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What is use of huge, near and far pointer in c language?

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What is use of huge, near and far pointer in c language?
posted Dec 6, 2015 by anonymous

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2 Answers

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In TURBO C there are three types of pointers. TURBO C works under DOS operating system which is based on 8085 microprocessor.

   1. Near pointer

    2. Far pointer

   3. Huge pointer

Near pointer:

The pointer which can points only 64KB data segment or segment number 8 is known as near pointer.

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

That is near pointer cannot access beyond the data segment like graphics video memory, text video memory etc. Size of near pointer is two byte. With help keyword near, we can make any pointer as near pointer.

Examples:

(1)

           #include<stdio.h>

          int main(){

         int x=25;

         int near* ptr;

          ptr=&x;

         printf(“%d”,sizeof ptr);

         return 0;

      }

Output: 2

(2)

#include<stdio.h>

int main(){

int near* near * ptr;

printf(“%d”,sizeof(ptr),sizeof(*ptr));

return 0;

}

Output: 2 2

Explanation: Size of any type of near pointer is two byte.

Near pointer only hold 16 bit offset address. Offset address varies from 0000 to FFFF (in hexadecimal).

Note: In printf statement to print the offset address in hexadecimal, %p is used.

Example:

#include<stdio.h>

int main(){

int i=10;

int *ptr=&i;

printf("%p",ptr);

return 0;

}

Output: Offset address in hexadecimal number format.

%p is also used to print any number in hexadecimal number format.

Example:

#include<stdio.h>

int main(){

int a=12;

printf("%p",a);

return 0;

}

Output: 000C

Explanation: Hexadecimal value of 12 is C.

Consider the following two c program and analyze its output:

(1)

#include<stdio.h>

int main(){

int near * ptr=( int *)0XFFFF;

ptr++;

ptr++;

printf(“%p”,ptr);

return 0;

}

Output: 0003

(2)

#include<stdio.h>

int main(){

int i;

char near *ptr=(char *)0xFFFA;

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

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

ptr++;

}

return 0;

}

Output:

FFFA

FFFB
FFFC
FFFD
FFFE
FFFF
0000

0001

0002

0003

0004

Explanation: When we increment or decrement the offset address from maximum and minimum value respectively then it repeats the same value in cyclic order. This property is known as cyclic nature of offset address.

Cyclic property of offset address.

If you increment the near pointer variable then move clockwise direction. If you decrement the near pointer then move anti clockwise direction.

What is default type of pointer in C?

Answer: It depends upon memory model.

What is memory model in C?

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
  1. Far pointer doesn’t normalize.

What is normalization of pointer?

Huge pointer in c programming

Huge pointer:

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

Size of huge pointer is 4 byte or 32 bit.

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

#include<stdio.h>

int main(){

char huge * far *p;

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

return 0;

}

Output: 4 4 1

Explanation: p is huge pointer, *p is far pointer and **p is char type data variable.

Normalization of huge pointer:

Turbo C compiler is based on 8085 microprocessor in which physical address of memory is represented in 20 bit. Conversion of 4 byte or 32 bit huge address into 20 bit actual physical address is known as normalization.

Formula to calculate physical address:

Example:

(q) What will be physical address of huge address 0X59994444?

Answer:

Huge address: 0X59994444

Offset address: 0x4444

Segment address: 0x5999

Physical address= Segment address * 0X10 + Offset address

=0X5999 * 0X10 +0X4444

=0X59990 + 0X4444

=0X5DDD4

In binary: 0101 1101 1101 1101 0100

Note: Each hexadecimal digit is represented in 4 bit binary number.

When any relation operation is performed between two huge pointers first it normalizes in actual physical address.

Example:

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

#include<stdio.h>

int main(){

   int huge*p=(int huge*)0XC0563331;

   int huge*q=(int huge*)0xC2551341;

   if(p==q)

     printf("Equql");

   else

     printf("Not equal");

   return 0;

}

Output: Equal

Explanation:

As we know huge pointers compare its physical address.

Physical address of huge pointer p

Huge address: 0XC0563331

Offset address: 0x3331

Segment address: 0XC056

Physical address= Segment address * 0X10 + Offset address

=0XC056 * 0X10 +0X3331

=0XC0560 + 0X3331

=0XC3891

Physical address of huge pointer q

Huge address: 0XC2551341

Offset address: 0x1341

Segment address: 0XC255

Physical address= Segment address * 0X10 + Offset address

=0XC255 * 0X10 +0X1341

=0XC2550 + 0X1341

=0XC3891

Since both huge pointers p and q are pointing same physical address so if condition will true.

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

#include<stdio.h>

int main(){

double near *p,far *q;

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

return 0;

}

Output: 4 2 8

Explanation: q is far pointer, p is near pointer, *p is double data type constant.

Don’t use huge pointer:

If you will increment huge pointer it will increment both offset and segment address unlike to far pointer which only increments offset address. So if you have little knowledge about huge pointer and you are using huge pointer then you can easily access and modify the IVT, device driver memory, video memory etc. This might be dangerous for your computer.

(q)Why there are three types of pointer in Turbo c compiler?

Answer:

Turbo c compiler is based on Dos operating system which is based on 8085 microprocessors. In 8085 microprocessor actual physical address is represented in 20 bit. But there are not any pointers which can point 20 bit address. Considering simplicity of calculations, access to actual physical address, security etc. c has introduced three type of pointer i.e. near, far and huge pointer.

answer Dec 8, 2015 by Shivaranjini
0 votes

The concept of near, far, and huge is very old when you had CPUs with smaller address spaces than the amount of RAM on the device, such as a 16 bit CPU (with 65536 possible memory locations) and 1MB of RAM, so you had to use various tricks to do your own "memory paging" so the pointers would work.

Now coming to your question -
A near pointer is a 16 bit pointer to an object contained in the current segment, be it code segment, data segment, stack segment, or extra segment. The compiler can generate code with a near pointer and does not have to concern itself with segment addressing, so using near pointers is fastest, and generates smallest code. The limitation is that you can only access 64kb of data at a time, because that is the size of a segment - 64kb.

A far pointer is a 32 bit pointer to an object anywhere in memory. In order to use it, the compiler must allocate a segment register, load it with the segment portion of the pointer, and then reference memory using the offset portion of the pointer relative to the newly loaded segment register. This takes extra instructions and extra time, so it is the slowest and largest method of accessing memory.

Now the huge pointer. The far pointer suffers because you can not just add one to it and have it point the the next item in memory - you have to consider segment:offset rules, because of the 16 byte offset issue. The huge pointer is a monolithic pointer to some item with a large chunk of memory, and there are no segment:offset boundaries. Or in short a huge pointer is the actual memory address, without reference to any segment register. See the following example to see the difference between Huge and Far ...

Far Pointer
int main()
{
    char far* f=(char far*)0x0000ffff;
    printf("%Fp",f+0x1);
    return 0;
}
Output 
0x00000000

Huge Pointer
int main()
{
    char huge* f=(char huge*)0x0000ffff;
    printf("%Fp",f+0x1);
    return 0;
}
Output 
0x00010000
answer Dec 6, 2015 by Salil Agrawal
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