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Relocation (computing)
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<h2 id="segmentation">Segmentation</h2> <p><a href="/facts/Object_file/sFBvosE4">Object files</a> are typically segmented into various memory segment or section types. Example segment types include <a href="/facts/Code_segment/tRPrQU7G">code segment (.text)</a>, <a href="/facts/Data_segment/AR64BUzX">initialized data segment (.data)</a>, <a href="/facts/.bss/hM4yEzyU">uninitialized data segment (.bss)</a>, or others as established by the programmer, such as common segments, or named static segments. </p> <h2 id="relocation-table">Relocation table</h2> <p>The relocation table is a list of addresses created by a <a href="/facts/Compiler/WNfCDFJe">compiler</a> or <a href="/facts/Assembler_(computer_programming)/7XuV0cla">assembler</a> and stored in the object or executable file. Each entry in the table references 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. Entries in the relocation table are known as <i>fixups</i> 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.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> </p><p>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.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> </p> <h3>DOS and 16-bit Windows</h3> <p>Far <a href="/facts/Pointer_(computer_programming)/fPoNzy9b">pointers</a> (<a href="/facts/32-bit/TDMVce8s">32-bit</a> pointers with <a href="/facts/X86_memory_segmentation/7yD0VGfy">segment</a>:offset, used to address 20-bit 640 <a href="/facts/Kilobyte/jIe6t4yv">KB</a> <a href="/facts/Computer_storage/wPHKFpdx">memory</a> space available to <a href="/facts/DOS/sWwDXNmX">DOS</a> <a href="/facts/Computer_program/e38jH9N9">programs</a>), which point to code or data within a <a href="/facts/DOS_executable/W8vGH66t">DOS executable</a> (<a href="/facts/EXE/xgRXf4rt">EXE</a>), do not have absolute segments, because the actual <a href="/facts/Memory_address/7EsUmbea">address</a> of code/data depends on where the program is loaded in memory and this is not known until the program is loaded. </p><p>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 <a href="/facts/Loader_(computing)/eIK4f2xB">loader</a> uses a relocation table to find the segments which need to be adjusted. </p> <h3>32-bit Windows</h3> <p>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. </p><p>For both <a href="/facts/Dynamic_Link_Library/vySyC1dz">DLLs</a> and for EXEs which opt into <a href="/facts/Address_space_layout_randomization/K6lMxWTr">address space layout randomization</a> (ASLR), an <a href="/facts/Exploit_(computer_security)/EdlqsRJM">exploit</a> mitigation technique introduced with <a href="/facts/Windows_Vista/IASEUCTB">Windows Vista</a>, 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. </p> <h3>64-bit Windows</h3> <p>When running native 64-bit binaries on Windows Vista and above, ASLR is mandatory, and thus relocation sections cannot be omitted by the compiler. </p> <h3>Unix-like systems</h3> <p>The <a href="/facts/Executable_and_Linkable_Format/ro2CYqso">Executable and Linkable Format</a> (ELF) executable format and shared library format used by most Unix-like systems allows several types of relocation to be defined.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> </p> <h2 id="relocation-procedure">Relocation procedure</h2> <p>The linker reads segment information and relocation tables in the object files and performs relocation by: </p> <ul><li>merging all segments of common type into a single segment of that type</li> <li>assigning unique run time addresses to each section and each symbol, giving all code (functions) and data (global variables) unique run time addresses </li> <li>referring to the relocation table to modify[<i>why?</i>] symbol references so that they point to the correct run time addresses.