This is an ATA command F5h ("Security Freeze Lock") related question.
Dell Latitude C840 (BIOS revision A13--the latest) freezes all hard drives upon booting, so nobody could change any "ATA Security" hard drive settings. (That's the infamous feature that most of the computers have). ATA specifications state that the only way to unfreeze the hard drive is to turn the power off, or do a "cold boot" through a hardware reset.
However, I noticed that the BIOS has an ability to unfreeze the hard drive without rebooting. Here are the steps:
1. Boot up the computer into DOS, or into Windows 98 (but not into Windows XP, because [Fn-F1] doesn't work there).
2. While at the DOS prompt, press [Fn-F1], which will get you into BIOS Setup.
3. Start changing the hard drive's password.
4. After typing and re-typing a new password, press [ENTER] and then listen carefully. You'll hear that the hard drive stops spinning, stays quiet for a couple of seconds, and starts spinning again.
5. After that, the BIOS screen will show that a new hard drive password is in effect.
6. Press [ESCAPE] to return to DOS prompt and continue working on your projects.
So everything is fine--but wait a minute. When you entered the BIOS screen by pressing [Fn-F1], the hard drive was frozen. You were not supposed to be able to change the password--or any ATA Security settings--while it was frozen. You were supposed to turn the power off or "cold reboot" the computer to unfreeze the hard drive first. So, the BIOS must have somehow unfrozen the hard drive without rebooting the computer. How does it do that? I understand the answer can be found by disassembling the BIOS, but that's a very big project. Maybe somebody already knows the answer?

When you change the password, does it ask you for the current password first?
I assume that this is thought to say, well if you know the password, then it is ok to unfreeze the HDD. Probably with a software power down and power up routine.

If it froze the HDD everytime with no way to get out of the freeze, how could you ever change things?

I admit I haven't looked at Dell BIOS too much. I have around 7 different model Dells in a network I can look at if the behaviour is indeed suspicious or wrong(bug)

Yes, of course, it asks for a current password first.
But the software power down and power up routine you mentioned is probably what I am looking for. Would it be possible to post an Assembly language code of such routine?
I don't know any ATA commands that would unfreeze a hard drive. Commands like E1h "IDLE IMMEDIATE" and E0h "STANDBY IMMEDIATE" result in the hard drive spinning down, but don't unfreeze it. It stays frozen until you press the power button.
This is not a bug in the BIOS, this is a correct behavior; I just want to know how BIOS unfreezes the hard drive without rebooting the computer or physically turning off power.

VSC = Vendor Specific Commands I assume they can do with them whatever they please without conforming to any standard such as ATA commands.

I am not really into reversing HDD Firmware. Someone else may be able to help, or you could in essence MITM or sniff Hard disk communication. http://www.perisoft.net/bushound