.COM files in DOS set all x86 segment registers to the same value and the SP (stack pointer) register to the offset of the last word available in the first 64 KB segment (typically FFFEh) or the maximum size of memory available in the block the program is loaded into for both, the program plus at least 256 bytes stack, whatever is smaller, thus the stack begins at the very top of the corresponding memory segment and works down from there.[3][4]

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A COM file is a type of simple executable file. On the Digital Equipment Corporation (DEC) VAX operating systems of the 1970s, .COM was used as a filename extension for text files containing commands to be issued to the operating system (similar to a batch file).[1] With the introduction of Digital Research's CP/M (a microcomputer operating system), the type of files commonly associated with COM extension changed to that of executable files. This convention was later carried over to DOS. Even when complemented by the more general EXE file format for executables, the compact COM files remained viable and frequently used under DOS.

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Taking advantage of this default behaviour, virus writers and other malicious programmers have used names like notepad.com for their creations, hoping that if it is placed in the same directory as the corresponding EXE file, a command or batch file may accidentally trigger their program instead of the text editor notepad.exe. Again, these .com files may in fact contain a .exe format executable.

Under CP/M 3, if the first byte of a COM file is C9h, there is a 256-byte header;[2] since C9h corresponds to the 8080 instruction RET, this means that the COM file will immediately terminate if run on an earlier version of CP/M that does not support this extension. (Because the instruction sets of the 8085 and Z80 are supersets of the 8080 instruction set, this works on all three processors.) C9h is an invalid opcode on the 8088/8086, and it will cause a processor-generated interrupt 6 exception in v86 mode on the 386 and later x86 chips. Since C9h is the opcode for LEAVE since the 80188/80186 and therefore not used as the first instruction in a valid program, the executable loader in some versions of DOS rejects COM files that start with C9h, avoiding a crash.

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The COM format is the original binary executable format used in CP/M (including SCP and MSX-DOS) as well as DOS. It is very simple; it has no header (with the exception of CP/M 3 files),[2] and contains no standard metadata, only code and data. This simplicity exacts a price: the binary has a maximum size of 65,280 (FF00h) bytes (256 bytes short of 64 KB) and stores all its code and data in one segment.

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Files may have names ending in .COM, but not be in the simple format described above; this is indicated by a magic number at the start of the file. For example, the COMMAND.COM file in DR DOS 6.0 is actually in DOS executable format, indicated by the first two bytes being MZ (4Dh 5Ah), the initials of Mark Zbikowski.

In the original DOS 1.x API, which was a derivative of the CP/M API, program termination of a .COM file would be performed by calling the INT 20h (Terminate Program) function or else INT 21h Function 0, which served the same purpose, and the programmer also had to ensure that the code and data segment registers contained the same value at program termination to avoid a potential system crash. Although this could be used in any DOS version, Microsoft recommended the use of INT 21h Function 4Ch for program termination from DOS 2.x onward, which did not require the data and code segment to be set to the same value.

Windows NT-based operating systems use the .com extension for a small number of commands carried over from MS-DOS days although they are in fact presently implemented as .exe files. The operating system will recognize the .exe file header and execute them correctly despite their technically incorrect .com extension. (In fact any .exe file can be renamed .com and still execute correctly.) The use of the original .com extensions for these commands ensures compatibility with older DOS batch files that may refer to them with their full original filenames. These commands are CHCP, DISKCOMP, DISKCOPY, FORMAT, MODE, MORE and TREE.[6]

Some computer virus writers have hoped to take advantage of modern computer users' likely lack of knowledge of the .com file extension and associated binary format, along with their more likely familiarity with the .com Internet domain name. E-mails have been sent with attachment names similar to "www.example.com". Unwary Microsoft Windows users clicking on such an attachment would expect to begin browsing a site named http://www.example.com/, but instead would run the attached binary command file named www.example, giving it full permission to do to their machine whatever its author had in mind.[citation needed]

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There is nothing malicious about the COM file format itself; this is an exploitation of the coincidental name collision between .com command files and .com commercial web sites.

Although the file format is the same in DOS and CP/M, .COM files for the two operating systems are not compatible; DOS COM files contain x86 instructions and possibly DOS system calls, while CP/M COM files contain 8080 instructions and CP/M system calls (programs restricted to certain machines could also contain additional instructions for 8085 or Z80).

In the Intel 8080 CPU architecture, only 65,536 bytes of memory could be addressed (address range 0000h to FFFFh). Under CP/M, the first 256 bytes of this memory, from 0000h to 00FFh were reserved for system use by the zero page, and any user program had to be loaded at exactly 0100h to be executed.[nb 1] COM files fit this model perfectly. Before the introduction of MP/M and Concurrent CP/M, there was no possibility of running more than one program or command at a time: the program loaded at 0100h was run, and no other.

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On Windows NT and derivatives (Windows 2000, Windows XP, Windows Vista, and Windows 7), the PATHEXT variable is used to override the order of preference (and acceptable extensions) for calling files without specifying the extension from the command line. The default value still places .com files before .exe files. This closely resembles a feature previously found in JP Software's line of extended command line processors 4DOS, 4OS2, and 4NT.

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Under DOS there is no memory management provided for COM files by the loader or execution environment. All memory is simply available to the COM file. After execution, the operating system command shell, COMMAND.COM, is reloaded. This leaves the possibilities that the COM file can either be very simple, using a single segment, or arbitrarily complex, providing its own memory management system. An example of a complex program is COMMAND.COM, the DOS shell, which provided a loader to load other COM or EXE programs. In the .COM system, larger programs (up to the available memory size) can be loaded and run, but the system loader assumes that all code and data is in the first segment, and it is up to the .COM program to provide any further organization. Programs larger than available memory, or large data segments, can be handled by dynamic linking, if the necessary code is included in the .COM program. The advantage of using the .COM rather than .EXE format is that the binary image is usually smaller and easier to program using an assembler.[5] Once compilers and linkers of sufficient power became available, it was no longer advantageous to use the .COM format for complex programs.

Since it lacks relocation information, it is loaded by the operating system at a pre-set address, at offset 0100h immediately following the PSP, where it is executed (hence the limitation of the executable's size): the entry point is fixed at 0100h.[nb 1] This was not an issue on 8-bit machines since they can address 64k of memory max, but 16-bit machines have a much larger address space, which is why the format fell out of use.

The .COM file name extension has no relation to the .com (for "commercial") top-level Internet domain name. However, this similarity in name has been exploited by malware writers.

It is possible to make a .COM file to run under both operating systems in form of a fat binary. There is no true compatibility at the instruction level; the instructions at the entry point are chosen to be equal in functionality but different in both operating systems, and make program execution jump to the section for the operating system in use. It is basically two different programs with the same functionality in a single file, preceded by code selecting the one to use.

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In DOS, if a directory contains both a COM file and an EXE file with same name, when no extension is specified the COM file is preferentially selected for execution. For example, if a directory in the system path contains two files named foo.com and foo.exe, the following would execute foo.com:

The format is still executable on many modern Windows NT-based platforms, but it is run in an MS-DOS-emulating subsystem, NTVDM, which is not present in 64-bit variants. COM files can be executed also on DOS emulators such as DOSBox, on any platform supported by these emulators.

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