You must preserve ESI,EDI,EBX,EBP.
Others EAX,EDX,ECX are free to trash.
http://en.wikipedia.org/wiki/X86_calling_conventions#Intel_ABI
More links at the bottom...

IIRC, here's a thread somewhere in Windows that says some WinAPI functions thrash some regs they shouldn't. I'll look to see if I can find it.

Are you using 32 or 64 bit? 64's really hard, IMO.

There's a different convention for 64 bit API calls. It's on the page bitshifter provided.

Similar discussion here: Winapi will affect register?

It has to be very certain. Otherwise how do you think your browser's running? Think about how many calls to APIs it makes a second, but it remains uncrashed(I assume, since you posted.).

That thread I mentioned may be non existant. I found one where ManOfSteel challenged Pirata Derick(Sorry if I butchered your name.) to find a WinAPI that corrupts regs. He would've found it if one existed, I bet.

I've said somewhere that EnumThreadWindow (or whatever) clovered EBX or EDI in Win98SE, however when I've tried to reproduce the problem I couldn't so probably I was virused at that time without noticing it.

Sorry for the very long delay, but i had more serius thing to do.

I'm sad for the group of people in this forum who think they are like divinity:

They think to have all the knowledge at all, so they act to be superior from all the other users...

They think they are authorized (from their own godness) to answer
in a bad manner, joking with all the others users considered "inferior"
by them....

But with all them knowledge, they can't understand they are only
poor illused men.

Great forums like FASM BOARD are always ruined by these little group of men.
When i understood this i went away from here pacefully.

Casually i saw the upper post joke of Tyler and i wanted to answer for the last time.
Life teached me there're three ways to say concepts to people:

The first way is talking with them... like i did years ago.
The second way is explaining it cleary another time... like i'm doing now.

And i will explain that concept in a little strange polite way to this
group and specifically to 2 illused users in this wonderful board:

Offending people thought terminals is an easy way for cowards...
The Difference is only repeating it face to face.
Why don't you, Tiller (sorry if i butchered your name), and ManOfShit
(sorry if i butchered your idiot alias) both came to me, here in Rome at Quadraro neighbourhood.

I will "touch" your bones so you, Tiller, and the other "cabròn" ManOfShit
(I repeat i'm very sorry to butcher you idiot alias) will not be able to
use any keyboards to offend others users no more.

THIS IS THE THIRD WAY, when the last two ways didn't work.
Come here to me, or live in your illusions, poor men.
The other things are meaningless... simply i don't care.

Maybe your knowledge could save you from me, there... hahahahah

P.S.

ManOfSteel was too busy to start and looking for the function
in "an" old version of windows (because i'll not say to you where) that corrupt
the outgoing ESI register when exiting in that function documented as STDCALL.

Probably for a bug. It happens when you execute it with one of bool parameter set to FALSE.
But microsoft corrected it on the seguent version and the poor ManOfSteel is unable to
verify that, because he is STEEEEEEL too busy.

Now i can return to my researches on cybernetics.