</li></ul> <h2 id="example">Example</h2> <p>The following example uses <a href="/facts/Donald_Knuth/GozNzDM6">Donald Knuth</a>'s <a href="/facts/MIX_(abstract_machine)/C7JqgIMI">MIX</a> architecture and MIXAL assembly language. The principles are the same for any architecture, though the details will change. </p> <ul><li>(A) Program <i>SUBR</i> 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 <i>ST</i> in location 5.</li> <li>(C) If <i>SUBR</i> 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 <i>ST</i>, now 125. [1 61 shown in the instruction is the MIX machine code representation of 125].</li> <li>(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 <i>SUBR</i> 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 <i>ST</i>, 305. [4 49 is the MIX machine representation of 305].</li></ul> <h2 id="alternatives">Alternatives</h2> <p>Some architectures avoid relocation entirely by deferring address assignment to run time; as, for example, in stack machines with <a href="/facts/Zero_address_arithmetic/IBtk4NRA">zero address arithmetic</a> or in some segmented architectures where every compilation unit is loaded into a separate segment. </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Linker_(computing)/pn1hztYe">Linker (computing)</a></li> <li><a href="/facts/Library_(computing)/xRzL3P7q">Library (computing)</a></li> <li><a href="/facts/Object_file/sFBvosE4">Object file</a></li> <li><a href="/facts/Prebinding/BEgQJShv">Prebinding</a></li> <li><a href="/facts/Static_library/CB39voVa">Static library</a></li> <li><a href="/facts/Self-relocation/gMw61ATA">Self-relocation</a></li> <li><a href="/facts/Rebasing/mHo899gJ">Rebasing</a></li> <li><a href="/facts/Garbage_collection_(computing)/j2vlIu7g">Garbage collection</a></li> <li><a href="/facts/Pointer_swizzling/4nFmkbp4">Pointer swizzling</a>, a lazy form of pointer modification</li> <li><a href="/facts/Relocatable_Object_Module_Format/KV63XvwB">Relocatable Object Module Format</a></li></ul> <h2 id="further-reading">Further reading</h2> <ul><li>Johnson, Glenn (1975-12-21) [1975-11-13]. <a href="https://archive.org/stream/bitsavers_decpdp11xxAINDEC11DFKTAAD1134MEMORYMANAGEMENTBASIC_2477046/MAINDEC-11-DFKTA-A-D_1134_MEMORY_MANAGEMENT_BASIC_LOGIC_Dec75"><i>11/34 Memory Management Basic Logic test</i></a>. <a href="/facts/Digital_Equipment_Corporation/ztxs6keA">Digital Equipment Corporation</a> (DEC). MAINDEC-11-DFKTA-A-D. Retrieved 2017-08-19.</li> <li>Formaniak, Peter G.; Leitch, David (July 1977). <a href="https://archive.org/details/byte-magazine-1977-07/page/n35/mode/1up">"A Proposed Microprocessor Software Standard"</a>. <i><a href="/facts/BYTE/xWovF95Q">BYTE</a> - the small systems journal</i>. Technical Forum. Vol. 2, no. 7. Peterborough, New Hampshire, USA: <a href="/facts/Byte_Publications%2c_Inc./xWovF95Q">Byte Publications, Inc.</a> pp. 34, 62–63. ark:/13960/t32245485. Retrieved 2021-12-06. (3 pages) (NB. Describes a <a href="/facts/Mostek_hex_format/2LWl2u83">relocatable hex format</a> by <a href="/facts/Mostek/NIePx5jb">Mostek</a>.)</li> <li>Ogdin, Carol Anne; Colvin, Neil; Pittman, Tom; Tubb, Philip (November 1977). <a href="https://archive.org/details/byte-magazine-1977-11-rescan/page/n199/mode/1up">"Relocatable Object Code Formats"</a>. <i><a href="/facts/BYTE/xWovF95Q">BYTE</a> - the small systems journal</i>. Technical Forum. Vol. 2, no. 11. Peterborough, New Hampshire, USA: <a href="/facts/Byte_Publications%2c_Inc./xWovF95Q">Byte Publications, Inc.</a> pp. 198–205. ark:/13960/t59c88b4h, ark:/13960/t3kw76j24. Retrieved 2021-12-06. (8 pages) (NB. Describes a <a href="/facts/Technical_Design_Labs_Relocable_Object_Module_Format/2LWl2u83">relocatable hex format</a> by <a href="/facts/Technical_Design_Labs/Dc1MN8jS">TDL</a>.)</li> <li><a href="/facts/Gary_Arlen_Kildall/yzxo0mLU">Kildall, Gary Arlen</a> (February 1978) [1976]. <a href="https://archive.org/details/dr_dobbs_journal_vol_03/page/n67/mode/1up">"A simple technique for static relocation of absolute machine code"</a>. <i><a href="/facts/Dr._Dobb%2527s_Journal_of_Computer_Calisthenics_%2526_Orthodontia/6Qov1hUK">Dr. Dobb's Journal of Computer Calisthenics & Orthodontia</a></i>. 