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In
computer systems A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These program ...
a loader is the part of an
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also i ...
that is responsible for loading programs and
libraries A library is a collection of Document, materials, books or media that are accessible for use and not just for display purposes. A library provides physical (hard copies) or electronic media, digital access (soft copies) materials, and may be a ...
. 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 memory-mapping the contents of the
executable file In computing, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, causes a computer "to perform indicated tasks according to encoded instructions", as opposed to a data fil ...
containing the program instructions 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 highly specialized computer systems that only have a fixed set of specialized programs.
Embedded system An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is ''embedded'' ...
s typically do not have loaders, and instead, the code executes directly from ROM or similar. In order to load the operating system itself, as part of booting, a specialized
boot loader A bootloader, also spelled as boot loader or called boot manager and bootstrap loader, is a computer program that is responsible for booting a computer. When a computer is turned off, its softwareincluding operating systems, application code, a ...
is used. In many operating systems, the loader resides permanently in memory, though some operating systems that support
virtual memory In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very ...
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 A memory-mapped file is a segment of virtual memory that 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 b ...
.) 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
Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, an ...
, the loader is the handler for the
system call In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, acc ...
execve(). The Unix loader's tasks include: # validation (permissions, memory requirements etc.); # memory-mapping the executable object from the disk into main memory; # copying the
command-line argument A command-line interpreter or command-line processor uses a command-line interface (CLI) to receive commands from a user in the form of lines of text. This provides a means of setting parameters for the environment, invoking executables and pro ...
s into virtual memory; # initializing registers (e.g., the stack pointer); # jumping to the program entry point (_start). In Microsoft Windows 7 and above, the loader is the LdrInitializeThunk function contained in ntdll.dll, that does the following: # initialisation of structures in the DLL itself (i.e.
critical section In concurrent programming, concurrent accesses to shared resources can lead to unexpected or erroneous behavior, so parts of the program where the shared resource is accessed need to be protected in ways that avoid the concurrent access. One way to ...
s, module lists); # validation of executable to load; # creation of a heap (via the function RtlCreateHeap); # allocation of environment variable block and PATH block; # addition of executable and NTDLL to the module list (a
doubly-linked list In computer science, a doubly linked list is a linked data structure that consists of a set of sequentially linked records called nodes. Each node contains three fields: two link fields (references to the previous and to the next node in the s ...
); # loading of KERNEL32.DLL to obtain several important functions, for instance BaseThreadInitThunk; # loading of executable's imports (i.e.
dynamic-link libraries Dynamic-link library (DLL) is Microsoft's implementation of the shared library concept in the Microsoft Windows and OS/2 operating systems. These libraries usually have the file extension DLL, OCX (for libraries containing ActiveX controls), o ...
) recursively (check the imports' imports, their imports and so on); # in debug mode, raising of system breakpoint; # initialisation of DLLs; # garbage collection; # calling NtContinue on the context parameter given to the loader function (i.e. jumping to RtlUserThreadStart, that will start the executable)


Relocating loaders

Some operating systems need
relocating loader Relocation is the process of assigning load addresses for position-dependent code and data of a program and adjusting the code and data to reflect the assigned addresses. Prior to the advent of multiprocess systems, and still in many embedded ...
s, which adjust addresses (pointers) in the executable to compensate for variations in the address at which loading starts. The operating systems that need relocating loaders are those in which a program is not always loaded into the same location in the address space and in which pointers are absolute addresses rather than offsets from the program's
base address In computing, a base address is an address serving as a reference point ("base") for other addresses. Related addresses can be accessed using an ''addressing scheme''. Under the ''relative addressing'' scheme, to obtain an absolute address, the ...
. Some well-known examples are IBM's
OS/360 OS/360, officially known as IBM System/360 Operating System, is a discontinued batch processing operating system developed by IBM for their then-new System/360 mainframe computer, announced in 1964; it was influenced by the earlier IBSYS/IBJOB ...
for their System/360 mainframes, and its descendants, including
z/OS z/OS is a 64-bit operating system for IBM z/Architecture mainframes, introduced by IBM in October 2000. It derives from and is the successor to OS/390, which in turn was preceded by a string of MVS versions.Starting with the earliest: * ...
for the
z/Architecture z/Architecture, initially and briefly called ESA Modal Extensions (ESAME), is IBM's 64-bit complex instruction set computer (CISC) instruction set architecture, implemented by its mainframe computers. IBM introduced its first z/Architecture ...
mainframes.


