Relocation (computing): Difference between revisions

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{{Short description|Assigning or adjusting addresses at runtime}}

{{Distinguish|Renumbering}}

{{More footnotes needed|date=July 2024}}

{{Use dmy dates|date=May 2019|cs1-dates=y}}

{{Use list-defined references|date=December 2021}}

{{Use American English|date=January 2019}}

{{anchor|Static|Dynamic|Compile-time|Load-time|Run-Time|Virtual|Page boundary|Paragraph boundary|Segment|Offset|Line}}'''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.<ref name="Intel_iRMX"/><ref name="Levine_1999_CH1_CH3"/> Prior to the advent of multiprocess systems, and still in many embedded systems, the addresses for objects were [[absolute address|absolute]] 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 (computing)|linker]] usually performs relocation in conjunction with '''symbol resolution''', the process of searching files and libraries to replace symbolic references or names of [[Library (computer science)|libraries]] with actual usable addresses in [[primary storage|memory]] before running a program.

In [[software development]], '''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.<ref name="Intel_iRMX"/><ref name="Levine_1999_CH1_CH3"/>

Relocation is typically done by the linker at [[link time]], but it can also be done at [[load time]] by a relocating [[loader (computing)|loader]], or at [[Run time (program lifecycle phase)|run time]] by the running program [[self-relocation|itself]]. Some architectures avoid relocation entirely by deferring [[address assignment]] to run time; this is known as [[zero address arithmetic]].{{Which|date=March 2017}}

A [[Linkerlinker (computing)|linker]] usually performs relocation in conjunction with '''symbol resolution''', the process of searching files and libraries to replace symbolic references or names of [[Librarylibrary (computer science)|libraries]] with actual usable addresses in [[primary storage|memory]] before running a program.

Relocation is typically done by the linker at [[link time]], but it can also be done at [[load time]] by a relocating [[loader (computing)|loader]], or at [[Runrun time (program lifecycle phase)|run time]] by the running program [[self-relocation|itself]]. Some architectures avoid relocation entirely by deferring [[address assignment]] to run time; this is known as [[zero address arithmetic]].{{Which|date=March 2017}}

== Segmentation ==

[[Object file]]s are typically segmented into various [[Memory segmentation|memory segment]] or section types. Example segmentssegment types include [[code segment|code segment (.text)]], [[data segment|initialized data segment (.data)]], [[.bss|uninitialized data segment (.bss)]]), or others.{{clarify|date=November 2016}}as established by the programmer, such as common segments, or named static segments.

== Relocation table ==

The relocation table is a list of [[pointer (computer programming)|pointer]]saddresses created by the translator (a [[compiler]] or [[Assembler (computer programming)#Assembler|assembler]]) and stored in the object or executable file. Each entry in the table, or "fixup", is a [[pointer (computer programming)|pointer]] toreferences 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. FixupsEntries in the relocation table are known as ''fixups'' and 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.<ref name="Levine_1999_CH1_CH3"/>

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.<ref name="Borland_2007"/>

=== DOS and 16-bit Windows ===

Far [[pointer (computer programming)|pointer]]s ([[32-bit]] pointers with [[x86 memory segmentation|segment]]:offset, used to address 20-bit 640 &nbsp;[[kilobyte|KB]] [[computer storage|memory]] space available to [[DOS]] [[computer program|program]]sprograms), which point to code or data within a [[DOS executable]] ([[EXE]]), do not have absolute segments, because the actual [[memory address|address]] of code/ or 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 (computing)|loader]] uses a relocation table to find the segments whichthat 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 [[Dynamic Link Library|DLL]]s and for EXEs which opt into [[address space layout randomization]] (ASLR), an [[exploit (computer security)|exploit]] mitigation technique introduced with [[Windows Vista]], 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.

