How to write a debugger ?

Debugger tool is very mystery for me.
Who can explain the mechanism of a debugger?

I want to implement a simple debugger program for 8086 and how to start my trip?

Thanks a lot!

Play with RosAsm, it has a debugger which is written using RosAsm so you can see how it does what it does as well. This is the best way to see one that is open source to get an idea. You can get it here:

http://betov.free.fr/RosAsm.html

RosAsm is a little different feature wise than most assemblers, and actually allows you to hold your source inside the PE itself, but looking at the debugger will give you and idea of how ASM debuggers work.

Hope this helps.

Are you planning on using the Debug API? If so Icezelion has a set of tutorials on his site implementing it here: http://win32assembly.online.fr/tut28.html