union , enum , typedef :

union

The keyword union is useful in defining a new data type like a structure. Its usage is also similar in syntax to a structure. But conceptually union and structures differ in the way they manage memory.In a structure memory is allocated for each member . In Union same memory is shared by all data members

Thus a structure variable requires memory = sum of the memory requirements of all its members

A union variable requires memory = maximum memory requirement of one member

Thus union allows us to interpret the same memory location

in different ways .

union sample

{

int x ;

float y ;

char ch ;

} p ;

Thus here p occupies 4 bytes. Byte 0 is interpreted as char ch . Byte 0 , byte1 together as int x , .All the four bytes together are read as float y . Changing the value of one data member may affect value of all other members.

Program:

union A

{

int x ;

char ch[2] ;

};

void main( )

{

union A var ;

var.x = 260 ;

printf(“%d %d” , var.ch[0] , var.ch[1] );

var.ch[0] += 10 ;

printf(“\nx = 5d” , var.x );

var.ch[1] += 2 ;

printf(“\nx = %d” , var.x);

}

Memory allocated for var => 2 Bytes or 16 bits

var.x = 260 -> 0000 0001 0000 0100

var.ch[0] - > 0000 0100 /* prints 4 */

var.ch[1] - > 0000 0001 /* prints 1 */

var.ch[0] += 10

var.ch[0] - > 0000 1110 /* now 14 */

var.x -> 0000 0001 0000 1110 /* now 270 */

var.ch[1] += 2 ;

var.ch[1] - > 0000 0011 /* 3 */

var.x - > 0000 0011 0000 1110

/* 512 + 256 + 14 = 782 */

Application of unions :-

There is a frequent requirement while interacting with hardware that the same data be interpreted differently

For e.g : Unions are useful in reading and manipulating CPU registers and availing various BIOS services (calling int86( ) function)

CPU registers are 2 bytes in size .We may access the information they store as 2 bytes entirely or as two information of one byte each.

What is an interrupt ?

An interrupt means to stop doing something and instead do something else. Keyboard interrupts are generated by pressing control key , (and or) alt keys , shift key along with some other key. An operating system has a table of interrupt numbers (Interrupt Vector Table IVT) and actions to be taken when the interrupt is encountered.

For e.g

ctrl + C : by pressing ctrl + C (^C) you might stop some

process

PrintScreen : By pressing the PrintScreen key you are generating an interrupt

An interrupt is serviced by the Operating system

You can call various dos interrupt services using int86( ) function (include

Declaration:

int int86(int intno, union REGS *inregs, union REGS *outregs);

inregs holds the values of A , B , C, D registers that are set before the function call

outregs holds the values of A , B , C , D registers after the call

Return Value:

The functions returns the value of AX register after completion of the software interrupt.

Programs: int86( ) function

#include

#include

#include

#define VIDEO 0x10

void movetoxy(int x, int y)

{

union REGS regs;

regs.h.ah = 2; /* set cursor position */

regs.h.dh = y;

regs.h.dl = x;

regs.h.bh = 0; /* video page 0 */

int86(VIDEO, &regs, &regs);

}

­

REGS

REGS (union)

The union REGS is used to pass information to and from these functions . It is defined as follows :-

union REGS {

struct WORDREGS x;

struct BYTEREGS h;

};

BYTEREGS and WORDREGS

Structures for storing byte and word registers

struct BYTEREGS {

unsigned char al, ah, bl, bh;

unsigned char cl, ch, dl, dh;

};

struct WORDREGS {

unsigned int ax, bx, cx, dx;

unsigned int si, di, cflag, flags;

};

Enumerated Data Types :-

The keyword enum helps us to define named integral constants .The advantage :

Improved readability and program documentation.

We use #defines to define constants. Here the datatype is not specific .But with enums the constants are of type int .

Program:

enum status { false , true };

here the constant false is assigned a value 0 and the constant true is assigned a value 1

We may have a code line such as

return true ;

or found = false ;

Program:

enum colors { red , green , blue } ;

Here the constant red has a value 0 , green 1 , blue 2

You might write code like

COLOR background, foreground;
background = blue ;
foreground = red;

Values may be redefined as

enum E1 { c1 = 100 , c2 , c3 };

In this case c1 is assigned a value 100 , c2 = 101 , c3 = 102 and so on .

By default the named constants take incremental values starting with 0 ( 0 , 1 , 2 , 3 .... )
One is free to change these values while defining such as

enum sample { x = 1 , y = 10 , z = 20 } ;
or
enum sample { a , b , c = 10 , d , e , f = 100 , g , h };
here values of a to h are a = 0 , b = 1 , c = 10 , d = 11 , e = 12 , f = 100 , g = 101 , h = 102

The C std library includes lot of enums and one can freely use them in code. IF given a choice between #define and enum in code, one is adviced to use enums liberally instead of #defines as the latter do not pin down the data type while enums are always considered integral constants.

typedef :

The keyword typedef helps to rename datatypes or create datatypes to suit our needs . We can create user defined datatypes based on basic types like int, char, float, double or derived types like struct, union etc. Advantage is ability to call data type by a name that better defines its values and the code is better documented.

For e.g
typedef unsigned int number ;

/* program calls int by another name number */

number age ;
number quantity ;

The advantage is improved readability and program documentation .

typedef int boolean ;

typedef unsigned int WORD ;

You might now declare a variable as

Boolean status ;

WORD x ;

typedef are useful in structures

typedef struct

{

char *title ;

char *author ;

float cost ;

}Book ;

Book is a datatype we have defined .We might declare a variable

Book b1 , b2 ;

Note :

The keyword struct is not used while using data type Book to declare a variable.