Editing Fat binary (section)

===Combined COM-style binaries for CP/M-80 and DOS===
[[CP/M-80]], [[MP/M-80]], [[Concurrent CP/M]], [[CP/M Plus]], [[Personal CP/M-80]], [[SCP (operating system)|SCP]] and [[MSX-DOS]] executables for the [[Intel 8080]] (and [[Zilog]] [[Z80]]) processor families use the same [[.COM]] [[file extension]] as [[DOS]]-compatible operating systems for [[Intel 8086]] binaries.<ref group="nb" name="NB_CP/M-86"/> In both cases programs are loaded at offset +100h and executed by jumping to the first byte in the file.<ref name="Paul_2002_COM"/><ref name="Wilkinson_2005"/> As the [[opcode]]s of the two processor families are not compatible, attempting to start a program under the wrong operating system leads to incorrect and unpredictable behaviour.

In order to avoid this, some methods have been devised to build fat binaries which contain both a CP/M-80 and a DOS program, preceded by initial code which is interpreted correctly on both platforms.<ref name="Wilkinson_2005"/> The methods either combine two fully functional programs each built for their corresponding environment, or add [[code stub|stub]]s which cause the program to [[exit gracefully]] if started on the wrong processor. For this to work, the first few instructions (sometimes also called ''[[#Gadget|gadget header]]s''<ref name="Cha-Pak-Brumley-Lipton_2010"/>) in the .COM file have to be valid code for both 8086 and 8080 processors, which would cause the processors to branch into different locations within the code.<ref name="Cha-Pak-Brumley-Lipton_2010"/>
For example, the utilities in Simeon Cran's emulator MyZ80 start with the opcode sequence {{mono|EBh, 52h, EBh}}.<ref name="Wilkinson-Seligman-Drushel-Elliott-Harston_1999"/><ref name="Elliott_2009_PMARC"/> An 8086 sees this as a jump and reads its next instruction from offset +154h whereas an 8080 or compatible processor goes straight through and reads its next instruction from +103h.
A similar sequence used for this purpose is {{mono|EBh, 03h, C3h}}.<ref name="Christ_2012"/><ref name="Brehm_2016"/>
<!-- Using {{mono|EBh, 04h, EBh}} as start code, -->John C. Elliott's FATBIN<ref name="Elliott_1996_FATBIN"/><ref name="Elliott_1998_FATBIN"/><ref name="Elliott_2002_FBMAKE"/> is a utility to combine a CP/M-80 and a DOS .COM file into one executable.<ref name="Wilkinson-Seligman-Drushel-Elliott-Harston_1999"/><ref name="Elliott_2005_FATBIN-PMSFX"/> His derivative of the original [[PMsfx]] modifies archives created by Yoshihiko Mino's [[PMarc]] to be [[self-extractable archive|self-extractable]] under ''both'', CP/M-80 and DOS, starting with {{mono|EBh, 18h, 2Dh, 70h, 6Dh, 73h, 2Dh}} to also include the "-pms-" signature for self-extracting [[PMarc|PMA]] archives,<ref name="Elliott_1997_PMSFX2"/><ref name="Wilkinson-Seligman-Drushel-Elliott-Harston_1999"/><ref name="Elliott_2005_FATBIN-PMSFX"/><ref name="Elliott_2009_PMARC"/> thereby also representing a form of [[executable ASCII code]].

Another method to keep a DOS-compatible operating system from erroneously executing .COM programs for CP/M-80 and MSX-DOS machines<ref name="Wilkinson_2005"/> is to start the 8080 code with {{mono|C3h, 03h, 01h}}, which is decoded as a "RET" instruction by x86 processors, thereby gracefully exiting the program,<ref group="nb" name="NB_RET"/> while it will be decoded as "JP 103h" instruction by 8080 processors and simply jump to the next instruction in the program. Similar, the CP/M assembler Z80ASM+ by SLR Systems would display an error message when erroneously run on DOS.<ref name="Wilkinson-Seligman-Drushel-Elliott-Harston_1999"/>

Some [[CP/M-80 3.0]] .COM files may have one or more [[RSX (computing)|RSX]] overlays attached to them by [[GENCOM (CP/M command)|GENCOM]].<ref name="Elliott_CPM3_COM"/> If so, they start with an extra [[256 byte boundary|256-byte]] header (one [[page (computing)|page]]). In order to indicate this, the first byte in the header is set to [[magic byte]] {{mono|C9h}}, which works both as a signature identifying this type of COM file to the CP/M 3.0 [[executable loader]], as well as a "RET" instruction for 8080-compatible processors which leads to a graceful exit if the file is executed under older versions of CP/M-80.<ref group="nb" name="NB_RET"/>

{{mono|C9h}} is never appropriate as the first byte of a program for any x86 processor (it has different meanings for different generations,<ref group="nb" name="NB_C9"/> but is never a meaningful first byte); the executable loader in some versions of DOS rejects COM files that start with {{mono|C9h}}, avoiding incorrect operation.

Similar [[opcode overlapping|overlapping]] code sequences have also been devised for combined Z80/[[6502]],<ref name="Wilkinson-Seligman-Drushel-Elliott-Harston_1999"/> 8086/[[68000]]<ref name="Wilkinson-Seligman-Drushel-Elliott-Harston_1999"/> or x86/[[MIPS architecture|MIPS]]/[[ARM architecture|ARM]] binaries.<ref name="Cha-Pak-Brumley-Lipton_2010"/>