DOS executable (.com) - Revision history

Jsummers: /* See also */

2025-08-08T13:09:07Z

‎See also

Sembiance: /* Sample files */

2024-02-16T19:27:27Z

‎Sample files

Sembiance: Added sample files

2021-01-14T21:36:09Z

Added sample files

Jsummers: Category:MS-DOS

2017-03-29T16:25:21Z

Category:MS-DOS

Dan Tobias at 23:31, 7 October 2016

2016-10-07T23:31:44Z

Felix at 18:56, 27 August 2015

2015-08-27T18:56:56Z

Jsummers at 02:33, 9 May 2014

2014-05-09T02:33:29Z

Dan Tobias: /* Introduction */

2012-12-05T23:47:01Z

‎Introduction

Darkstar: completely rewritten the page

2012-12-05T23:02:11Z

completely rewritten the page

New page

{{FormatInfo
|subcat=Executables
|extensions={{ext|com}}
|mimetypes=
{{mimetype|application/octet-stream}}
}}
==Introduction==
On DOS and early Windows systems, files with extension .com are a special type of 16 bit executable which most of the time can only be run from the commandline. The extension comes from [[CP/M]] and stands for "command program" (or some variant thereof). As such they predate the more common [[EXE]] format, and are quite limited compared to the newer format:
*COM files contain only one area of combined code and data. There is no segmentation, no seperate code and data segments, and (more importantly) no relocation table
*They contain the raw program binary, without any header, which will be mapped into memory at a specific offset (<code>0x100</code>) in a (possibly random) segment. Thus they resemble some arbitrary [[ROM]] (.bin) format.
*They can be at most around 64kb big (65280 bytes to be exact)
*They can only use relative adressing, since the position in memory where they are loaded is not known to the program (but can be determined through an API call)
A good example is '''command.com''', the 16 bit command line interface of [[MS-DOS]].

Since COM files contain directly executable code, most of them start with a simple <code>0xe9</code> or <code>oxeb</code> byte, representing a short JUMP to the "real" entry point of the program. But this is not set in stone, and there are lots of COM files that have a different first byte

Sometimes, COM files are larger than 64kb and start with a "MZ" or "ZM" signature in the first 2 bytes. These are in fact simply renamed [[EXE]] files, the original DOS kernel did not care for the file extension but only looked at the first 2 bytes to determine the file format (COM or EXE) of the program to be run.

==References==
*[http://en.wikipedia.org/wiki/COM_file Wikipedia entry] on COM files
*[http://www.fileformat.info/format/com/corion.htm Entry] on fileformat.info
[[category:File formats with extension .com]]

@@ Line 10: / Line 10: @@
 == See also ==
-* [[TLD .com]]
+* [[Com]] (disambiguation page)
 == Sample files ==

@@ Line 13: / Line 13: @@
 == Sample files ==
-* https://telparia.com/fileFormatSamples/executable/com/
+* {{DexvertSamples|executable/com}}
 == References ==

@@ Line 11: / Line 11: @@
 == See also ==
 * [[TLD .com]]
 == References ==

@@ Line 3: / Line 3: @@
   | extensions = {{ext|com}}
 }}
-'''COM''' ("command file") is the native executable format of [[wikipedia:CP/M|CP/M]]. The format was inherited by the DOS family of operating systems; MS-DOS 1.0 introduced [[MS-DOS EXE|MZ]] as a replacement. COM files are raw, flat binaries without relocations; there are no identifying headers or magic numbers, except for a variant introduced in CP/M 3 which contains a 256-byte header.
+'''COM''' ("command file") is the native executable format of [[wikipedia:CP/M|CP/M]]. The format was inherited by the DOS family of operating systems; [[MS-DOS]] 1.0 introduced [[MS-DOS EXE|MZ]] as a replacement. COM files are raw, flat binaries without relocations; there are no identifying headers or magic numbers, except for a variant introduced in CP/M 3 which contains a 256-byte header.
 On PC computers, COM files are loaded at fixed offset 0x100 in an arbitrary [[wikipedia:x86 memory segmentation#Real mode|real mode memory segment]]. The entry point is located at the start of the file. Many COM files start with a jump instruction (opcode <code>0xe9</code> or <code>0xeb</code>), but this isn't required. Since the executable is limited to a single segment, the maximum size of a COM file is 65280 (0xff00) bytes.
-Some files carrying the <code>.com</code> extension are in fact in the [[MS-DOS EXE|MZ]] format: for example, the <code>COMMAND.COM</code> and <code>EDIT.COM</code> binaries distributed with Windows 95 and later are MZ executables. The DOS loader can detect this and load the file appropriately.
+Some files carrying the <code>.com</code> extension are in fact in the [[MS-DOS EXE|MZ]] format: for example, the <code>COMMAND.COM</code> and <code>EDIT.COM</code> binaries distributed with [[Windows 95]] and later are MZ executables. The DOS loader can detect this and load the file appropriately.
 == See also ==
@@ Line 15: / Line 15: @@
 * [[Wikipedia: COM file]]
 * [http://www.fileformat.info/format/com/corion.htm Entry] on fileformat.info

