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 21 bits[Note 1]. 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 linear representation.
On C 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.