Loader

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In computing, a loader is the part of an operating system that is responsible for one of the essential stages in the process of starting a program, loading programs, that is, starting up programs by reading the contents of executable files (executables- files containing program text) into memory, then carrying out other required preparatory tasks, after which the program code is finally allowed to run and is started when the operating system passes control to the loaded program code.

All operating systems that support program loading have loaders, apart from systems where code executes directly from ROM or in the case of highly specialized computer systems that only have a fixed set of specialised programs.

In many operating systems the loader is permanently resident in memory, although some operating systems that support virtual memory may allow the loader to be located in a region of memory that is pageable.

In the case of operating systems that support virtual memory, the loader may not actually copy the contents of executable files into memory, but rather may simply declare to the virtual memory subsystem indicate that there is a mapping between a region of memory allocated to contain the running program's code and the contents of the associated executable file. (See memory-mapped file.) The virtual memory subsystem is then be made aware that pages with that region of memory need to be filled on demand if and when program execution actually hits those areas of unfilled memory. This may mean parts of a program's code are not actually copied into memory until they are actually used, and unused code may never be loaded into memory at all.

Responsibilities

In Unix, the loader is the handler for the system call execve().^[1] The Unix loader's tasks 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 the program entry point (_start).

Relocating loaders

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

Dynamic linking loaders are another type of loader that load and link shared libraries (like .dll files) to already loaded running programs.

See also

• Library (computing)
• Linker (computing)
• Prebinding

References

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