Manual The BIOS companion

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Error Messages Codes. Arche Technologies. Chips and Technologies. The BIOS. Dell Phoenix. Wait states. Shadow RAM. Bus Types. Expansion Cards. Base IO Address. Open Sesame.

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Setup Programs. Note that the copy isn't done by some magic circuitry, it is just done by the bios itself when it starts executing out of ROM initially, it just copies itself to RAM and then continues executing from there.

Solution: BIOS Dashboard

This is another case where the received folk wisdom on the subject, as unfortunately exemplified by psusi 's answer and indeed part of the question, is stuck in the world as it was around , despite the wealth of technical references available explaining how it is now otherwise. The world that the author of the WWW page that you pointed to, S.

Ebrahim Shubbar, erroneously lives in, despite writing in , is even older. The changed this to FFFF0. But the world itself is the highly out of date world of the late s that folk wisdom still circulates. There are some additional details that I'll skip over here, but that's the gist of it.

Even though the initial value of the CS register after reset is F , the segment descriptor associated with that register initially holds FFFF as its base address. And that's where the machine firmware is principally mapped into physical address space on bit and bit x86 machines.

How To Get Into The BIOS On Windows 10

There's a KiB window onto the firmware down in the "conventional memory" area, but the NVRAM holding the machine firmware can be up to 16MiB although this varies by chipset on modern PCs and is principally mapped into the 16MiB of physical address space immediately below the 4GiB line — i. The code initially executed by the processor immediately after reset lives in the top 64KiB of this 16MiB address range.

So the firmware's code and read-only data ended up being accessed a lot at run-time. So the fact that code executing out of the NVRAM, and read-only data in the same, come to the processor more slowly than when shadowed into system RAM is less of a problem than it used to be. Moreover, what firmware code and data they do rely upon don't necessarily live in the part of NVRAM mapped to the portion of physical address space, the aforementioned KiB "conventional memory" window, that is necessarily even shadowable in the first place.

Staff Bios | Environmental Traveling Companions

Protected mode firmware services don't all need to live below the 1MiB line in physical address space as real mode firmware services do, and some do not. And of course it would only be possible to do the same trick with the area of physical address space that they do live in if there's at least 4GiB of system RAM.

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Ironically, a more accurate source for information on this than S. What is this??

The BIOS Companion : The Book That Doesn't Come with Your Motherboard!

Well, as mentioned on Bugtraq mailing list [11], there is a way to reset the CMOS password by creating a boot disk whose first sectors contains the string "KEY" followed by 0x This is it for my brief description of the Bios. If you look back at the figures mentionned above, you'll realise that most informations concerning your hardware is stored inside the CMOS or the BDA. Well, there is an even much complete way to gather informations on a computer. Describing the SMBIOS structure is off topic since we won't use it in this paper, refer to those links for more infos.

Enougth description, let's move to a more practical point of view It made use of debug to access physical ports. Under Linux, this requires special permissions that are given using ioperm. As seen earlier,CMOS is not loaded on memory : it is set on a different chip.


Interraction with the CMOS is done through physical ports 0x70 and 0x All physical ports operations follow the same scheme only the port numbers change. The first one is used to seek a pointer within the chip, and the other one is used to read or write at this position. Here is how to interract with a CMOS chip : Writing to 0x70 with a given value will in return allow us to read the actual content of the CMOS chip at this offset on physical port 0x The way this checksum is calculated depends on the model of the CMOS.

The main idea to reset CMOS is to make the checksum fail.

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To do so, we will use a trick from the "Bios Companion" [4] : writing on port 0x70 with a value of 0x2e corresponding to the CMOS checksum offset and then writing on port 0x71 with an arbitrary value which will replace the actual checksum. Christophe Grenier www. I can only suppose Bios manufacturers decided that the algorithmes would have to be made so that such values are impossible in any CMOS configuration.

As explained in the first section of this paper : When entering a Bios Password at command prompt, the input is stored at adress 0x41e. It is then compared to the cyphered one stored in CMOS for validation. As Christophe Grenier explained me by mail , reversing the BIOS ROM is unecessary : one can build a conversion table by using a diffing approche ie : entering a password and dump the CMOS, then change one letter in the password and see what has changed and so on Christophe even told me this was the methodology he used to build his password cracking tools.

But the keyboard Buffer is a circular one, whish means that once a character is read it is flushed. At least it should be In fact, I realized that Bioses did not flushed this buffer after use. In other terms, the flags at 0x1A and 0x1C in DBA are not updated after the user enters the password. Hence, the buffer used by the password is never flushed Therefore, the password remains in plain text at physical adress 0x41e. Note that this done by Bios functions and is OS independant. If you experiment the code below, you will notice that other softwares do not always use those flags correctly.

For instance, I noticed that grub and lilo did not read the 0x1A flag and use the whole buffer, even if it has not been flushed! I've not been able to find out any way to use this fact, but if you do, please send me a mail.