Loader (computing)

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In computing, a loader is the part of an operating system that is responsible for loading programs from executables (i.e., executable files) into memory, preparing them for execution and then executing them. The loader is usually a part of the operating system's kernel and usually is loaded at system boot time and stays in memory until the system is rebooted, shut down, or powered off. Some operating systems that have a pageable kernel may have the loader in the pageable part of memory and thus the loader sometimes may be swapped out of memory. All operating systems that support program loading have loaders. Some embedded operating systems in highly specialized computers run only one program and have no program loading capabilities and thus no loaders, for example embedded systems in cars or stereo equipment.

In Unix, the loader is the handler for the system call execve(). The loader's tasks under Unix include: (1) validation (permissions, memory requirements etc.); (2) copying the program image from the disk into main memory; (3) copying the command-line arguments on the stack; (4) initializing registers (e.g., the stack pointer); (5) jumping to main().

Some computers need relocating loaders, which adjust addresses (pointers) in the executable to compensate for variations in the address at which loading starts. The computers which need relocating loaders are those in which pointers are absolute addresses rather than offsets from the program's base address. One well-known example is IBM's System/360 mainframes and their descendants, including the System z9 series.

Dynamic linkers are another type of loader that load and link shared libraries (like dlls) to already loaded running programs.

Loader programs are useful for prototyping, testing, and one-off applications. One such program was an integral part of Gene Amdahl's original OS/360 operating system, and this loader facility was continued through OS/360's descendants including MVT, MVS and z/OS.