Loader (computing)
Encyclopedia
In computing
, a loader is the part of an operating system
that is responsible for loading programs. It is one of the essential stages in the process of starting a program, as it places programs into memory and prepares them for execution. Loading a program involves reading the contents of executable file
, the file containing the program text, into memory, and then carrying out other required preparatory tasks to prepare the executable for running. Once loading is complete, the operating system starts the program by passing 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 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 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.
, the loader is the handler
for the system call
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.
are another type of loader that load and link shared libraries (like .dll files) to already loaded running programs.
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
, a loader is the part of an operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
that is responsible for loading programs. It is one of the essential stages in the process of starting a program, as it places programs into memory and prepares them for execution. Loading a program involves reading the contents of executable file
Executable
In computing, an executable file causes a computer "to perform indicated tasks according to encoded instructions," as opposed to a data file that must be parsed by a program to be meaningful. These instructions are traditionally machine code instructions for a physical CPU...
, the file containing the program text, into memory, and then carrying out other required preparatory tasks to prepare the executable for running. Once loading is complete, the operating system starts the program by passing 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
Virtual memory
In computing, virtual memory is a memory management technique developed for multitasking kernels. This technique virtualizes a computer architecture's various forms of computer data storage , allowing a program to be designed as though there is only one kind of memory, "virtual" memory, which...
may allow the loader to be located in a region of memory that is pageable
Paging
In computer operating systems, paging is one of the memory-management schemes by which a computer can store and retrieve data from secondary storage for use in main memory. In the paging memory-management scheme, the operating system retrieves data from secondary storage in same-size blocks called...
.
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 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
Memory-mapped file
A memory-mapped file is a segment of virtual memory which has been assigned a direct byte-for-byte correlation with some portion of a file or file-like resource. This resource is typically a file that is physically present on-disk, but can also be a device, shared memory object, or other resource...
.) The virtual memory subsystem is then 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 UnixUnix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
, the loader is the handler
Callback (computer science)
In computer programming, a callback is a reference to executable code, or a piece of executable code, that is passed as an argument to other code. This allows a lower-level software layer to call a subroutine defined in a higher-level layer....
for the system call
System call
In computing, a system call is how a program requests a service from an operating system's kernel. This may include hardware related services , creating and executing new processes, and communicating with integral kernel services...
execve. The Unix loader's tasks include:- validation (permissions, memory requirements etc.);
- copying the program image from the disk into main memory;
- copying the command-line arguments on the stackCall stackIn computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, control stack, run-time stack, or machine stack, and is often shortened to just "the stack"...
; - initializing registersProcessor registerIn computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...
(e.g., the stack pointer); - jumping to the program entry point (
_start).
Relocating loaders
Some computers need relocatingRelocation (computer science)
"Relocation is the process of assigning load addresses to various parts of [a] program and adjusting the code and data in the program to reflect the assigned addresses."...
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
Base address
In computing, a base address is an address serving as a reference point for other addresses.In computers using relative addressing scheme, to obtain an absolute address, the relevant base address is taken and offset is added to it....
. One well-known example is IBM's System/360
System/360
The IBM System/360 was a mainframe computer system family first announced by IBM on April 7, 1964, and sold between 1964 and 1978. It was the first family of computers designed to cover the complete range of applications, from small to large, both commercial and scientific...
mainframes and their descendants, including the System z9
System z9
IBM System z9 is a line of IBM mainframe. It was announced on July 25, 2005 and the first models were available on September 16, 2005. The System z9 also marks the end of the previously used eServer zSeries naming convention, and it is the last z/Architecture 1 machine.- Background :System z9 is a...
series.
Dynamic linkers
Dynamic linking loadersDynamic linker
In computing, a dynamic linker is the part of an operating system that loads and links the shared libraries for an executable when it is executed. The specific operating system and executable format determine how the dynamic linker functions and how it is implemented...
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)
- Dynamic linkerDynamic linkerIn computing, a dynamic linker is the part of an operating system that loads and links the shared libraries for an executable when it is executed. The specific operating system and executable format determine how the dynamic linker functions and how it is implemented...
- PrebindingPrebindingPrebinding is a method for reducing the time it takes to launch executables in the Mach-O file format. For example, this is what Mac OS X is doing when in the "Optimizing" stage of installing system software or certain applications....