3 (2). <a href="/facts/People%2527s_Computer_Company/Dwhp3hsx">People's Computer Company</a>: 10–13 (66–69). <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-8104-5490-4. #22 ark:/13960/t8hf1g21p. Retrieved 2017-08-19. <a href="https://web.archive.org/web/20170819141800/http://www.retrotechnology.com/dri/d_dri_refs.html">[4]</a><a href="https://web.archive.org/web/20170819173516/http://archive.computerhistory.org/resources/access/text/2016/12/102762506-05-01-acc.pdf">[5]</a><a href="https://archive.today/20170909091943/https://groups.google.com/forum/%23!msg/comp.os.cpm/TLHgIi16yTo/gupNB1ai8UQJ#!topic/comp.os.cpm/TLHgIi16yTo">[6]</a>. Originally presented at: <a href="/facts/Gary_Arlen_Kildall/yzxo0mLU">Kildall, Gary Arlen</a> (1977) [22–24 November 1976]. "A Simple Technique for Static Relocation of Absolute Machine Code". Written at <a href="/facts/Naval_Postgraduate_School/K7CDD8XD">Naval Postgraduate School</a>, Monterey, California, USA. In Titus, Harold A. (ed.). <a href="https://books.google.com/books?id=cyBGAQAAIAAJ&q=relocation"><i>Conference Record: Tenth Annual Asilomar Conference on Circuits, Systems and Computers: Papers Presented November 22–24, 1976</i></a>. <i>Asilomar Conference on Signals, Systems & Computers</i>. Asilomar Hotel and Conference Grounds, Pacific Grove, California, USA: Western Periodicals Company. pp. 420–424. <a href="/facts/ISSN_(identifier)/DPAflDvU">ISSN</a> <a href="https://search.worldcat.org/issn/1058-6393">1058-6393</a>. Retrieved 2021-12-06. (609 pages). (This "resize" method, named <i>page boundary relocation</i>, could be applied statically to a <a href="/facts/CP%2fM-80/n5rTgXdC">CP/M-80</a> disk image using <a href="/facts/MOVCPM/n5rTgXdC">MOVCPM</a> [pl] in order to maximize the <a href="/facts/Transient_Program_Area/n5rTgXdC">TPA</a> for programs to run. It was also utilized dynamically by the CP/M debugger <a href="/facts/Dynamic_Debugging_Tool/jmRMJtmj">Dynamic Debugging Tool</a> (DDT) to <a href="/facts/Self-relocation/gMw61ATA">relocate itself</a> into higher memory. The same approach was independently developed by <a href="/facts/Bruce_H._Van_Natta/zLkwNa0R">Bruce H. Van Natta</a> of <a href="/facts/IMS_Associates/zLkwNa0R">IMS Associates</a> to produce relocatable <a href="/facts/PL%2fM/GaCkqILu">PL/M</a> code. As <i>paragraph boundary relocation</i>, <a href="/facts/Self-relocation/gMw61ATA">another variant</a> of this method was later utilized by dynamically <a href="/facts/High_memory_area/NPW4zCVl">HMA</a> self-relocating <a href="/facts/Terminate-and-stay-resident_program/6croHFxf">TSRs</a> like <a href="/facts/KEYB_(DOS_command)/d72UqhJ3">KEYB</a>, <a href="/facts/SHARE_(DOS_command)/uNdsI1PF">SHARE</a>, and <a href="/facts/NLSFUNC_(DOS_command)/d72UqhJ3">NLSFUNC</a> under <a href="/facts/DR_DOS_6.0/GcCTKutW">DR DOS 6.0</a> and higher. A much more sophisticated and <a href="/facts/Byte_alignment/Bqk6zMNS">byte-level granular</a> method based on a somewhat similar approach was independently conceived and implemented by Matthias R. Paul and Axel C. Frinke for their <a href="/facts/Dynamic_dead-code_elimination/waQRsajr">dynamic dead-code elimination</a> to dynamically minimize the runtime footprint of resident drivers and TSRs (like FreeKEYB).)</li> <li>Mossip, Richard H. (September–October 1980). Written at Bloomingdale, New Jersey, USA. <a href="http://bitsavers.informatik.uni-stuttgart.de/magazines/S-100_Microsystems/v01n05.pdf">"Relocatable Code"</a> (PDF). <i><a href="/facts/S-100_Microsystems_(magazine)/u8j9G4aT">S-100 Microsystems</a></i>. Vol. 1, no. 5. Mountainside & Springfield, New Jersey, USA: <a href="/facts/Libes%2c_Inc./u8j9G4aT">Libes, Inc.</a> pp. 54–55. <a href="/facts/ISSN_(identifier)/DPAflDvU">ISSN</a> <a href="https://search.worldcat.org/issn/0199-7955">0199-7955</a>. ark:/13960/s2cfgkmxcwg. ark:/13960/s2qdm1t01nr. <a href="https://web.archive.org/web/20231127182118/http://bitsavers.informatik.uni-stuttgart.de/magazines/S-100_Microsystems/v01n05.pdf">Archived</a> (PDF) from the original on 2023-11-27. Retrieved 2023-11-27. <a href="https://archive.org/details/bitsavers_S100Micros_46725899/page/54/mode/1up">[7]</a><a href="https://archive.org/details/v1n5_20230524/page/54/mode/1up">[8]</a> (2 pages) (NB. Describes page boundary relocation and relocating assemblers.)