we will not be using multiplexer as used generally. Instead we will design a new signal named Op_ code. This helps in reducing power consumption and power dissipation and reducing area. All the modules will be simulated in models im ISim (P.40vd) and synthesized using Xilinx ISE 14.3.The adder which will be used in this design is ripple carry adder which effectively combines two-32-bit-in-one-group and gives us sum, which makes the basic addition unit increased in speed, reduced in size, and eventually lowered in power consumption.

Keywords: ALU; Verilog; AND; OR; Op_code; modelsim SE 6.4c; Xilinx ISE 14.7; ripple carry adder; NOT; NOR; Simulation.

INTRODUCTION

32-BIT ALU Due to widespread use of microprocessors and signal processors, operation of high performance and low power hardware has always been a design problem. Arithmetic Logic Unit (ALU) is the most important part of microprocessors controlling the speed of operation of the processor. Better is the design of ALU better the microprocessor works. The main objective of this project is to improve the design of the ALU and reduce the power usage and area covered by it. ALU is getting smaller and more compound to enable the development of a more powerful but smaller systems. The need for high speed, less power consumption and compatible systems

Saif Abbas et. al., / International Journal of Engineering & Science Research

Copyright © 2018 Published by IJESR. All rights reserved 60

has been increasing as a result of computer, digital signal processing and networking applications. Arithmetic operations such as, addition, addition with carry and subtraction, subtraction with borrow and logical operations such as AND, OR, NOT, XOR, XNOR, NAND, NOR and right shift and left shift in shift operations are using all type of operations done in ALU. In some computer processors, the ALU is divided into three separate parts, the AU (Arithmetic Unit) and the LU (Logical Unit) and SU (Shift Unit). The AU performs the arithmetic operations and the LU performs the logical operations and SU performs Shift operations.

An Arithmetic unit does the following function:

1. Addition 2. Addition with carry 3. Subtraction 4. Subtraction with borrow

A Logic unit does the following function:

1. AND 4. NOT 7.XNOR 2. OR 5. NAND 3. XOR 6. NOR

A Shift unit does the following task:

1. Right shift 2. Left shift

DESIGN OF 32‐ BIT ALU

While designing the ALU we will follow the principle "Divide and conquer" in order to make a design that has smaller, more manageable blocks. Besides designing the 32‐bit ALU as one circuit we will first design one bit ADDER, SUBTRACTOR. These bits can then be put together to make a 32‐bit ADDER, SUBTRACTOR, OR, AND, NOT, XOR, LEFT SHIFT, RIGHT SHIFT UNIT.

32‐BIT ARITHMETIC UNITS

An Arithmetic unit does the following task: Addition, Addition with carry, Subtraction, Subtraction with borrow. Firstly, we start by making one bit Full Adder, and similarly make 32 full adders. In this we will use ripple carry adders.

To have small area coverage and fewer number of interconnections and area efficiency our ALU has been designed using ripple carry adder. A 16 -bit, 13.9ns, 13.9mV (Ripple carry adder). Some other types of adders are ripple-carry adder, carry- look ahead adder, carry-skip adder and Manchester carry chain adder. Carry- look ahead adders first compute carry propagate and generate and then computes SUM and CARRY from them. A 16- Bit, 24 ns, 17mV (Carry Look ahead adder).

Two 32‐bit inputs A and B have been given and 33‐bit output is RESULT. C_in is the carry input of the adder and the carry out is C_out. The output of the circuit is calculated from the following :

RESULT = A + B + C_in

Where A is a 32‐bit number, B is a 32‐bit number and C‐in is the carry input bit to the ripple carry adder.

In case of output with Op_code

Output => add_out when”1001”

Sub_out when”1010”



RIPPLE CARRY ADDERARITHMATIC UNIT

SIMULATED TIMING DIAGRAM OF ARITHMETIC UNIT

(ADDITION)

WITHOUT CARRY

WITH CARRY



(SUBSTRACTION)

WITHOUT BORROW

WITH BORROW

32-BITS LOGIC UNIT

A Logic unit does the following functions AND, OR, NOT, NAND, NOR, XOR, XNOR. We design a logic unit that can perform the four basic logic operations: OR, AND, XOR and NOT, because from these four operations, all other logic micro‐operations can be derived that is NAND, NOR, XNOR.

