Use in Data Structures
When setting up data structures like lists, queues and trees, it is necessary to have pointers to help manage how the structure is implemented and controlled. Typical examples of pointers are start pointers, end pointers, and stack pointers. These pointers can either be absolute (the actual physical address or a virtual address in virtual memory) or relative (an offset from an absolute start address ("base") that typically uses fewer bits than a full address, but will usually require one additional arithmetic operation to resolve).
A two-byte offset, containing a 16-bit, unsigned integer, can be used to provide relative addressing for up to 64 kilobytes of a data structure. This can easily be extended to 128K, 256K or 512K if the address pointed to is forced to be on a half-word, word or double-word boundary (but, requiring an additional "shift left" bitwise operation—by 1,2 or 3 bits—in order to adjust the offset by a factor of 2,3 or 4, before its addition to the base address). Generally, though, such schemes are a lot of trouble, and for convenience to the programmer a flat address space is preferred.
A one byte offset, such as the hexadecimal ASCII value of a character (e.g. X'29') can be used to point to an alternative integer value (or index) in an array (e.g. X'01'). In this way, characters can be very efficiently translated from 'raw data' to a usable sequential index and then to an absolute address without a lookup table.
Read more about this topic: Pointer (computer Programming)
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