</li> <li>Huitt, Robert; <a href="/facts/Gordon_Eubanks/fvBMbbhe">Eubanks, Gordon</a>; <a href="/facts/Thomas_Alan_Rolander/27HPXfpM">Rolander, Thomas "Tom" Alan</a>; Laws, David; Michel, Howard E.; Halla, Brian; <a href="/facts/John_Harrison_Wharton/WdMrX7qD">Wharton, John Harrison</a>; Berg, Brian; Su, Weilian; <a href="/facts/Scott_Kildall/TcHcEDAj">Kildall, Scott</a>; Kampe, Bill (2014-04-25). Laws, David (ed.). <a href="https://archive.computerhistory.org/resources/access/text/2014/06/102746909-05-01-acc.pdf">"Legacy of Gary Kildall: The CP/M IEEE Milestone Dedication"</a> (PDF) (video transscription). Pacific Grove, California, USA: <a href="/facts/Computer_History_Museum/Y9Q5XevR">Computer History Museum</a>. CHM Reference number: X7170.2014. <a href="https://web.archive.org/web/20141227142045/http://archive.computerhistory.org/resources/access/text/2014/06/102746909-05-01-acc.pdf">Archived</a> (PDF) from the original on 2014-12-27. Retrieved 2020-01-19. […] Laws: […] "dynamic relocation" of the OS. Can you tell us what that is and why it was important? […] <a href="/facts/Gordon_Eubanks/fvBMbbhe">Eubanks</a>: […] what <a href="/facts/Gary_Arlen_Kildall/yzxo0mLU">Gary</a> did […] was […] mind boggling. […] I remember the day at the <a href="/facts/Naval_Postgraduate_School/K7CDD8XD">school</a> 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 <a href="/facts/High_order_byte/wbrQnyu3">high order byte</a>. And so he created a <a href="/facts/Bitmap/Fr80ySnT">bitmap</a>. […] 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 <a href="/facts/CP%2fM/n5rTgXdC">CP/M</a> 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. […] <a href="/facts/Intel/SMF0gJJX">Intel</a> […] had the <a href="/facts/Little-endian/wbrQnyu3">bytes reversed</a>, right, for the memory addresses. But they were always in the same place, so you could relocate it on a <a href="/facts/256_byte_boundary/Bqk6zMNS">256 byte boundary</a>, 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 […] <a href="https://ethw.org/Milestones:The_CP/M_Microcomputer_Operating_System,_1974">[9]</a><a href="https://www.youtube.com/watch?v=HO6IPpL0y8g">[10]</a> (33 pages)</li> <li>Lieber, Eckhard; von Massenbach, Thomas (1987). "CP/M 2 lernt dazu. Modulare Systemerweiterungen auch für das 'alte' CP/M". <i><a href="/facts/C%2527t_-_magazin_f%25C3%25BCr_computertechnik/6pbWadaU">c't - magazin für computertechnik</a></i> (part 1) (in German). 1987 (1). <a href="/facts/Heise_Verlag/jbFl3SV2">Heise Verlag</a>: 124–135; Lieber, Eckhard; von Massenbach, Thomas (1987). "CP/M 2 lernt dazu. Modulare Systemerweiterungen auch für das 'alte' CP/M". <i><a href="/facts/C%2527t_-_magazin_f%25C3%25BCr_computertechnik/6pbWadaU">c't - magazin für computertechnik</a></i> (part 2) (in German). 1987 (2). <a href="/facts/Heise_Verlag/jbFl3SV2">Heise Verlag</a>: 78–85; Huck, Alex (2016-10-09). <a href="http://hc-ddr.hucki.net/wiki/doku.php/cpm:rsm:ct">"RSM für CP/M 2.2"</a>. <i>Homecomputer DDR</i> (in German). <a href="https://web.archive.org/web/20161125023237/http://hc-ddr.hucki.net/wiki/doku.php/cpm%3Arsm%3Act">Archived</a> from the original on 2016-11-25. Retrieved 2016-11-25.</li> <li>Guzis, Charles "Chuck" P. (2015-03-16). <a href="http://www.vcfed.org/forum/showthread.php?46767-CP-M-assembly-language-programming&p=360328#post360328">"Re: CP/M assembly language programming"</a>. <i>Vintage Computer Forum</i>. Genre: CP/M and MP/M. <a href="https://web.archive.org/web/20200201052736/http://www.vcfed.org/forum/showthread.php?46767-CP-M-assembly-language-programming&p=360328">Archived</a> from the original on 2020-02-01. Retrieved 2020-02-01. […] Ever wonder how <a href="/facts/MOVCPM/n5rTgXdC">MOVCPM</a> [pl] works? Since the <a href="/facts/BDOS/n5rTgXdC">BDOS</a> and <a href="/facts/Console_Command_Processor/n5rTgXdC">CCP</a> 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 <a