DOS executable (.com)
Dan Tobias (Talk | contribs) (→Introduction) |
m |
||
| Line 6: | Line 6: | ||
}} | }} | ||
==Introduction== | ==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: | + | 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 | *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 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. | ||
| Line 15: | Line 15: | ||
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. | 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 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. | + | 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. |
==References== | ==References== | ||
Revision as of 02:33, 9 May 2014
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 (
0x100) 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 0xe9 or oxeb 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 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.
References
- Wikipedia entry on COM files
- Entry on fileformat.info
