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In a segmented architecture computer, a far pointer is a pointer to memory in a specific context,[1] such as a segment selector making it possible to point to addresses outside of the default segment.
Comparison and arithmetic on far pointers is problematic: there can be several different segment-offset address pairs pointing to one physical address.
For example, in an Intel 8086, as well as in later processors running 16-bit code, a far pointer has two parts: a 16-bit segment value, and a 16-bit offset value. A linear address is obtained by shifting the binary segment value four times to the left, and then adding the offset value. Hence the effective address is 20 bits (actually 21-bit,[which?] which led to the address wraparound and the Gate A20).[clarification needed] There can be up to 4096 different segment-offset address pairs pointing to one physical address. To compare two far pointers, they must first be converted (normalized) to their 20-bit linear representation.
OnC compilers targeting the 8086 processor family, far pointers were declared using a non-standard far qualifier; e.g., char far *p; defined a far pointer to a char. The difficulty of normalizing far pointers could be avoided with the non-standard huge qualifier. On other compilers it was done using an equally non-standard __far qualifier.[2]
Example of far pointer:
#include <stdio.h>
int main() {
char far *p =(char far *)0x55550005;
char far *q =(char far *)0x53332225;
*p = 80;
(*p)++;
printf("%d",*q);
return 0;
}
p is : 0x5555 * 0x10 + 0x0005 = 0x55555q is : 0x5333 * 0x10 + 0x2225 = 0x55555p and q both point to the same location 0x55555.
