What is the far pointer in c?
Explanation:
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.
Size of far
pointer is 4 byte or 32 bit. Examples:
(1) What will be output
of following c program?
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?
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?
int main(){
int far *p,far *q;
printf("%d %d",sizeof(p)
,sizeof(q));
}
Output: 4 4
First 16 bit
stores: Segment number
Next 16 bit
stores: Offset address
Example:
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"
int main(){
int i=25;
int far*ptr=&i;
printf("%X %X",FP_SEG(ptr),FP_OFF(ptr));
}
Output: Any
segment and offset address in hexadecimal number format respectively.
(2)What will be output
of following c program?
#include "dos.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?
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?
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?
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 a cyclic property of data type
in c? Explain with any example.
Explanation:
#include<stdio.h>
int main(){
signed char c1=130;
signed char c2=-130;
printf("%d %d",c1,c2);
return 0;
}
Output:
-126 126 (why?)
This
situation is known as overflow of signed char.
Range
of unsigned char is -128 to 127. If we will assign a value greater than 127
then value of variable will be changed to a value if we will move clockwise
direction as shown in the figure according to number. If we will assign a
number which is less than -128 then we have to move in anti-clockwise
direction.
