Far pointer


In a segmented architecture computer, a far pointer is a pointer which includes 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.

In 16-bit x86

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. 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 to their 20-bit linear representation.
On C compilers targeting the 8086 processor family, far pointers were declared using a non-standard far qualifier. For example, 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.
Example of far pointer:

  1. include
int main