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In computing, position-independent code (PIC) or position-independent executable (PIE) is a body of machine code that, being placed somewhere in the primary memory, executes properly regardless of its absolute address. PIC is commonly used for shared libraries, so that the same library code can be loaded in a location in each program address space where it will not overlap any other uses of memory (for example, other shared libraries). PIC was also used on older computer systems lacking an MMU,〔 〕 so that the operating system could keep applications away from each other even within the single address space of an MMU-less system. Position-independent code can be executed at any memory address without modification. This differs from relocatable code, in which a link editor or program loader modifies a program before execution so it can be run only from a particular memory location. Generating position-independent code is often the default behavior for compilers, but they may place restrictions on the use of some language features, such as disallowing use of absolute addresses (position-independent code has to use relative addressing). Instructions that refer directly to specific memory addresses sometimes execute faster, and replacing them with equivalent relative-addressing instructions may result in slightly slower execution, although modern processors make the difference practically negligible. == History == In early computers, code was position-dependent: each program was built to be loaded into, and run from, a particular address. In order to run multiple jobs using separate programs at the same time jobs had to be scheduled such that no two concurrent jobs would run programs that required the same load addresses. For example, both a payroll program and an accounts receivable program built to run at address 32K could not both be run at the same time. Sometimes multiple versions of a program were maintained, each built for a different load address. This was the situation on IBM DOS/360 (1966) except for a special class of self-relocating programs. An improvement on this situation was the ability to relocate executable programs when they were loaded into memory. Only one copy of the program was required, but once loaded the program could not be moved. IBM OS/360 programs (1966) fit this model. In segmented systems such as Multics (1964), code is inherently position-independent, since addresses in a program are relative to the current segment rather than absolute. Position-independent code was developed to eliminate these restrictions for non-segmented systems. A position-independent program could be loaded at any address in memory. The invention of dynamic address translation (the function provided by an MMU) originally reduced the need for position-independent code because every process could have its own independent address space (range of addresses). However, multiple simultaneous jobs using the same code created a waste of virtual memory. If two jobs run entirely identical programs, dynamic address translation provides a solution by allowing the system simply to map two different jobs' address 32K to the same bytes of real memory, containing the single copy of the program. Different programs may share common code. For example, the payroll program and the accounts receivable program may both contain an identical sort subroutine. A shared module (a shared library is a form of shared module) gets loaded once and mapped into the two address spaces. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Position-independent code」の詳細全文を読む スポンサード リンク
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