The reason for interrupting the POST process during boot-up is generally to make changes to the equipment list of the CMOS. You may need to be speedy to do this, for a modern machine will speed through the POST. What you need to do is hit a magic key, or combination of keys.
Different BIOS chips will have different interrupt keys. The most frequent seems to be just hold the "Delete" or "Insert" key. But the Phoenix BIOS will use "Control-Alt-Escape" -- meaning you have to hold all three keys at once.
Here is a list of some BIOS chip interrupt keys..
|BIOS chip||Interrupt keys|
|AMI BIOS||Del key during the POST|
|DTK BIOS||Esc key during the POST|
|IBM PS/2 BIOS||Ctrl-Alt-Ins after Ctrl-Alt-Del|
|Phoenix BIOS||Ctrl-Alt-Esc or Ctrl-Alt-S|
From: mike at pencom.com (Mike Heath)
Some 286 machines don't have a CMOS configuration menu in the BIOS. They require a software CMOS setup program. If you don't have the Installation and/or Diagnostics diskette for your machine, you can try using a shareware/freeware program. Try looking in:
If there is no reset button, or (as happens) the BIOS does not respond to the reset button, you will have to power down and start up again. PLEASE: allow time for the hard drives to come to a stop, since powering up hard drives while they are still slowing down could damage them.
If things are so screwed up that nothing responds (perhaps you or someone has set the CMOS to ignore the keyboard?), or the POST encounters an inexplicable error and goes catatonic, know that the whole of the CMOS can be reset to "default" values, by moving a jumper on the motherboard temporarily to another position.
This is generally a three pin jumper set at the rear of the board, but near the on-board battery. You move the jumper from 2-3 to 1-2, and wait a minute. Don't confuse this with the 4-pin jumper set which accepts an auxillary battery.
At times the CMOS battery will be dead. These are either a small black case with a 4 pin connector, or a slip-in flat battery. Replace it.
You may find some batteries (as Ni-Cads) soldered to the board. At times they go dead, for Ni-Cads don't hold charge if the computer is frequently shut down by the owner.
At times, also, the battery has started to leak and corrode. Clip it off, and see if there is a 4-pin battery connection near by. A replacement battery can either be plugged in, or leads can be soldered to where the on-board battery was.
Lastly, a CMOS could have been set up with a password. Don't do this. If someone else has done it, reset the whole of the CMOS, or try some of the default passwords. The default password for the AMI BIOS is "American Megatrends" without the quotes. You can probably get this information off the internet also.