Far pointer in c programming

The pointer which can point or access whole the residence memory of RAM i.e. which can access all 16 segments is known as far pointer.

Far pointer:

(If you don’t know what is segment the click here)

Size of far pointer is 4 byte or 32 bit.

Examples:

(1) What will be output of following c program?

#include<stdio.h>

int main(){

int x=10;

int far *ptr;

ptr=&x;

printf("%d",sizeof ptr);

return 0;

}

Output: 4

(2)What will be output of following c program?

#include<stdio.h>

int main(){

int far *near*ptr;

printf("%d %d",sizeof(ptr) ,sizeof(*ptr));

return 0;

}

Output: 4 2

Explanation: ptr is far pointer while *ptr is near pointer.

(3)What will be output of following c program?

#include<stdio.h>

int main(){

int far *p,far *q;

printf("%d %d",sizeof(p) ,sizeof(q));

return 0;

}

Output: 4 4

First 16 bit stores: Segment number

Next 16 bit stores: Offset address

What is segment number and offset address?

Example:

#include<stdio.h>

int main(){

int x=100;

int far *ptr;

ptr=&x;

printf("%Fp",ptr);

return 0;

}

Output: 8FD8:FFF4

Here 8FD8 is segment address and FFF4 is offset address in hexadecimal number format.

Note: %Fp is used for print offset and segment address of pointer in printf function in hexadecimal number format.

In the header file dos.h there are three macro functions to get the offset address and segment address from far pointer and vice versa.

1. FP_OFF(): To get offset address from far address.

2. FP_SEG(): To get segment address from far address.

3. MK_FP(): To make far address from segment and offset address.

Examples:

(1)What will be output of following c program?

#include <dos.h>
#include<stdio.h>

int main(){

int i=25;

int far*ptr=&i;

printf("%X %X",FP_SEG(ptr),FP_OFF(ptr));

return 0;

}

Output: Any segment and offset address in hexadecimal number format respectively.

(2)What will be output of following c program?

#include <dos.h>
#include<stdio.h>

int main(){

int i=25;

int far*ptr=&i;

unsigned int s,o;

s=FP_SEG(ptr);

o=FP_OFF(ptr);

printf("%Fp",MK_FP(s,o));

return 0;

}

Output: 8FD9:FFF4 (Assume)

Note: We cannot guess what will be offset address, segment address and far address of any far pointer .These address are decided by operating system.

Limitation of far pointer:

We cannot change or modify the segment address of given far address by applying any arithmetic operation on it. That is by using arithmetic operator we cannot jump from one segment to other segment. If you will increment the far address beyond the maximum value of its offset address instead of incrementing segment address it will repeat its offset address in cyclic order.

Example:

(q)What will be output of following c program?

#include<stdio.h>

int main(){

int i;

char far *ptr=(char *)0xB800FFFA;

for(i=0;i<=10;i++){

printf("%Fp \n",ptr);

ptr++;

}

return 0;

}

Output:

B800:FFFA

B800:FFFB

B800:FFFC

B800:FFFD

B800:FFFE

B800:FFFF

B800:0000

B800:0001

B800:0002

B800:0003

B800:0004

This property of far pointer is called cyclic nature of far pointer within same segment.

Important points about far pointer:

1. Far pointer compares both offset address and segment address with relational operators.

Examples:

(1)What will be output of following c program?

#include<stdio.h>

int main(){

int far *p=(int *)0X70230000;

int far *q=(int *)0XB0210000;

if(p==q)

printf("Both pointers are equal");

else

printf("Both pointers are not equal");

    return 0;
}

Output: Both pointers are not equal

(2)What will be output of following c program?

#include<stdio.h>

int main(){

int far *p=(int *)0X70230000;

int far *q=(int *)0XB0210000;

int near *x,near*y;

x=(int near *)p;

y=(int near *)q;

if(x==y)

printf("Both pointer are equal");

else

printf("Both pointer are not equal");

    
    return 0;
}

Output: Both pointers are equal

2. Far pointer doesn’t normalize.

What is normalization of pointer?

Generic pointer
Null pointer
Wild pointer
Dangling pointer
Near pointer
Far pointer
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Text video memory
Huge pointer
Memory model in C
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