library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
entity fadf is
Port ( ain : in STD_LOGIC;
bin : in STD_LOGIC;
cin : in STD_LOGIC;
sum : out STD_LOGIC;
cout : out STD_LOGIC);
end fadf;
architecture df of fadf is
begin
sum<= ain xor bin xor cin;
cout<= (ain and bin) or ( bin and cin) or (ain and cin);
end df;
As I understand it, a database schema is a physical entity, it describes the structure of exactly how the data is stored and is itself stored by DBMS for reference. Data model, on the other hand, is an abstract representation of database.
A number of extensions to the relational data model have been proposed in the three decades or so since its invention. Many of these extensions have been implemented in commercial DBMS. What is termed the post-relational data model here is not strictly a data model in that no coherent theory has been developed. Nevertheless it is useful to discuss it here in terms of a set of mech- anisms found in many contemporary DBMS. Such a data model is also referred to by the terms extended-relational and object-relational data model. In Chapter 18 we discuss how the proposed SQL3 standard addresses many of these features. In Chapter 34 we also consider how the ORACLE DBMS supports some of these features. In the first half of the chapter we consider two extensions to the data defin- ition part of the relational data model: abstract data types and nested relations. In the second half of the chapter we consider two constructs - triggers and stored procedures - that have been used both for data manipulation and data integrity purposes. The incorporation of these features into a relational DBMS provides it with the ability to handle complex objects and behaviour. Hence many of the DBMS with these features have termed themselves object-rela- tional systems.
Access Work Area
Use of primary keys less data redundancy compatible with inconsistencies associated with database anomalies
ADO is active x data object. It is used to access database with the help of data controls and objects as well. it is an extended form of RDO and DAO. RDO is remote data object which is used to access server site data. data in this case reside on the sql server. and we use sql queries to access data. in this we write a sql command and then squ server processes it and gives back the result. DAO is data access object. it is used to access data with the help of program and some data controls. it provides an extention to data controls and data bound controls. DAO helps in accessing data with various conditions or quries.
Vhdl has got three models - programming styles. 1. data flow model 2. behavioral model 3. structural model.
Half adder is a combinational circuit which can add two bits. It contains two inputs and two outputs. The same is implemented in entity declaration of VHDL program. The outputs are related to inputs as follows: SUM output is obtained by XORing the inputs and CARRY output is obtained by ANDing the inputs i. e. multiplication. The VHDL code for half adder using data flow model is given below: library IEEE; use IEEE.STD_LOGIC_1164.ALL; use IEEE.STD_LOGIC_ARITH.ALL; use IEEE.STD_LOGIC_UNSIGNED.ALL; entity had is Port ( a : in STD_LOGIC; b : in STD_LOGIC; s : out STD_LOGIC; c : out STD_LOGIC); end had; architecture df of had is begin s<= a xor b; c<= a and b; end df;
VHDL program follows IEEE library. This means that all the data types, commands, keywords etc. used in a VHDL program are stored in a library called IEEE library. This library will be available in the EDA tool which is executing the VHDL program. 1164 is a package where all the logic gates are defined. This is a sub part of IEEE library. As encoder program requires logic gates, we need to use 1164 package in the code.
VHDL is a hardware description language which is used to describe digital circuits or systems. The data involved digital systems is logical data i. e. 0 or 1. Hence, VHDL uses logical data as input and provides the same type of data in output.
VHDL can be written in three different models. They are calleddata flow modelbehavioral modelstructural modelBefore attempting a VHDL program, one should know the steps involced in these modeling styles.Data flow model:In this model, the input data simply flows into the output. THat is, we will be implementing the relation between input and output terminals directly.For example,c < = a and b;Here, the output c is an ANDing of a and b. We are actually implementing the direct relation between inputs and outputs. That is, c = a + b.Hence, we need not write any complex conditional statements here in data flow model.Simply implement the output expression. Thats all.Here, we are implementing the code at a very basic level i. e. circuit level or gate level.Behavioral model:Here, in behavioral model, one needs to code the behavior of the system to be designed. If we consider the same above example, the behavior is that, the output should be one (1) whenever both the inputs are one (1).we can code it like this:if (a=1 and b=1) thenc
VHDL can be written in three different ways. They are calleddata flow modelbehavioral modelstructural modelBefore attempting a VHDL program, one should know the steps involced in these modeling styles.Data flow model:In this model, the input data simply flows into the output. THat is, we will be implementing the relation between input and output terminals directly.For example,c < = a and b;Here, the output c is an ANDing of a and b. We are actually implementing the direct relation between inputs and outputs. That is, c = a + b.Hence, we need not write any complex conditional statements here in data flow model.Simply implement the output expression. Thats all.Here, we are implementing the code at a very basic level i. e. circuit level or gate level.Behavioral model:Here, in behavioral model, one needs to code the behavior of the system to be designed. If we consider the same above example, the behavior is that, the output should be one (1) whenever both the inputs are one (1).we can code it like this:if (a=1 and b=1) thenc
In VHDL, std_logic is a data type. It is assigned to input and / or output variables. It means that the variable is a standard logic type i. e. a logic bit which accepts or provides one bit data, either 1 or 0.
And when the ASIC industry needed a standard way to convey gatelevel design data and timing information in VHDL, one of Accelleras progenitors (VHDL International) sponsored the IEEE VHDL team to
Below code can implement NOT gate in VHDL. The code is written in data flow model. Library ieee; use ieee.std_logic_1164.all; Entity gates is port (a : in std_logic; c : out std_logic); end gates ; architecture and1 of gates is begin c<=not a; end and1;
These are predefined words in VHDL standards. Bit indicates that the data type is a bit i. e. 0 or 1. A bit_vector is an array of bits. example: a: in bit; b: in bit_vector(1 downto 0);
Below code can implement AND gate in VHDL. The code is written in data flow model. Library ieee; use ieee.std_logic_1164.all; Entity gates is port (a,b : in std_logic; c : out std_logic); end gates ; architecture and1 of gates is begin c<=a and b; end and1;
Frank A. Scarpino has written: 'VHDL and AHDL digital system implementation' -- subject(s): Computer-aided design, Logic circuits, Electronic digital computers, Data processing, System design, Circuits, VHDL (Computer hardware description language)