Windows executables can be marked as ASLR-compatible. The ability exits in Windows 8 and newer to enable ASLR even for applications not marked as compatible.<ref>{{cite web |url=https://cloud.google.com/blog/topics/threat-intelligence/six-facts-about-address-space-layout-randomization-on-windows |title=Six Facts about Address Space Layout Randomization on Windows |date=March 17, 2020 |access-date=July 24, 2020}}</ref> To run successfully in this envoronment the relocation sections cannot be omitted by the compiler.<!--[[User:Kvng/RTH]]-->

=== 64-bit Windows ===

When running native 64-bit binaries on Windows Vista and above, ASLR is mandatory{{Citation needed|date=January 2014}}, and thus relocation sections cannot be omitted by the compiler.

=== Unix-like systems ===

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* 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 {{clarify|date=July 2021}}

* referring to the '''relocation table''' to modify{{why|date=November 2016}} symbolssymbol references so that they point to the correct{{clarify|date=November 2016}} run time addresses.

==Example==

The following example uses [[Donald Knuth]]'s [[MIX (abstract machine)|MIX]] architecture and MIXAL assembly language. The principles are the same for any architecture, though the details will change.

[[File:relocation example.tif|frameless|right|334px]]

* (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. [1 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. [4 49 is the MIX machine representation of 305].

==Alternatives==

Some architectures avoid relocation entirely by deferring address assignment to run time; as, for example, in stack machines with [[zero address arithmetic]] or in some segmented architectures where every compilation unit is loaded into a separate segment.

== See also ==

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* [[Static library]]

* [[Self-relocation]]

* [[Position-independent code]] (PIC)

* [[Rebasing]]

* [[Garbage collection (computing)|Garbage collection]]

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== References ==

{{Reflist|refs=

<ref name="Intel_iRMX">{{cite book |title=iRMX 86 Application Loader Reference Manual |publisher=[[Intel]] |url=http://bitsavers.informatik.uni-stuttgart.de/pdf/intel/iRMX/iRMX_86_Rev_6_Mar_1984/146196_Burst/iRMX_86_Application_Loader_Reference_Manual.pdf |access-date=2020-01-11 |url-status=live |archive-url=https://web.archive.org/web/20200111215002/http://bitsavers.informatik.uni-stuttgart.de/pdf/intel/iRMX/iRMX_86_Rev_6_Mar_1984/146196_Burst/iRMX_86_Application_Loader_Reference_Manual.pdf |archive-date=2020-01-11 |chapter=Types of Object Code |pages=((1{{hyphen}}2, 12–1{{hyphen}}3)) |quote=[…] ''Absolute code'', and an absolute object module, is code that has been processed by LOC86 to run only at a specific ___location in memory. The [[Loader (computing)|Loader]] loads an absolute object module only into the specific ___location the module must occupy. ''[[Position-independent code]]'' (commonly referred to as PIC) differs from absolute code in that PIC can be loaded into any memory ___location. The advantage of PIC over absolute code is that PIC does not require you to reserve a specific block of memory. When the Loader loads PIC, it obtains [[iRMX&nbsp;86]] memory segments from the pool of the calling task's job and loads the PIC into the segments. A restriction concerning PIC is that, as in the [[PL/M-86]] COMPACT model of segmentation […], it can have only one code segment and one data segment, rather than letting the base addresses of these segments, and therefore the segments themselves, vary dynamically. This means that PIC programs are necessarily less than 64K bytes in length. PIC code can be produced by means of the BIND control of LINK86. ''Load-time locatable code'' (commonly referred to as LTL code) is the third form of object code. LTL code is similar to PIC in that LTL code can be loaded anywhere in memory. However, when loading LTL code, the Loader changes the base portion of pointers so that the pointers are independent of the initial contents of the registers in the microprocessor. Because of this fixup (adjustment of base addresses), LTL code can be used by tasks having more than one code segment or more than one data segment. This means that LTL programs may be more than 64K bytes in length. [[FORTRAN 86]] and [[Pascal 86]] automatically produce LTL code, even for short programs. LTL code can be produced by means of the BIND control of LINK86. […]}}</ref>

