After I run a com file many times, it always gives the same output.
SO although the source is changed and recompiled, I get the same answer
which waste me a lot of time to find out what's wrong.

???

What are you trying to run? what are the expected results?
what is your input files? do you have multiple executables with the same name in your path? Maybe another one is being executed.

Maybe I encountered a rare case but that phenomena can be reproduced.
What I am doing in the com file is to perform FPU instructions and print out some results.

Well, sometimes i find myself doing the same thing:
-executing an old compiled file instead of the new one... an my hours of debug based on wrong assumptions go to waste or "experience"

The real problem is not with the DOS window i presume...

Generally speaking: the problem is with the fact that computers do:
"exactly what you tell them to do"
and not:
"what you intend them to do"
neither:
"what you wanted to tell them to do"

Calling them "stupid" dosent help, they do not react and do not get better

Usually finding the fault requires understanding of every little aspect, and most of the times it is a wrong assumption on the programmers side OR of the programmer that programmed the tool that you are using --much more rarely, but yes it happens

So keep up the good work, check every little assumption you have made steep by steep (streamlineing) and sooner or later you will find the fault location and cause...

LiuJunfeng,
I suppose you're using FAT32 and long filenames. If so, it's "well-known" bug of this filesystem.
I write it with "", because nearly no-one, but MS knows about it. It appears in cmd/xcopy and other such stuff, but is worked-arround in windos explorer, so usually is masked.
(I've just recived some info about it, and if I'm right with this supposition, I'll try to translate some more details from Russian to post here)

well, I now just open a new command window and execute my app on the command line when I feel that DOS is executing an old compiled file instead of the new one.

Thanks for all your reply.