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.
FP_OFF(): To get offset address from far address.
FP_SEG(): To get segment address from far address.
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:
- 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
- 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.