<ref name="Levine_1999_CH1_CH3">{{cite book |author-last=Levine |author-first=John R. |author-link=John R. Levine |title=Linkers and Loaders |date=2000 |orig-date=October 1999 |edition=1 |publisher=[[Morgan Kaufmann]] |series=The Morgan Kaufmann Series in Software Engineering and Programming |___location=San Francisco, California, USA |isbn=1-55860-496-0 |oclc=42413382 |chapter=Chapter 1: Linking and Loading & Chapter 3: Object Files |pages=5<!-- , --> |url=https://www.iecc.com/linker/ |access-date=2020-01-12 |url-status=live |archive-url=https://archive.today/20121205032107/http://www.iecc.com/linker/ |archive-date=2012-12-05}} Code: [https://archive.today/20200114225034/https://linker.iecc.com/code.html][ftp://ftp.iecc.com/pub/linker/]{{dead link|date=May 2025|bot=medic}}{{cbignore|bot=medic}} Errata: [https://linker.iecc.com/<!-- https://archive.today/20200114224817/https://linker.iecc.com/ 2020-01-14 -->]</ref>

<ref name="Borland_2007">{{cite web |author=Borland |author-link=Borland |title=Borland article #15961: Coping with 'Fixup Overflow' messages. |orig-date=1998-07-02 |date=1999-09-01 |series=Technical Information Database - Product: Borland C++ 3.1 |work=community.borland.com |id=TI961C.txt #15961 |url=http://vmlinux.org/~jakov/community.borland.com/15961.html |access-date=2007-01-15 |url-status=live |archive-url=https://web.archive.org/web/20080707030831/http://vmlinux.org/~jakov/community.borland.com/15961.html |archive-date=2008-07-07}}</ref>

<ref name="ELF">{{cite web |title=Executable and Linkable Format (ELF) |series=Tool Interface Standards (TIS) Portable Formats Specification, Version 1.1 |website=skyfree.org |url=http://www.skyfree.org/linux/references/ELF_Format.pdf |access-date=2018-10-01 |url-status=live |archive-url=https://web.archive.org/web/20191224125108/http://www.skyfree.org/linux/references/ELF_Format.pdf |archive-date=2019-12-24}}</ref>

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== Further reading ==

* {{cite book |title=11/34 Memory Management Basic Logic test |author-first=Glenn |author-last=Johnson |publisher=[[Digital Equipment Corporation]] (DEC) |date=1975-12-21 |orig-date=1975-11-13 |id=MAINDEC-11-DFKTA-A-D |url=https://archive.org/stream/bitsavers_decpdp11xxAINDEC11DFKTAAD1134MEMORYMANAGEMENTBASIC_2477046/MAINDEC-11-DFKTA-A-D_1134_MEMORY_MANAGEMENT_BASIC_LOGIC_Dec75 |access-date=2017-08-19}}

* {{cite journalmagazine |title=A Proposed Microprocessor Software Standard |author-first1=Peter G. |author-last1=Formaniak |author-first2=David |author-last2=Leitch |series=Technical Forum |magazine=[[BYTE]] - the small systems journal |publisher=[[Byte Publications, Inc.]] |publication-place=Peterborough, New Hampshire, USA |volume=2 |number=7 |date=July 1977 |id=ark:/13960/t32245485 |pages=34, 62–63 |url=https://archive.org/details/byte-magazine-1977-07/page/n35/mode/1up |access-date=2021-12-06}} (3 pages) (NB. Describes a [[Mostek hex format|relocatable hex format]] by [[Mostek]].)

