HOME

TheInfoList



OR:

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 systems, the addresses for objects were
absolute Absolute may refer to: Companies * Absolute Entertainment, a video game publisher * Absolute Radio, (formerly Virgin Radio), independent national radio station in the UK * Absolute Software Corporation, specializes in security and data risk manag ...
starting at a known location, often zero. Since multiprocessing systems dynamically link and switch between programs it became necessary to be able to relocate objects using position-independent code. A linker usually performs relocation in conjunction with symbol resolution, the process of searching files and libraries to replace symbolic references or names of
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 ...
with actual usable addresses in
memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remember ...
before running a program. Relocation is typically done by the linker at
link time In computer science, link time refers to the period of time, during the creation of a computer program, in which a linker is being applied to that program. Link time occurs after compile time and before runtime (when a program is executed). I ...
, but it can also be done at load time by a relocating
loader Loader can refer to: * Loader (equipment) * Loader (computing) ** LOADER.EXE, an auto-start program loader optionally used in the startup process of Microsoft Windows ME * Loader (surname) * Fast loader * Speedloader * Boot loader ** LOADER.COM ...
, or at run time by the running program itself. Some architectures avoid relocation entirely by deferring address assignment to run time; as, for example, in stack machines with
zero address arithmetic In computer science, computer engineering and programming language implementations, a stack machine is a computer processor or a virtual machine in which the primary interaction is moving short-lived temporary values to and from a push down st ...
or in some segmented architectures where every compilation unit is loaded into a separate segment.


Segmentation

Object files are segmented into various memory segment types. Example segments include code segment (.text), initialized data segment (.data), uninitialized data segment (.bss), or others.


Relocation table

The relocation table is a list of pointers created by the translator (a
compiler In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs tha ...
or assembler) and stored in the object or executable file. Each entry in the table, or "fixup", is a pointer to an absolute address in the object code that must be changed when the loader relocates the program so that it will refer to the correct location. Fixups are designed to support relocation of the program as a complete unit. In some cases, each fixup in the table is itself relative to a base address of zero, so the fixups themselves must be changed as the loader moves through the table. In some architectures a fixup that crosses certain boundaries (such as a segment boundary) or that is not aligned on a word boundary is illegal and flagged as an error by the linker.


DOS and 16-bit Windows

Far pointers (
32-bit In computer architecture, 32-bit computing refers to computer systems with a processor, memory, and other major system components that operate on data in 32- bit units. Compared to smaller bit widths, 32-bit computers can perform large calculati ...
pointers with segment:offset, used to address 20-bit 640 KB
memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remember ...
space available to
DOS DOS is shorthand for the MS-DOS and IBM PC DOS family of operating systems. DOS may also refer to: Computing * Data over signalling (DoS), multiplexing data onto a signalling channel * Denial-of-service attack (DoS), an attack on a communicat ...
program Program, programme, programmer, or programming may refer to: Business and management * Program management, the process of managing several related projects * Time management * Program, a part of planning Arts and entertainment Audio * Programm ...
s), which point to code or data within a DOS executable (
EXE Exe or EXE may refer to: * .exe, a file extension * exe., abbreviation for executive Places * River Exe, in England * Exe Estuary, in England * Exe Island, in Exeter, England Transportation and vehicles * Exe (locomotive), a British locomotiv ...
), do not have absolute segments, because the actual address of code/data depends on where the program is loaded in memory and this is not known until the program is loaded. Instead, segments are relative values in the DOS EXE file. These segments need to be corrected, when the executable has been loaded into memory. The EXE
loader Loader can refer to: * Loader (equipment) * Loader (computing) ** LOADER.EXE, an auto-start program loader optionally used in the startup process of Microsoft Windows ME * Loader (surname) * Fast loader * Speedloader * Boot loader ** LOADER.COM ...
uses a relocation table to find the segments which need to be adjusted.


32-bit Windows

With 32-bit Windows operating systems, it is not mandatory to provide relocation tables for EXE files, since they are the first image loaded into the virtual address space and thus will be loaded at their preferred base address. For both DLLs and for EXEs which opt into address space layout randomization (ASLR), an exploit mitigation technique introduced with
Windows Vista Windows Vista is a major release of the Windows NT operating system developed by Microsoft. It was the direct successor to Windows XP, which was released five years before, at the time being the longest time span between successive releases of ...
, relocation tables once again become mandatory because of the possibility that the binary may be dynamically moved before being executed, even though they are still the first thing loaded in the virtual address space.


64-bit Windows

When running native 64-bit binaries on Windows Vista and above, ASLR is mandatory, and thus relocation sections cannot be omitted by the compiler.


