Position independent code

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.

In early computers such as the IBM System/360 (1965), code was position-dependent: each program was built to be loaded into, and run from, a particular address. Where a multitasking operating system allowed multiple jobs to be run using separate programs at the same time, operations 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.

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