* {{cite journalmagazine |title=Relocatable Object Code Formats |author-first1=Carol Anne |author-last1=Ogdin |author-first2=Neil |author-last2=Colvin |author-first3=Tom |author-last3=Pittman |author-first4=Philip |author-last4=Tubb |series=Technical Forum |magazine=[[BYTE]] - the small systems journal |publisher=[[Byte Publications, Inc.]] |publication-place=Peterborough, New Hampshire, USA |volume=2 |number=11 |date=November 1977 |id=ark:/13960/t59c88b4h, ark:/13960/t3kw76j24 |pages=198–205 |url=https://archive.org/details/byte-magazine-1977-11-rescan/page/n199/mode/1up |access-date=2021-12-06}} (8 pages) (NB. Describes a [[Technical Design Labs Relocable Object Module Format|relocatable hex format]] by [[Technical Design Labs|TDL]].)

* {{cite journal |author-first=Gary Arlen |author-last=Kildall |author-link=Gary Arlen Kildall |title=A simple technique for static relocation of absolute machine code |journal=[[Dr. Dobb's Journal of Computer Calisthenics & Orthodontia]] |publisher=[[People's Computer Company]] |volume=3 |issue=2 |id=#22<!-- |number=22 --> ark:/13960/t8hf1g21p |date=February 1978 |orig-date=<!-- November -->1976 |pages=10–13<!-- in the issue --> (66–69<!-- in the volume -->) |isbn=0-8104-5490-4<!-- of the volume --> |url=https://archive.org/details/dr_dobbs_journal_vol_03/page/n67/mode/1up |access-date=2017-08-19}} [https://web.archive.org/web/20170819141800/http://www.retrotechnology.com/dri/d_dri_refs.html][https://web.archive.org/web/20170819173516/http://archive.computerhistory.org/resources/access/text/2016/12/102762506-05-01-acc.pdf][https://archive.today/20170909091943/https://groups.google.com/forum/%23!msg/comp.os.cpm/TLHgIi16yTo/gupNB1ai8UQJ#!topic/comp.os.cpm/TLHgIi16yTo]. Originally presented at: {{cite conference |title=Conference Record: Tenth Annual Asilomar Conference on Circuits, Systems and Computers: Papers Presented November 22–24, 1976 |editor-first=Harold A. |editor-last=Titus |chapter=A Simple Technique for Static Relocation of Absolute Machine Code |author-first=Gary Arlen |author-last=Kildall |journal=Asilomar Conference on Signals, Systems & Computers |author-link=Gary Arlen Kildall |<!-- written-at -->___location=[[Naval Postgraduate School]], Monterey, California, USA |publisher=Western Periodicals Company |publication-place=Asilomar Hotel and Conference Grounds, Pacific Grove, California, USA |date=1977 |orig-date=22–24 November 1976 |issn=1058-6393 |pages=420–424 |url=https://books.google.com/books?id=cyBGAQAAIAAJ&q=relocation |access-date=2021-12-06}} (609 pages). (This "resize" method, named ''page boundary relocation'', could be applied statically to a [[CP/M-80]] disk image using {{ill|MOVCPM|pl|MOVCPM (CP/M)}} in order to maximize the [[Transient Program Area|TPA]] for programs to run. It was also utilized dynamically by the CP/M debugger [[Dynamic Debugging Tool]] (DDT) to [[self-relocation|relocate itself]] into higher memory. The same approach was independently developed by [[Bruce H. Van Natta]] of [[IMS Associates]] to produce relocatable [[PL/M]] code. As ''paragraph boundary relocation'', [[Self-relocation#Ref-Paul-2002-Drivers|another variant]] of this method was later utilized by dynamically [[high memory area|HMA]] self-relocating [[Terminate-and-Staystay-Residentresident program|TSRTSRs]]s like [[KEYB (DOS command)|KEYB]], [[SHARE (DOS command)|SHARE]], and [[NLSFUNC (DOS command)|NLSFUNC]] under [[DR DOS&nbsp;6.0]] and higher. A much more sophisticated and [[byte alignment|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).)

