Hello,
I would like to know how are virtual devices (like NUL), implemented in MS-DOS.
Thanks in advance.

I looked on wikipedia, didn't find much except this: http://en.wikipedia.org/wiki/Virtual_Device

Hope that helps any.

weren't MS-DOS sources released / leaked ?

SHHHHHHHHHHHHHHHH....

http://www.singlix.org/trdos/specs.html

You could also take a look at how http://www.freedos.org/ DOeS it.

If your looking for another examples (other than Dos), i think there was one in TITAN-OS, the OS written by Tomasz Grysztar.
But i maybe wrong.

A virtual device in DOS is just a device driver like the ones that load from the CONFIG.SYS file - they have a device name (like NUL and CON) and usually process I/O (NUL just ignores the I/O and returns, for example).

As a side note, this is why you can't call a file 'nul' or 'con' - even in win32 (?not sure) - because it already exists.