href="/facts/8080/32FvNTcD">8080</a> code, since <a href="/facts/Relative_addressing/Qv0N3nND">relative addressing</a> 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 <a href="/facts/MP%2fM/slxQMszr">MP/M</a> even uses this scheme to construct its page-relocatable files. You simply assemble the source program <i>twice</i> with the second assembly origin 100H (256 bytes) higher than the first. The two binary images are then compared, byte for byte, and a <a href="/facts/Bitmap/Fr80ySnT">map</a> 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. […]</li> <li>Guzis, Charles "Chuck" P. (2015-07-29). <a href="http://www.vcfed.org/forum/showthread.php?48532-How-does-MOVCPM-COM-work&p=376438#post376438">"Re: How does MOVCPM.COM work?"</a>. <i>Vintage Computer Forum</i>. Genre: CP/M and MP/M. <a href="https://web.archive.org/web/20200201053054/http://www.vcfed.org/forum/showthread.php?48532-How-does-MOVCPM-COM-work=&p=376438">Archived</a> from the original on 2020-02-01. Retrieved 2020-02-01. […] <a href="/facts/MOVCPM/n5rTgXdC">MOVCPM</a> [pl] uses an early type of PRL format. Basically, <a href="/facts/CP%2fM/n5rTgXdC">CP/M</a> is assembled twice; the second time is 100H bytes offset. The two binaries are compared and a <a href="/facts/Bitmap/Fr80ySnT">bitmap</a> constructed. A set bit implies that the <a href="/facts/High-order_byte/wbrQnyu3">high-order byte</a> of an address is to be adjusted. Low order address bytes are not affected; hence, "Page relocatable 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. […]</li> <li>Guzis, Charles "Chuck" P. (2016-11-08). <a href="http://www.vcfed.org/forum/showthread.php?54822-Is-it-safe-to-use-RST-28h-in-CP-M-assembly-programs&p=434954#post434954">"Re: Is it safe to use RST 28h in CP/M assembly programs?"</a>. <i>Vintage Computer Forum</i>. Genre: CP/M and MP/M. <a href="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">Archived</a> from the original on 2020-02-01. Retrieved 2020-02-01. […] I've referenced PRL files and how they originally got their start with <a href="/facts/MOVCPM/n5rTgXdC">MOVCPM</a> [pl], but became an integral part of <a href="/facts/MP%2fM/slxQMszr">MP/M</a> and <a href="/facts/CP%2fM_3.0/n5rTgXdC">CP/M 3.0</a>. But PRL files use a <a href="/facts/Bitmap/Fr80ySnT">bit map</a> 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 <a href="/facts/8080/32FvNTcD">8080</a> code which doesn't have relative jumps, but may be a consideration for <a href="/facts/Z80/Jeqe2h55">Z80</a> 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 <a href="/facts/Runtime_(program_lifecycle_phase)/lCxKsrzB">run-time</a> 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. […]</li> <li>Roth, Richard L. (February 1978) [1977]. <a href="https://archive.org/stream/dr_dobbs_journal_vol_03/dr_dobbs_journal_vol_03_djvu.txt">"Relocation Is Not Just Moving Programs"</a>. <i><a href="/facts/Dr._Dobb%2527s_Journal_of_Computer_Calisthenics_%2526_Orthodontia/6Qov1hUK">Dr. Dobb's Journal of Computer Calisthenics & Orthodontia</a></i>. 3 (2). Ridgefield, California, USA: <a href="/facts/People%2527s_Computer_Company/Dwhp3hsx">People's Computer Company</a>: 14–20 (70–76). <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-8104-5490-4. #22. <a href="https://web.archive.org/web/20190420010941/https://archive.org/stream/dr_dobbs_journal_vol_03/dr_dobbs_journal_vol_03_djvu.txt">Archived</a> from the original on 2019-04-20. Retrieved 2019-04-19.</li> <li>Calingaert, Peter (1979) [1978-11-05]. "8.2.2 Relocating Loader". Written at <a href="/facts/University_of_North_Carolina_at_Chapel_Hill/Wrd3mBWz">University of North Carolina at Chapel Hill</a>. In <a href="/facts/Ellis_Horowitz/zWdrtU2s">Horowitz, Ellis</a> (ed.). <a href="https://archive.org/details/assemblerscompil00cali"><i>Assemblers, Compilers, and Program Translation</i></a>. Computer software engineering series (1st printing, 1st ed.). Potomac, Maryland, USA: <a href="/facts/Computer_Science_Press%2c_Inc./sJ3zg32G">Computer Science Press, Inc.</a> pp. <a href="https://archive.org/details/assemblerscompil00cali/page/237">237</a>–241. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-914894-23-4. <a href="/facts/ISSN_(identifier)/DPAflDvU">ISSN</a> <a href="https://search.worldcat.org/issn/0888-2088">0888-2088</a>. <a href="/facts/LCCN_(identifier)/5hb5Cw2V">LCCN</a> <a href="https://lccn.loc.gov/78-21905">78-21905</a>. Retrieved 2020-03-20. (2+xiv+270+6 pages)</li> <li><a href="https://www.fileformat.info/format/ms-obj/corion.htm"><i>The Microsoft OBJ File Format</i></a>. <a href="/facts/Microsoft/nGIDDXdx">Microsoft</a>, Product Support Services. Application Note SS0288. <a href="https://archive.today/20170909092856/http://www.fileformat.info/format/ms-obj/corion.htm">Archived</a> from the original on 2017-09-09. Retrieved 2017-08-21.</li> <li><a href="/facts/Andrew_Stuart_Tanenbaum/fFuDWfaR">Tanenbaum, Andrew Stuart</a>; Bos, Herbert (2015). <i>Modern Operating Systems</i> (4 ed.). <a href="/facts/Pearson_Education_Inc./UNq9rds8">Pearson Education Inc.</a> <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-13359162-0.</li> <li>Elliott, John C. (2012-06-05) [2000-01-02]. <a href="https://www.seasip.info/Cpm/prl.html">"PRL file format"</a>. <i>seasip.info</i>. <a href="https://web.archive.org/web/20200126182027/https://www.seasip.info/Cpm/prl.html">Archived</a> from the original on 2020-01-26. Retrieved 2020-01-26. […] A PRL file is a relocatable binary file, used by <a href="/facts/MP%2fM/slxQMszr">MP/M</a> and <a href="/facts/CP%2fM_Plus/n5rTgXdC">CP/M Plus</a> for various modules other than <a href="/facts/.COM_file/3SXSyIxa">.COM files</a>. The file format is also used for FID files on the <a href="/facts/Amstrad_PCW/eUHIck5p">Amstrad PCW</a>. There are several file formats which use versions of PRL: SPR (System PRL), RSP (Resident System Process). LINK-80 can also produce <a href="/facts/.OVL/4njAmCUj">OVL</a> (<a href="/facts/Overlay_(programming)/4njAmCUj">overlay</a>) files, which have a PRL header but are not relocatable. <a href="/facts/Graphics_System_Extension/oE8BpWAq">GSX</a> drivers are in PRL format; so are <a href="/facts/Resident_System_Extension/n5rTgXdC">Resident System Extensions</a> (.RSX). […] <a href="https://www.seasip.info/Cpm/amsfid.html#PRL%20file%20format">[11]</a></li> <li>Elliott, John C. (2012-06-05) [2000-01-02]. <a href="https://www.seasip.info/Cpm/rel.html">"Microsoft REL format"</a>. <i>seasip.info</i>. <a href="https://web.archive.org/web/20200126181737/https://www.seasip.info/Cpm/rel.html">Archived</a> from the original on 2020-01-26. Retrieved 2020-01-26. […] The REL format is generated by <a href="/facts/Microsoft/nGIDDXdx">Microsoft</a>'s M80 and <a href="/facts/Digital_Research/9bYQl2pI">Digital Research</a>'s RMAC. […]</li> <li>feilipu (2018-09-05) [2018-09-02]. <a href="https://github.com/z88dk/z88dk/issues/935">"Support for PRL, page relocatable executable for MP/M"</a>. <i>z88dk</i>. <a href="https://web.archive.org/web/20200201053652/https://github.com/z88dk/z88dk/issues/935">Archived</a> from the original on 2020-02-01. Retrieved 2020-01-26. […] Out of the assembled <a href="/facts/Microsoft/nGIDDXdx">Microsoft</a> .REL files the linker has to generate a .PRL format executable for <a href="/facts/MP%2fM/slxQMszr">MP/M</a>. The .PRL format is essentially a <a href="/facts/.COM_file/3SXSyIxa">.COM file</a> 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. […]</li> <li>Brothers, Hardin (April 1983). <a href="https://archive.org/details/80-microcomputing-magazine-1983-04">"Understanding Relocatable Code"</a>. <i><a href="/facts/80_Micro/4TnaCcuN">80 Micro</a></i>. The Next Step (39). <a href="/facts/1001001%2c_Inc./4TnaCcuN">1001001, Inc.</a>: <a href="https://archive.org/details/80-microcomputing-magazine-1983-04/page/n37">38</a>, 40, 42, 45. <a href="/facts/ISSN_(identifier)/DPAflDvU">ISSN</a> <a href="https://search.worldcat.org/issn/0744-7868">0744-7868</a>. Retrieved 2020-02-06. <a href="https://archive.org/stream/80-microcomputing-magazine-1983-04/80Microcomputing_0483_djvu.txt">[12]</a><a href="https://archive.org/download/80-microcomputing-magazine-1983-04/80Microcomputing_0483_text.pdf">[13]</a></li> <li>Brothers, Hardin (April 1985). <a href="https://archive.org/details/80-microcomputing-magazine-1985-04">"Relocatable Programs: Microcomputing's Hoboes"</a>. <i><a href="/facts/80_Micro/4TnaCcuN">80 Micro</a></i>. The Next Step (63). <a href="/facts/CW_Communications%2fPeterborough%2c_Inc./fEvubHhX">CW Communications/Peterborough, Inc.