* {{cite magazine |title=Relocatable Code |author-first=Richard H. |author-last=Mossip |___location=Bloomingdale, New Jersey, USA |publisher=[[Libes, Inc.]] |publication-place=Mountainside & Springfield, New Jersey, USA |magazine=[[S-100 Microsystems (magazine)|S-100 Microsystems]] |issn=0199-7955 |volume=1 |number=5 |date=September–October 1980 |id=ark:/13960/s2cfgkmxcwg. ark:/13960/s2qdm1t01nr |pages=54–55 |url=http://bitsavers.informatik.uni-stuttgart.de/magazines/S-100_Microsystems/v01n05.pdf |access-date=2023-11-27 |url-status=live |archive-url=https://web.archive.org/web/20231127182118/http://bitsavers.informatik.uni-stuttgart.de/magazines/S-100_Microsystems/v01n05.pdf |archive-date=2023-11-27}} [https://archive.org/details/bitsavers_S100Micros_46725899/page/54/mode/1up][https://archive.org/details/v1n5_20230524/page/54/mode/1up] (2 pages) (NB. Describes page boundary relocation and relocating assemblers.)<!-- In the article the author announces a more detailed article at a later stage - this still has to be found -->

* {{cite news |title=unknown |editor-first=Sol |editor-last=Libes |publisher=Libes Inc. |publication-place=Mountainside, New Jersey, USA |newspaper=[[S-100 Micro Systems]] |volume=1 |date=1980–1981 |issue= |page=54 |url=https://books.google.com/books?id=vEosAQAAIAAJ&dq=page+boundary+relocation&focus=searchwithinvolume&q=page+boundary+relocation |access-date=2021-12-06}}

* {{cite web |title=Legacy of Gary Kildall: The CP/M IEEE Milestone Dedication |author-first1=Robert |author-last1=Huitt |author-first2=Gordon |author-last2=Eubanks |author-link2=Gordon Eubanks |author-first3=Thomas "Tom" Alan |author-last3=Rolander |author-link3=Thomas Alan Rolander |author-first4=David |author-last4=Laws |author-first5=Howard E. |author-last5=Michel |author-first6=Brian |author-last6=Halla |author-first7=John Harrison |author-last7=Wharton |author-link7=John Harrison Wharton |author-first8=Brian |author-last8=Berg |author-first9=Weilian |author-last9=Su |author-first10=Scott |author-last10=Kildall |author-link10=Scott Kildall |author-first11=Bill |author-last11=Kampe |editor-first=David |editor-last=Laws |date=2014-04-25 |___location=Pacific Grove, California, USA |type=video transscription |id=CHM Reference number: X7170.2014 |publisher=[[Computer History Museum]] |url=https://archive.computerhistory.org/resources/access/text/2014/06/102746909-05-01-acc.pdf |access-date=2020-01-19 |url-status=live |archive-url=https://web.archive.org/web/20141227142045/http://archive.computerhistory.org/resources/access/text/2014/06/102746909-05-01-acc.pdf |archive-date=2014-12-27 |quote=[…] Laws: […] "dynamic relocation" of the OS. Can you tell us what that is and why it was important? […] [[Gordon Eubanks|Eubanks]]: […] what [[Gary Arlen Kildall|Gary]] did […] was […] mind boggling. […] I remember the day at the [[Naval Postgraduate School|school]] he came bouncing into the lab and he said, I have figured out how to relocate. He took advantage of the fact that the only byte was always going to be the [[high order byte]]. And so he created a [[bitmap]]. […] it didn't matter how much memory the computer had, the operating system could always be moved into the high memory. Therefore, you could commercialize this […] on machines of different amounts of memory. […] you couldn't be selling a 64K [[CP/M]] and a 47K CP/M. It'd just be ridiculous to have a hard compile in the addresses. So Gary figured this out one night, probably in the middle of the night thinking about some coding thing, and this really made CP/M possible to commercialize. I really think that without that relocation it would have been a very tough problem. To get people to buy it, it'd seem complicated to them, and if you added more memory you'd have to go get a different operating system. […] [[Intel]] […] had the [[little-endian|bytes reversed]], right, for the memory addresses. But they were always in the same place, so you could relocate it on a [[256 byte boundary]], to be precise. You could therefore always relocate it with just a bitmap of where those […] Laws: Certainly the most eloquent explanation I've ever had of dynamic relocation […]}} [https://ethw.org/Milestones:The_CP/M_Microcomputer_Operating_System,_1974][https://www.youtube.com/watch?v=HO6IPpL0y8g] (33 pages)

