What is the difference between far and near?

Answer:

Compilers for PC compatibles use two types of pointers.

near pointers are 16 bits long and can address a 64KB range. far pointers are 32 bits long and can address a 1MB range.

near pointers operate within a 64KB segment. There's one segment for function addresses and one segment for data.

far pointers have a 16-bit base (the segment address) and a 16-bit offset. The base is multiplied by 16, so a far pointer is effectively 20 bits long. For example, if a far pointer had a segment of 0x7000 and an offset of 0x1224, the pointer would refer to address 0x71224. A far pointer with a segment of 0x7122 and an offset of 0x0004 would refer to the same address.

Before you compile your code, you must tell the compiler which memory model to use. If you use a small- code memory model, near pointers are used by default for function addresses. That means that all the functions need to fit in one 64KB segment. With a large-code model, the default is to use far function addresses. You'll get near pointers with a small data model, and far pointers with a large data model. These are just the defaults; you can declare variables and functions as explicitly near or far.

far pointers are a little slower. Whenever one is used, the code or data segment register needs to be swapped out. far pointers also have odd semantics for arithmetic and comparison. For example, the two far pointers in the preceding example point to the same address, but they would compare as different! If your program fits in a small-data, small-code memory model, your life will be easier. If it doesn't, there's not much you can do.

If it sounds confusing, it is. There are some additional, compiler-specific wrinkles. Check your compiler manuals for details.