Unix-like systems

The
Executable and Linkable Format In computing, the Executable and Linkable FormatTool Interface Standard (TIS) Portable Formats SpecificationVersion 1.1'' (October 1993) (ELF, formerly named Extensible Linking Format), is a common standard file format for executable files, ...
(ELF) executable format and shared library format used by most Unix-like systems allows several types of relocation to be defined.


Relocation procedure

The linker reads segment information and relocation tables in the object files and performs relocation by: * merging all segments of common type into a single segment of that type * assigning unique run time addresses to each section and each symbol, giving all code (functions) and data (global variables) unique run time addresses * referring to the relocation table to modify symbols so that they point to the correct run time addresses.


Example

The following example uses
Donald Knuth Donald Ervin Knuth ( ; born January 10, 1938) is an American computer scientist, mathematician, and professor emeritus at Stanford University. He is the 1974 recipient of the ACM Turing Award, informally considered the Nobel Prize of computer sc ...
's MIX architecture and MIXAL assembly language. The principles are the same for any architecture, though the details will change. * (A) Program ''SUBR'' is compiled to produce object file (B), shown as both machine code and assembler. 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. The address in the jump instruction, which is now at location 133, must be relocated to point to the new location of the code for statement ''ST'', now 125. 61 shown in the instruction is the MIX machine code representation of 125 * (D) When the program is loaded into memory to run it may be loaded at some location other than the one assigned by the linker. This example shows ''SUBR'' now at location 300. The address in the jump instruction, now at 313, needs to be relocated again so that it points to the updated location of ''ST'', 305. 49 is the MIX machine representation of 305


See also

*
Linker (computing) 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 simp ...
*
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 sub ...
* Object file * Prebinding * Static library * Self-relocation * Position-independent code (PIC) * Rebasing *
Garbage collection Waste collection is a part of the process of waste management. It is the transfer of solid waste from the point of use and disposal to the point of treatment or landfill. Waste collection also includes the curbside collection of recyclabl ...
* Pointer swizzling, a lazy form of pointer modification * Relocatable Object Module Format


References


Further reading

* * (3 pages) (NB. Describes a relocatable hex format by Mostek.) * (8 pages) (NB. Describes a relocatable hex format by TDL.) *

https://web.archive.org/web/20170819173516/http://archive.computerhistory.org/resources/access/text/2016/12/102762506-05-01-acc.pd

Originally presented at: (609 pages). (This "resize" method, named ''page boundary relocation'', could be applied statically to a CP/M-80 disk image using in order to maximize the TPA for programs to run. It was also utilized dynamically by the CP/M debugger Dynamic Debugging Tool (DDT) to relocate itself into higher memory. The same approach was independently developed by Bruce H. Van Natta of
IMS Associates IMS Associates, Inc., or IMSAI, was a microcomputer company, responsible for one of the earliest successes in personal computing, the IMSAI 8080. The company was founded in 1973 by William Millard and was based in San Leandro, California. The ...
to produce relocatable PL/M code. As ''paragraph boundary relocation'', another variant of this method was later utilized by dynamically HMA self-relocating TSRs like
KEYB KEYB (107.9 FM, "108 Key FM") is a radio station licensed to Altus, Oklahoma Altus () is a city in and the county seat of Jackson County, Oklahoma, United States. The population was 19,813 at the 2010 census, a loss of 7.7 percent compared ...
, SHARE, and NLSFUNC under DR DOS 6.0 and higher. A much more sophisticated and byte-level granular method based on a somewhat similar approach was independently conceived and implemented by Matthias R. Paul and Axel C. Frinke for their dynamic dead-code elimination to dynamically minimize the runtime footprint of resident drivers and TSRs (like FreeKEYB).) * *

https://www.youtube.com/watch?v=HO6IPpL0y8g] (33 pages) * * * * * * (2+xiv+270+6 pages) * * *

* * *

https://archive.org/download/80-microcomputing-magazine-1983-04/80Microcomputing_0483_text.pdf] *

https://archive.org/download/80-microcomputing-magazine-1985-04/80Microcomputing_0485_text.pdf] *

https://archive.org/download/the-computer-journal-32/tcj_32_May-June_1988_text.pdf] *

https://archive.org/download/the-computer-journal-33/tcj_33_July-August_1988_text.pdf] *

https://archive.org/download/the-computer-journal-34/tcj_34_September-October_1988_text.pd

*

https://archive.org/download/the-computer-journal-54/tcj_54_January-February_1992_text.pd

*

https://archive.org/download/the-computer-journal-55/tcj_55_March-June_1992_text.pdf] * {{Executables Computer libraries Assignment operations