* {{cite journal |title=CP/M 2 lernt dazu. Modulare Systemerweiterungen auch für das 'alte' CP/M |language=de |type=part 1 |author-first1=Eckhard |author-last1=Lieber |author-first2=Thomas |author-last2=von Massenbach |journal=[[c't - magazin für computertechnik]] |publisher=[[Heise Verlag]] |date=1987 |volume=1987 |issue=1 |pages=124–135 |postscript=;}} {{cite journal |title=CP/M 2 lernt dazu. Modulare Systemerweiterungen auch für das 'alte' CP/M |language=de |type=part 2 |author-first1=Eckhard |author-last1=Lieber |author-first2=Thomas |author-last2=von Massenbach |journal=[[c't - magazin für computertechnik]] |publisher=[[Heise Verlag]] |date=1987 |volume=1987 |issue=2 |pages=78–85 |postscript=;}} {{cite web |title=RSM für CP/M 2.2 |language=de |date=2016-10-09 |author-first=Alex |author-last=Huck |work=Homecomputer DDR |url=http://hc-ddr.hucki.net/wiki/doku.php/cpm:rsm:ct |access-date=2016-11-25 |url-status=live |archive-url=https://web.archive.org/web/20161125023237/http://hc-ddr.hucki.net/wiki/doku.php/cpm%3Arsm%3Act |archive-date=2016-11-25}}

* {{cite web |title=Re: CP/M assembly language programming |author-first=Charles "Chuck" P. |author-last=Guzis |date=2015-03-16 |work=Vintage Computer Forum |series=Genre: CP/M and MP/M |url=http://www.vcfed.org/forum/showthread.php?46767-CP-M-assembly-language-programming&p=360328#post360328 |access-date=2020-02-01 |url-status=live |archive-url=https://web.archive.org/web/20200201052736/http://www.vcfed.org/forum/showthread.php?46767-CP-M-assembly-language-programming&p=360328 |archive-date=2020-02-01 |quote=[…] Ever wonder how {{ill|MOVCPM|pl|MOVCPM (CP/M)}} works? Since the [[BDOS]] and [[Console Command Processor|CCP]] is in high memory, above the user application, addresses have to be changed every time the system memory size is changed. Now that requires relocating addresses in [[8080]] code, since [[relative addressing]] is not part of the hardware. Without implementing a full-blown relocating assembler and loader, how does one go about this? It's actually pretty clever and [[MP/M]] even uses this scheme to construct its page-relocatable files. You simply assemble the source program ''twice'' with the second assembly origin 100H (256 bytes) higher than the first. The two binary images are then compared, byte for byte, and a [[bitmap|map]] constructed of where pairs of bytes differ in value by exactly 100H. The result is a list of locations where the relocation value needs to be adjusted if the ___location of a program in memory is to be moved. MP/M calls this sort of file PRL (page relocatable), but I don't know that CP/M 2.2 ever coined a name for it. […]}}

