.com

.com has come to mean a certain kind of business model, rabidly over used in the late 90s as companies scrambled to get internet presence, often to the exclusion of any other kind, which caused the inflating of the dot com bubble and it's inevitable bursting. The pattern often followed these lines:

• Set up website and offer some kind of service.
• Instantly make vast amounts of money as over-excited users try out every web based service under the cyber sun.
• "Float" on the Nasdaq (a market set up exclusively for tech. companies).
• Start to perform badly because either the infrastructure of the company isn't really geared towards such massive use or users single out one favourite service provider (eg. Amazon) that isn't you.
• Go bankrupt or be bought out by a much larger company, which often then goes bankrupt itself..

Now a by-word for "Bad Idea"

The top-level international commercial domain.

*.com is technically reserved for commercial ventures which have an international presence or client base.

Using *.com for strictly national business concerns is tantamount DNS banditry - it denies valid domains to those who have a better use for them. Unfortunately, *.com has become something of a status symbol so that every corner shop or old lady knitting sweaters think they need one to claim a valid net-presence. Hopefully, education will rememdy this. Before the DNS runs out.

In MS-DOS, .com was the extension of executables - as far as I know, .com files were just raw binary with no headers, so they could be created with things like debug easily. IIRC, they were also limited to 64 kilobytes in size, which is why they were later largely replaced with .exe files. DOS 6 had only a handful of COM files.

One of the most widely remembered .com file is, of course, COMMAND.COM. (Also registered as a domain name? You betcha. =)

.COM files are loaded into a new segment with offset 0x100. You can write .COM files if you have any 16bit x86 assembler that can output binary files, just remember to offset 0x100 at the beginning.

A ret instruction will return to the operating system, as will the widely used mov ax, 4c00h; int 21h. IIRC, you have all memory auto-allocated to your program from the segment you're loaded in, to the top of 640kb.

.COM files can still be fun and/or useful to write since they have a smaller (0 byte) header, compared to .exe files.

If you search the internet, you should be able to find several size coding competitions where the winner is the person to write the smallest program to do some specific task, such as displaying the flag of Japan, a Brainfuck interpreter, etc.