@@ Line 8: / Line 8: @@
 Some files carrying the <code>.com</code> extension are in fact in the [[MS-DOS EXE|MZ]] format: for example, the <code>COMMAND.COM</code> and <code>EDIT.COM</code> binaries distributed with Windows 95 and later are MZ executables. The DOS loader can detect this and load the file appropriately.
 == References ==
 * [[Wikipedia: COM file]]
 * [http://www.fileformat.info/format/com/corion.htm Entry] on fileformat.info

@@ Line 1: / Line 1: @@
 {{FormatInfo
-|subcat=Executables
+ | subcat = Executables
-|extensions={{ext|com}}
+ | extensions = {{ext|com}}
-|mimetypes=
-{{mimetype|application/octet-stream}}
 }}
-==Introduction==
+'''COM''' ("command file") is the native executable format of [[wikipedia:CP/M|CP/M]]. The format was inherited by the DOS family of operating systems; MS-DOS 1.0 introduced [[MS-DOS EXE|MZ]] as a replacement. COM files are raw, flat binaries without relocations; there are no identifying headers or magic numbers, except for a variant introduced in CP/M 3 which contains a 256-byte header.
-On DOS and early Windows systems, files with extension .com are a special type of 16 bit executable which most of the time can only be run from the commandline. The extension comes from [[CP/M]] and stands for "command program" (or some variant thereof). As such they predate the more common [[MS-DOS EXE|EXE]] format, and are quite limited compared to the newer format:
-*COM files contain only one area of combined code and data. There is no segmentation, no seperate code and data segments, and (more importantly) no relocation table
-*They contain the raw program binary, without any header, which will be mapped into memory at a specific offset (<code>0x100</code>) in a (possibly random) segment. Thus they resemble some arbitrary [[ROM]] (.bin) format.
-*They can be at most around 64kb big (65280 bytes to be exact)
-*They can only use relative adressing, since the position in memory where they are loaded is not known to the program (but can be determined through an API call)
-A good example is '''command.com''', the 16 bit command line interface of [[MS-DOS]].
-Since COM files contain directly executable code, most of them start with a simple <code>0xe9</code> or <code>oxeb</code> byte, representing a short JUMP to the "real" entry point of the program. But this is not set in stone, and there are lots of COM files that have a different first byte.
+On PC computers, COM files are loaded at fixed offset 0x100 in an arbitrary [[wikipedia:x86 memory segmentation#Real mode|real mode memory segment]]. The entry point is located at the start of the file. Many COM files start with a jump instruction (opcode <code>0xe9</code> or <code>0xeb</code>), but this isn't required. Since the executable is limited to a single segment, the maximum size of a COM file is 65280 (0xff00) bytes.
-Sometimes, COM files are larger than 64kb and start with a "MZ" or "ZM" signature in the first 2 bytes. These are in fact simply renamed [[MS-DOS EXE|EXE]] files, the original DOS kernel did not care about the file extension but only looked at the first 2 bytes to determine the file format (COM or EXE) of the program to be run.
+Some files carrying the <code>.com</code> extension are in fact in the [[MS-DOS EXE|MZ]] format: for example, the <code>COMMAND.COM</code> and <code>EDIT.COM</code> binaries distributed with Windows 95 and later are MZ executables. The DOS loader can detect this and load the file appropriately.
-==References==
+== References ==
-*[http://en.wikipedia.org/wiki/COM_file Wikipedia entry] on COM files
+* [[Wikipedia: COM file]]
-*[http://www.fileformat.info/format/com/corion.htm Entry] on fileformat.info
+* [http://www.fileformat.info/format/com/corion.htm Entry] on fileformat.info
-[[category:File formats with extension .com]]