* {{cite web |title=Re: How does MOVCPM.COM work? |author-first=Charles "Chuck" P. |author-last=Guzis |date=2015-07-29 |work=Vintage Computer Forum |series=Genre: CP/M and MP/M |url=http://www.vcfed.org/forum/showthread.php?48532-How-does-MOVCPM-COM-work&p=376438#post376438 |access-date=2020-02-01 |url-status=live |archive-url=https://web.archive.org/web/20200201053054/http://www.vcfed.org/forum/showthread.php?48532-How-does-MOVCPM-COM-work=&p=376438 |archive-date=2020-02-01 |quote=[…] {{ill|MOVCPM|pl|MOVCPM (CP/M)}} uses an early type of PRL format. Basically, [[CP/M]] is assembled twice; the second time is 100H bytes offset. The two binaries are compared and a [[bitmap]] constructed. A set bit implies that the [[high-order byte]] of an address is to be adjusted. Low order address bytes are not affected; hence, "Page relocatblerelocatable file". Each byte in the bitmap corresponds to 8 bytes in the binary data. […] So everything to be moved in MOVCPM is part of the image and its relocation bitmap. […]}}

* {{cite web |title=Re: Is it safe to use RST 28h in CP/M assembly programs? |author-first=Charles "Chuck" P. |author-last=Guzis |date=2016-11-08 |work=Vintage Computer Forum |series=Genre: CP/M and MP/M |url=http://www.vcfed.org/forum/showthread.php?54822-Is-it-safe-to-use-RST-28h-in-CP-M-assembly-programs&p=434954#post434954 |access-date=2020-02-01 |url-status=live |archive-url=https://web.archive.org/web/20200201053327/http://www.vcfed.org/forum/showthread.php?54822-Is-it-safe-to-use-RST-28h-in-CP-M-assembly-programs&p=434954 |archive-date=2020-02-01 |quote=[…] I've referenced PRL files and how they originally got their start with {{ill|MOVCPM|pl|MOVCPM (CP/M)}}, but became an integral part of [[MP/M]] and [[CP/M 3.0]]. But PRL files use a [[bitmap|bit map]] in which every bit corresponds to a memory ___location; one bits indicate that a page relocation offset should be added to the corresponding memory ___location. If you have very few absolute memory references (as opposed to relative ones) you may want to employ a pointer list (2 bytes per reference) rather than a bitmap. This is unlikely in [[8080]] code which doesn't have relative jumps, but may be a consideration for [[Z80]] code. The trick to quickly find this out is to assemble your program twice; the second time offset by 100H, then compare the two binaries. The advantage of [[runtime (program lifecycle phase)|run-time]] relocation is that you don't have to incur a penalty for code that attempts to get around the relocation issue--no "tricks"; just write straight code. […]}}

* {{cite journal |author-first=Richard L. |author-last=Roth |title=Relocation Is Not Just Moving Programs |___location=Ridgefield, CACalifornia, USA |journal=[[Dr. Dobb's Journal of Computer Calisthenics & Orthodontia]] |publisher=[[People's Computer Company]] |id=#22 |volume=3 |number=2 |date=February 1978 |orig-date=1977 |pages=14–20<!-- in the issue --> (70–76<!-- in the volume -->) |isbn=0-8104-5490-4<!-- of the volume --> |url=https://archive.org/stream/dr_dobbs_journal_vol_03/dr_dobbs_journal_vol_03_djvu.txt |access-date=2019-04-19 |url-status=live |archive-url=https://web.archive.org/web/20190420010941/https://archive.org/stream/dr_dobbs_journal_vol_03/dr_dobbs_journal_vol_03_djvu.txt |archive-date=2019-04-20}}

* {{cite book |title=Assemblers, Compilers, and Program Translation |chapter=8.2.2 Relocating Loader |pages=[https://archive.org/details/assemblerscompil00cali/page/237 237]–241 |author-first=Peter |author-last=Calingaert |editor-first=Ellis |editor-last=Horowitz |editor-link=Ellis Horowitz |date=1979 |orig-date=1978-11-05 |series=Computer software engineering series |publisher=[[Computer Science Press, Inc.]] |publication-place=Potomac, Maryland, USA |___location=[[University of North Carolina at Chapel Hill]] |edition=1st printing, 1st |isbn=0-914894-23-4 |issn=0888-2088 |lccn=78-21905 |url=https://archive.org/details/assemblerscompil00cali |url-access=registration |access-date=2020-03-20}} (2+xiv+270+6 pages)

