Chapter 3 Loaders and Linkers. — Loader Design Options. Page 2. System Programming. 2. Loaders. ▫ Linkage editor. ▫ Linking before loading. ▫ Dynamic . LINKERS ANDLOADERSAkshay KhatriCSE 6th semester Translation . Dynamic linking• Many operating system environments allow dynamic. loader is executed. □ In PC, BIOS acts as a bootstrap loader. ▫ This bootstrap loads the first program to be run by the computer — usually an operating system.
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In computing, a linker or link editor is a computer utility program that takes one or more object files generated by a compiler and combines them into a looaders executable file, library file, or another ‘object’ file. A simpler version that writes its output directly to memory is called the loaderthough loading is typically considered a separate process.
Loader (computing) – Wikipedia
Typically, an object file can contain three kinds of symbols:. For most compilers, each object file is the result of compiling one input source code file. When a program comprises multiple object files, the linker combines these files into a unified executable sofware, resolving the symbols as it goes along.
Linkers can take objects from a collection called a library or runtime library. Most linkers do not include the whole library in the output; they include only the files that are referenced by other object files or libraries.
Library linking may thus be an iterative process, with some modules included requiring additional modules to be linked, and so on. Libraries exist for diverse purposes, and one or more system libraries are usually linked in by default.
Linker (computing) – Wikipedia
The linker also takes care of arranging the objects in a program’s address space. This may involve relocating code that assumes a specific base address into another base. Since a compiler seldom knows where an object will reside, it often assumes a fixed base location for example, zero. Relocating machine code may involve re-targeting of absolute jumps, loads and stores.
The executable output by the linker may need another relocation pass when it is finally loaded into memory just before execution. This pass is usually omitted on hardware offering virtual memory: This pass may also be omitted if the executable is a position independent executable. Many operating system environments allow dynamic linking, deferring the resolution of some undefined symbols until a aoftware is run.
That means that the executable code still contains undefined symbols, plus softwarr list of objects or libraries that will provide definitions for these. Static linking is the result of the linker copying all library routines used in the program into the executable image. This may require more disk space and memory than dynamic linking, but is more portable, since it does not require the presence of the library on the system where it runs.
Static linking also prevents ” DLL Hell “, since each program includes exactly the versions of library routines that it requires, with no conflict with other programs. A program using just a few routines from a library does not require the entire library to be installed.
As the compiler has no information on the layout of objects in the final output, it cannot take advantage of shorter or more efficient instructions that place a requirement on the address of another object. For example, a jump instruction can reference an absolute address or an offset from the current location, and the offset could be expressed with different lengths depending on the distance to the target. By generating the most conservative instruction usually the largest relative or absolute variant, depending on platform and adding relaxation hintsit is possible to substitute shorter or more efficient instructions during the final link.
This step can be performed only after all input objects have been read and assigned temporary addresses; the linker relaxation pass subsequently reassigns addresses, which may in turn allow more relaxations to occur. In general, the substituted sequences are shorter, which allows this process to always converge on the best solution given a fixed order of objects; if this is not the case, relaxations can conflict, and the linker needs to weigh the advantages of either option.
While instruction relaxation typically occurs at link-time, inner-module relaxation can already take place as part of the optimising process at compile-time. In some cases, relaxation can also occur at load-time as part of the relocation process or combined with dynamic dead-code elimination techniques. One advantage of this is that it allows a program to be maintained without having to keep all of the intermediate object files, or without having to re-compile program sections that haven’t changed.
It also permits program updates to be distributed in the form of small files originally card deckscontaining only the object module to be replaced. In such systems, object code is in the form and format of byte punched-card images, so that updates can be introduced into a system using that medium.
The term “linkage editor” should not be construed as implying that the program operates in a user-interactive mode like a text editor. It is intended for batch-mode execution, with the editing commands being supplied by the user in sequentially organized files, such as punched cardsDASDor magnetic tapeand tapes were often used during the initial installation of the OS. From Wikipedia, the free encyclopedia.
David William Barron, Assemblers and Loaders. Hanson, A Machine Independent Linker. Software-Practice and Experience 12, 4 April JonesAssembly Language as Object Code. Application binary interface Un. Binary code compatibility Foreign function interface Language binding Linker dynamic Loader Year problem.
Executable and object file formats. Unix command-line interface programs and shell builtins.
Look up linker in Wiktionary, the free dictionary.