OS/360 & Derivatives

In OS/360 and descendant systems, the (privileged) operating system facility is called IEWFETCH, and is an internal component of the OS Supervisor, whereas the (non-privileged) LOADER application can perform many of the same functions, plus those of the Linkage Editor, and is entirely external to the OS Supervisor (although it certainly uses many Supervisor services). IEWFETCH utilizes highly specialized channel programs, and it is theoretically possible to load and to relocate an entire executable within one revolution of the DASD media (about 16.6 ms maximum, 8.3 ms average, on "legacy" 3,600 rpm drives). For load modules which exceed a track in size, it is also possible to load and to relocate the entire module without losing a revolution of the media. IEWFETCH also incorporates facilities for so-called overlay structures, and which facilitates running potentially very large executables in a minimum memory model (as small as 44 KB on some versions of the OS, but 88 KB and 128 KB are more common). The OS's
nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom * Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...
(the always resident portion of the Supervisor) itself is formatted in a way that is compatible with a stripped-down version of IEWFETCH. Unlike normal executables, the OS's nucleus is "scatter loaded": parts of the nucleus are loaded into different portions of memory; in particular, certain system tables are required to reside below the initial 64 KB, while other tables and code may reside elsewhere. The system's
Linkage Editor In computing, a linker or link editor is a computer system program that takes one or more object files (generated by a compiler or an assembler) and combines them into a single executable file, library file, or another "object" file. A simple ...
application is named IEWL. IEWL's main function is to associate load modules (executable programs) and object modules (the output from, say, assemblers and compilers), including "automatic calls" to libraries (high-level language "built-in functions"), into a format which may be most efficiently loaded by IEWFETCH. There are a large number of editing options, but for a conventional application only a few of these are commonly employed. The load module format includes an initial "text record", followed immediately by the "relocation and/or control record" for that text record, followed by more instances of text record and relocation and/or control record pairs, until the end of the module. The text records are usually very large; the relocation and/or control records are small as IEWFETCH's three relocation and/or control record buffers are fixed at 260 bytes (smaller relocation and/or control records are certainly possible, but 260 bytes is the maximum possible, and IEWL ensures that this limitation is complied with, by inserting additional relocation records, as required, before the next text record, if necessary; in this special case, the sequence of records may be: ..., text record, relocation record, ..., control record, text record, ...). A special byte within the relocation and/or control record buffer is used as a "disabled bit spin" communication area, and is initialized to a unique value. The Read CCW for that relocation and/or control record has the Program Controlled Interrupt bit set. The processor is thereby notified when that CCW has been accessed by the channel via a special
IOS iOS (formerly iPhone OS) is a mobile operating system created and developed by Apple Inc. exclusively for its hardware. It is the operating system that powers many of the company's mobile devices, including the iPhone; the term also include ...
exit. At this point the processor enters the "disabled bit spin" loop (sometimes called "the shortest loop in the world"). Once that byte changes from its initialized value, the CPU exits the bit spin, and relocation occurs, during the "gap" within the media between the relocation and/or control record and the next text record. If relocation is finished before the next record, the NOP CCW following the Read will be changed to a TIC, and loading and relocating will proceed using the next buffer; if not, then the channel will stop at the NOP CCW, until it is restarted by IEWFETCH via another special IOS exit. The three buffers are in a continuous circular queue, each pointing to its next, and the last pointing to the first, and three buffers are constantly reused as loading and relocating proceeds. IEWFETCH can, thereby, load and relocate a load module of any practical size, and in the minimum possible time.


Dynamic linkers

Dynamic linking loaders are another type of loader that load and link
shared libraries In computer science, a library is a collection of non-volatile resources used by computer programs, often for software development. These may include configuration data, documentation, help data, message templates, pre-written code and su ...
(like .so files, .dll files or .dylib files) to already loaded running programs.


See also

*
Compile and go system In computer programming, a compile and go system, compile, load, and go system, assemble and go system, or load and go system is a programming language processor in which the compilation, assembly, or link steps are not separated from program ex ...
*
DLL hell In computing, DLL Hell is a term for the complications that arise when one works with dynamic-link libraries (DLLs) used with Microsoft Windows operating systems, particularly legacy 16-bit editions, which all run in a single memory space. DLL Hel ...
* Direct binding * Dynamic binding (computing) *
Dynamic dead code elimination In compiler theory, dead-code elimination (also known as DCE, dead-code removal, dead-code stripping, or dead-code strip) is a compiler optimization to remove code which does not affect the program results. Removing such code has several benefits: ...
*
Dynamic dispatch In computer science, dynamic dispatch is the process of selecting which implementation of a polymorphic operation (method or function) to call at run time. It is commonly employed in, and considered a prime characteristic of, object-oriented ...
* Dynamic library *
Dynamic linker In computing, a dynamic linker is the part of an operating system that loads and links the shared libraries needed by an executable when it is executed (at " run time"), by copying the content of libraries from persistent storage to RAM, filling ...
* Dynamic loading * Dynamic-link library *
GNU linker In computing, a linker or link editor is a computer system program that takes one or more object files (generated by a compiler or an assembler) and combines them into a single executable file, library file, or another "object" file. A simpler ...
*
Library (computing) In computer science, a library is a collection of non-volatile resources used by computer programs, often for software development. These may include configuration data, documentation, help data, message templates, pre-written code and su ...
* Linker (computing) *
Name decoration In compiler construction, name mangling (also called name decoration) is a technique used to solve various problems caused by the need to resolve unique names for programming entities in many modern programming languages. It provides a way of ...
* Prebinding * Prelinking *
Relocation (computer science) Relocation is the process of assigning load addresses for position-dependent code and data of a program and adjusting the code and data to reflect the assigned addresses. Prior to the advent of multiprocess systems, and still in many embedded ...
* Relocation table *
Shebang (Unix) In computing, a shebang is the character sequence consisting of the characters number sign and exclamation mark () at the beginning of a script. It is also called sharp-exclamation, sha-bang, hashbang, pound-bang, or hash-pling. When a text fil ...
*
Static library In computer science, a static library or statically-linked library is a set of routines, external functions and variables which are resolved in a caller at compile-time and copied into a target application by a compiler, linker, or binder, produ ...
* gold (linker) *
prelink In computing, prebinding, also called prelinking, is a method for optimizing application load times by resolving library symbols prior to launch. Background Most computer programs consist of code that requires external shared libraries to exec ...
*
Bug compatibility Computer hardware or software is said to be bug compatible if it exactly replicates even an undesirable feature of a previous version. The phrase is found in the Jargon File. An aspect of maintaining backward compatibility with an older system i ...


References

{{DEFAULTSORT:Loader (Computing) Operating system kernels Computer libraries