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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 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).
The compiler may start the compiled code at an arbitrary location, often location 1 as shown. Location 13 contains the machine code for the jump instruction to statement ST in location 5. (C) If SUBR is later linked with other code it may be stored at a location other than 1. In this example the linker places it at location 120.
1. The real mode compiler generates an instruction like this: CALL <segment>:<offset>-> 9A <offlow><offhigh><seglow><seghigh> with <seglow><seghigh> normally being defined as an address that must be fixed up at load time depending on the address where the code has been placed. […] 2. The Geos linker turns this into something else: INT 8xh ...
In most Unix-like systems, most of the machine code that makes up the dynamic linker is actually an external executable that the operating system kernel loads and executes first in a process address space newly constructed as a result of calling exec or posix_spawn functions. At link time, the path of the dynamic linker that should be used is ...
This differs from absolute code, [1] which must be loaded at a specific location to function correctly, [1] and load-time locatable (LTL) code, [1] in which a linker or program loader modifies a program before execution, so it can be run only from a particular memory location. [1]
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 ...
Dynamic loading is a mechanism by which a computer program can, at run time, load a library (or other binary) into memory, retrieve the addresses of functions and variables contained in the library, execute those functions or access those variables, and unload the library from memory.
Some formats can contain machine code for different processors, with the correct one chosen by the operating system when the program is loaded. [3] [4] The design and/or choice of an object file format is a key part of overall system design. It affects the performance of the linker and thus programmer turnaround while a program is being developed.