The 4 gates input are applied and they are selected according to the op_code. As follows

Output => not_out when”0000” and_out when”0001” or_out when”0010” nand_out when”0011” nor_out when”0100” xor_out when”0101” xnor_out when”0110”



Diagram of Logic Unit

SIMULATED TIMING DIAGRAM OF LOGIC UNIT

NOT

AND



OR

NAND

NOR



XOR

XNOR

32-BITS SHIFT UNIT

Shift unit is used to perform shift operations such as Right and Left shift. The shifting of bits of a register can be in any direction‐ left or right. The bit provided to a register that has to be shifted is first placed onto common bus.When load_data_signal is lowthe data is loaded and when start_shift_sig is high then shifting take place. When the shifting unit is activated the register is shifted left or right according to the selection fo op_code.

with op_code select

load_data_sig <= '0’ when "0111",

'0' when "1000",

'1' when others;

with op_code select

start_shift_sig <= ‘1’when "0111",

'1' when "1000",

'0' when others;

The input is applied and they are selected according to the op_code. As follows

Output =>shift_out when "0111", -- Left shift

shift_out when "1000", -- Right shift



Diagram of SHIFT Unit

SIMULATED TIMING DIAGRAM OF SHIFT UNIT

LEFT SHIFT

RIGHT SHIFT



CONCLUSION

In our paper “Design of 32‐bit ALU using VHDL” we have designed and simulated a 32 bit ALU. Arithmetic Logic Unit is the part of a device that performs all arithmetic operations, such as addition and subtraction, addition with carry, subtraction with borrow, shifting and all sorts of logical operations AND, OR, NOT, NAND, NOR, XOR, XNOR. The ALU is one component of the CPU (Central Processing Unit). The logical gates used are AND and OR and NOT and XOR for each 32- bit ALU circuit and using them other gates such as NOR, NAND, XNOR are designed. In this project we have not used multiplexer as used generally in deigning ALU. Instead we have designed a new signal named Op_code. This helps in reducing power consumption and power dissipation and reduces area. All the modules are simulated in modelsim ISim (P.40vd) and synthesised using Xilinx ISE 14.3.The adder used in this design is ripple carry adder. The comparison between other designed ALU and our ALU can be seen clearly bellow (5).

With Op select

Output <= not_out when "0000",

and_out when "0001",

or_out when "0010",

nand_out when "0011",

nor_out when "0100",

xor_out when "0101",

xnor_out when "0110",

shift_out when "0111", -- Left shift

shift_out when "1000", -- Right shift

add_out when "1001",

sub_out when "1010",

(others => '0') when others;

AREA OCCUPIED: 412 LTU TIMECONSUMED: 43.32 secs

\

AREA OCCUPIED: 715 LTU(1, 7) TIMECONSUMED: 63.410(1, 7)

COMPARRISON BETWEEN DESIGNED ALU AND OTHER ALUs

DESIGNED ALU OTHER ALUs Ø No use of mux Ø Used mux Ø Use of a new signal op_code to improve performance. Ø No use of any signal. Ø Reduced area coverage. Ø 412 LTU.

Ø Larger area coverage. Ø 715 LTU.

Ø Reduced time taken for implementation. 43.32 secs.

Ø Larger time consumption for implementation. 63.410 secs.

Ø Used only four basic gates for implementation of Logic Unit. Namely : AND, OR, NOT, XOR

Ø Uses all 7 gates for implementation of Logic Unit. Namely : AND, OR, XOR, XNOR, NOT NAND, NOR



REFERENCES

[1] Ghosh TK, Pal AJ. Computer Organization and Architecture, Tata McGraw‐ Hill Publishing Company Limited, New Delhi, 2013.

[2] Perry DL. VHDL Programming by Example, 4th ed., Tata McGraw‐Hill Publishing Company Limited, New Delhi, 2002.

[3] Munjal S, Bhardwaj S, Malik S. Dept. of Computer science and Engineering, Dronacharya College of Engineering, Gurgaon, Haryana, India, ALU USING VHDL DESIGNING A 4-BIT LOW POWER ALU USING VHDL, International Journal For Technological Research In Engineering 2015; 2(8).

[4] Singh RPP, Kumar P, Singh B. Performance Analysis of Fast Adders Using VHDL, Advances in Recent Technologies in Communication and Computing, 2009. ARTCom '09. International Conference, 17 November 2009.

[5] Comparative study and designing of Look Ahead Carry Adder and Ripple Carry Adder, January 2013.

[6] Z. GRODZKI College of Engineering, Lublin, Dqbrowshiego 13, Poland, the Theory of Shift-Registers, Information and Control 1972; 21: 196-205.

[7] Malik D, Rathore RS. 32‐bit Arithmetic Logical Unit (ALU) using VHDL. International Journal of Science, Engineering and Technology, 2013.

BIT ARITHMETIC LOGIC UNIT (ALU) USING VHDL - ijesr.orgijesr.org/admin/upload_journal/journal_IJESR Saif abbas 12ap18sr.pdf · An Arithmetic logic Unit (ALU) is used for arithmetic,

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