Resetting the BIOS Supervisor Password With Debug Command
To access the BIOS setup when the machine fails to operate, occasionally a drastic move is required. In older computers with battery-backed RAM, removal of the battery and short circuiting the battery input terminals for a while did the job; in some more modern machines this move only resets the real-time clock (RTC). Some motherboards offer a CMOS-reset jumper or a reset button. In yet other cases, the EEPROM chip has to be desoldered and the data in it manually edited using a programmer. Sometimes it is enough to ground the CLK or DTA line of the I²C (Inter-Integrated Circuit) bus of the EEPROM at the right moment during boot, this requires some precise soldering on SMD parts.
If the machine lets one boot but does not want to let the user into the BIOS setup utility, because of the BIOS supervisor password one possible recovery is to deliberately "damage" the CMOS checksum by doing direct port writes using DOS debug.exe command-line tool, corrupting some bytes of the checksum-protected area of the CMOS RAM; at the next boot, the CMOS typically resets its setting to factory defaults.
The Debug command can be executed from booted Windows Command Prompt or from FreeDOS .
1. Install FreeDOS on a USB flash drive using Rufus or UNetbootin.
2. Download the debug.exe tool and save it to the root of your USB flash drive.
3. Boot up the computer with your USB flash drive and type the following commands: (take note that the second and third commands start with the letter O and not the number zero.)
debug o 70 2E o 71 FF quit
4. Restart your computer with Ctrl + Alt + Del and you should encounter an error message like “CMOS checksum error” which is normal. You can now access the CMOS setup without entering a password.
Within Windows Command Prompt type the following commands: (take note that the second and third commands start with the letter O and not the number zero.)
debug -o 70 10 -o 71 aa -q
That will write to CMOS (Offset 10h) with the value 0AAh.
Restart your computer. You can now access the CMOS setup without entering a password.
Edited by FreeBooter, 13 June 2016 - 01:37 AM.