</a>: <a href="https://archive.org/details/80-microcomputing-magazine-1985-04/page/n99">98</a>, 100, 102–103. <a href="/facts/ISSN_(identifier)/DPAflDvU">ISSN</a> <a href="https://search.worldcat.org/issn/0744-7868">0744-7868</a>. Retrieved 2020-02-06. <a href="https://archive.org/stream/80-microcomputing-magazine-1985-04/80Microcomputing_0485_djvu.txt">[14]</a><a href="https://archive.org/download/80-microcomputing-magazine-1985-04/80Microcomputing_0485_text.pdf">[15]</a></li> <li>Sage, Jay (May–June 1988). Carlson, Art (ed.). <a href="https://archive.org/details/the-computer-journal-32">"ZCPR 3.4 - Type-4 Programs"</a>. <i>The Computer Journal (TCJ) - Programming, User Support, Applications</i>. ZCPR3 Corner (32). Columbia Falls, Montana, USA: <a href="https://archive.org/details/the-computer-journal-32/page/n10/mode/1up">10</a>–17 [<a href="https://archive.org/details/the-computer-journal-32/page/n15/mode/1up">15</a>–16]. <a href="/facts/ISSN_(identifier)/DPAflDvU">ISSN</a> <a href="https://search.worldcat.org/issn/0748-9331">0748-9331</a>. ark:/13960/t1wd4v943. Retrieved 2021-11-29. <a href="https://archive.org/stream/the-computer-journal-32/tcj_32_May-June_1988_djvu.txt">[16]</a><a href="https://archive.org/download/the-computer-journal-32/tcj_32_May-June_1988_text.pdf">[17]</a></li> <li>Mitchell, Bridger (July–August 1988). Carlson, Art (ed.). <a href="https://archive.org/details/the-computer-journal-33">"Z3PLUS & Relocation - Information on ZCPR3PLUS, and how to write self relocating Z80 code"</a>. <i>The Computer Journal (TCJ) - Programming, User Support, Applications</i>. Advanced CP/M (33). Columbia Falls, Montana, USA: <a href="https://archive.org/details/the-computer-journal-33/page/n10/mode/1up">9</a>–15. <a href="/facts/ISSN_(identifier)/DPAflDvU">ISSN</a> <a href="https://search.worldcat.org/issn/0748-9331">0748-9331</a>. ark:/13960/t36121780. Retrieved 2020-02-09. <a href="https://archive.org/stream/the-computer-journal-33/tcj_33_July-August_1988_djvu.txt">[18]</a><a href="https://archive.org/download/the-computer-journal-33/tcj_33_July-August_1988_text.pdf">[19]</a></li> <li>Sage, Jay (September–October 1988). Carlson, Art (ed.). <a href="https://archive.org/details/the-computer-journal-34">"More on relocatable code, PRL files, ZCPR34, and Type-4 programs"</a>. <i>The Computer Journal (TCJ) - Programming, User Support, Applications</i>. ZCPR3 Corner (34). Columbia Falls, Montana, USA: <a href="https://archive.org/details/the-computer-journal-34/page/n20/mode/1up">20</a>–25. <a href="/facts/ISSN_(identifier)/DPAflDvU">ISSN</a> <a href="https://search.worldcat.org/issn/0748-9331">0748-9331</a>. ark:/13960/t0ks7pc39. Retrieved 2020-02-09. <a href="https://archive.org/stream/the-computer-journal-34/tcj_34_September-October_1988_djvu.txt">[20]</a><a href="https://archive.org/download/the-computer-journal-34/tcj_34_September-October_1988_text.pdf">[21]</a><a href="https://web.archive.org/web/20170831184307/http://gaby.de/ftp/pub/cpm/znode51/articles/tcj/tcj34.ws">[22]</a></li> <li>Sage, Jay (January–February 1992). Carlson, Art; McEwen, Chris (eds.). <a href="https://archive.org/details/the-computer-journal-54">"Ten Years of ZCPR"</a>. <i>The Computer Journal (TCJ) - Programming, User Support, Applications</i>. Z-System Corner (54). S. Plainfield, New Jersey, USA: Socrates Press: <a href="https://archive.org/details/the-computer-journal-54/page/n4/mode/1up">3</a>–7. <a href="/facts/ISSN_(identifier)/DPAflDvU">ISSN</a> <a href="https://search.worldcat.org/issn/0748-9331">0748-9331</a>. ark:/13960/t89g6n689. Retrieved 2021-11-29. <a href="https://archive.org/stream/the-computer-journal-54/tcj_54_January-February_1992_djvu.txt">[23]</a><a