* {{cite book |title=The Microsoft OBJ File Format |publisher=[[Microsoft]], Product Support Services |id=Application Note SS0288 |url=https://www.fileformat.info/format/ms-obj/corion.htm |access-date=2017-08-21 |url-status=live |archive-url=https://archive.today/20170909092856/http://www.fileformat.info/format/ms-obj/corion.htm |archive-date=2017-09-09}}

* {{cite book |author-first1=Andrew Stuart |author-last1=Tanenbaum |author-link1=Andrew Stuart Tanenbaum |author-first2=Herbert |author-last2=Bos |title=Modern Operating Systems |edition=4 |date=2015 |publisher=[[Pearson Education Inc.]] |isbn=978-0-13359162-0}}

* {{cite web |author-first=John C. |author-last=Elliott |title=PRL file format |date=2012-06-05 |orig-date=2000-01-02 |website=seasip.info |url=https://www.seasip.info/Cpm/prl.html |access-date=2020-01-26 |url-status=live |archive-url=https://web.archive.org/web/20200126182027/https://www.seasip.info/Cpm/prl.html |archive-date=2020-01-26 |quote=[…] A PRL file is a relocatable binary file, used by [[MP/M]] and [[CP/M Plus]] for various modules other than [[.COM file]]s. The file format is also used for FID files on the [[Amstrad PCW]]. There are several file formats which use versions of PRL: SPR (System PRL), RSP (Resident System Process). LINK-80 can also produce [[.OVL|OVL]] ([[overlay (programming)|overlay]]) files, which have a PRL header but are not relocatable. [[Graphics System Extension|GSX]] drivers are in PRL format; so are [[Resident System Extension]]s (.RSX). […]}} [https://www.seasip.info/Cpm/amsfid.html#PRL%20file%20format]

* {{cite web |author-first=John C. |author-last=Elliott |title=Microsoft REL format |date=2012-06-05 |orig-date=2000-01-02 |website=seasip.info |url=https://www.seasip.info/Cpm/rel.html |access-date=2020-01-26 |url-status=live |archive-url=https://web.archive.org/web/20200126181737/https://www.seasip.info/Cpm/rel.html |archive-date=2020-01-26 |quote=[…] The REL format is generated by [[Microsoft]]'s M80 and [[Digital Research]]'s RMAC. […]}}

* {{cite web |title=Support for PRL, page relocatable executable for MP/M |date=2018-09-05 |orig-date=2018-09-02 |author=feilipu |work=z88dk |url=https://github.com/z88dk/z88dk/issues/935 |access-date=2020-01-26 |url-status=live |archive-url=https://web.archive.org/web/20200201053652/https://github.com/z88dk/z88dk/issues/935 |archive-date=2020-02-01 |quote=[…] Out of the assembled [[Microsoft]] .REL files the linker has to generate a .PRL format executable for [[MP/M]]. The .PRL format is essentially a [[.COM file]] with some additional information to enable the program and its data to be relocated onto any page. What does a .PRL file look like? The first bytes are size of the program, followed by the program origin at 0x0100. Following the program, there is a bit-for-byte mask appended to allow the MP/M system to know which bytes in the program need to be changed when the program is relocated. How does the linker do that without disassembling the whole application? In advance the program is linked for two different origins 0x0100 and 0x0200, from the .REL objects. The linker trick is simply recognising which bytes in the two versions of the executable differ. These bytes are then recorded in the bit mask stored following the executable, and the final .PRL program is designed to run from 0x0100 plus its page offset. The same trick is done for the .RSP and .SPR executable files, except that both these formats forego the offset, and run from 0x0000 plus their page offset. […]}}