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

segment selector x86 memory segmentation refers to the implementation of memory segmentation in the Intel x86 computer instruction set architecture. Segmentation was introduced on the Intel 8086 in 1978 as a way to allow programs to address more than 64 K ...

, 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 computing, a physical address (also real address, or binary address), is a memory address that is represented in the form of a binary number on the address bus circuitry in order to enable the data bus to access a ''particular'' storage cell o ...

In 16-bit x86

For example, in an

Intel 8086 The 8086 (also called iAPX 86) is a 16-bit microprocessor chip designed by Intel between early 1976 and June 8, 1978, when it was released. The Intel 8088, released July 1, 1979, is a slightly modified chip with an external 8-bit data bus (allowi ...

, as well as in later processors running

16-bit 16-bit microcomputers are microcomputers that use 16-bit microprocessors. A 16-bit register can store 216 different values. The range of integer values that can be stored in 16 bits depends on the integer representation used. With the two mos ...

code, a far pointer has two parts: a 16-bit segment value, and a 16-bit

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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 led to the address wraparound and the

Gate A20 The A20, or address line 20, is one of the electrical lines that make up the system bus of an x86-based computer system. The A20 line in particular is used to transmit the 21st bit on the address bus. A microprocessor typically has a number of ...

). 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. On C

compiler In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs that ...

s 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

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. The difficulty of normalizing far pointers could be avoided with the non-standard huge qualifier. Example of far pointer: #include int main() :Output of the following program: 81; Because both addresses point to same location. :Physical Address = (value of segment register) * 0x10 + (value of offset). :Location pointed to by pointer 'p' is : 0x5555 * 0x10 + 0x0005 = 0x55555 :Location pointed to by pointer 'q' is : 0x5333 * 0x10 + 0x2225 = 0x55555 :So, p and q both point to the same location 0x55555.

References

{{DEFAULTSORT:Far Pointer Computer memory