href="https://archive.org/download/the-computer-journal-54/tcj_54_January-February_1992_text.pdf">[24]</a><a href="https://web.archive.org/web/20170831184437/http://gaby.de/ftp/pub/cpm/znode51/articles/tcj/tcj54.ws">[25]</a></li> <li>Sage, Jay (May–June 1992) [March–June 1992]. Carlson, Art; McEwen, Chris (eds.). <a href="https://archive.org/details/the-computer-journal-55">"Type-3 and Type-4 Programs"</a>. <i>The Computer Journal (TCJ) - Programming, User Support, Applications</i>. Z-System Corner - Some New Applications of Type-4 Programs (55). S. Plainfield, New Jersey, USA: Socrates Press: <a href="https://archive.org/details/the-computer-journal-55/page/n14/mode/1up">13</a>–19. <a href="/facts/ISSN_(identifier)/DPAflDvU">ISSN</a> <a href="https://search.worldcat.org/issn/0748-9331">0748-9331</a>. ark:/13960/t4dn54d22. Retrieved 2021-11-29. <a href="https://archive.org/stream/the-computer-journal-55/tcj_55_March-June_1992_djvu.txt">[26]</a><a href="https://archive.org/download/the-computer-journal-55/tcj_55_March-June_1992_text.pdf">[27]</a></li> <li>Ganssle, Jack (February 1992). <a href="http://www.ganssle.com/articles/arelocat.htm">"Writing Relocatable Code - Some embedded code must run at more than one address"</a>. <i>Embedded Systems Programming</i>. The Ganssle Group - Perfecting the Art of Building Embedded Systems / TGG. <a href="https://web.archive.org/web/20190718232931/http://www.ganssle.com/articles/arelocat.htm">Archived</a> from the original on 2019-07-18. Retrieved 2020-02-20.</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"Types of Object Code". iRMX 86 Application Loader Reference Manual (PDF). Intel. pp. 1-2–1-3. Archived (PDF) from the original on 2020-01-11. Retrieved 2020-01-11. […] 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 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 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. […] <a href="http://bitsavers.informatik.uni-stuttgart.de/pdf/intel/iRMX/iRMX_86_Rev_6_Mar_1984/146196_Burst/iRMX_86_Application_Loader_Reference_Manual.pdf" target="_blank">http://bitsavers.informatik.uni-stuttgart.de/pdf/intel/iRMX/iRMX_86_Rev_6_Mar_1984/146196_Burst/iRMX_86_Application_Loader_Reference_Manual.pdf</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Levine, John R. (2000) [October 1999]. "Chapter 1: Linking and Loading & Chapter 3: Object Files". Linkers and Loaders. The Morgan Kaufmann Series in Software Engineering and Programming (1 ed.). San Francisco, California, USA: Morgan Kaufmann. p. 5. ISBN 1-55860-496-0. OCLC 42413382. Archived from the original on 2012-12-05. Retrieved 2020-01-12. Code: [1][2] Errata: [3] <a href="1-55860-496-0" target="_blank">1-55860-496-0</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Levine, John R. (2000) [October 1999]. "Chapter 1: Linking and Loading & Chapter 3: Object Files". Linkers and Loaders. The Morgan Kaufmann Series in Software Engineering and Programming (1 ed.). San Francisco, California, USA: Morgan Kaufmann. p. 5. ISBN 1-55860-496-0. OCLC 42413382. Archived from the original on 2012-12-05. Retrieved 2020-01-12. Code: [1][2] Errata: [3] <a href="1-55860-496-0" target="_blank">1-55860-496-0</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Borland (1999-09-01) [1998-07-02]. "Borland article #15961: Coping with 'Fixup Overflow' messages". community.borland.com. Technical Information Database - Product: Borland C++ 3.1. TI961C.txt #15961. Archived from the original on 2008-07-07. Retrieved 2007-01-15. <a href="/wiki/Borland" target="_blank">/wiki/Borland</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>"Executable and Linkable Format (ELF)" (PDF). skyfree.org. Tool Interface Standards (TIS) Portable Formats Specification, Version 1.1. Archived (PDF) from the original on 2019-12-24. Retrieved 2018-10-01. <a href="http://www.skyfree.org/linux/references/ELF_Format.pdf" target="_blank">http://www.skyfree.org/linux/references/ELF_Format.pdf</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> </ol>