Question

how to access BIOS ROM

Asked by: gdstaff

hi,

I have a PC motherboard with intel P3, 810 chipset & 1MByte of ROM keeping the BIOS.

I would like to read & write a byte to a specific location of the BIOS ROM. I belief the location range should be
0x???f0000 to 0x???fffff.  

Any idea how I can write a assembly code to do that?
Please point out  any register/IO involved?
Any simple assembly code you can provide...? If yes, is your code hardware dependant?

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Asked On
2002-11-14 at 02:07:24ID20398170
Tags

bios

,

access

Topic

Assembly Programming Language

Participating Experts
3
Points
140
Comments
8

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Answers

 

by: Alisher_NPosted on 2002-11-14 at 02:30:59ID: 7447459

to read a BIOS area you don't need any specific instructions: just read a memory from location you need (and you know that ROM is there)

mov   ax,0xF000
mov   es,ax
xor   ax,ax
mov   al,es:ax

this must be done under pure DOS of course!

remebmber, that modern BIOS are all compressed, so reading a couple of bytes will not give you any useful data...

you can use BIOS FLASH utility for your motherborad to get a whole BIOS image

 

by: dimitryPosted on 2002-11-14 at 09:20:30ID: 7449194

Alisher_N is completely right. I want only to clarify
one point:
BIOS is placed on Flash (once it was on ROM). You can
update it only with the help of some BIOS Flash utility
and you need to be very careful - because this is the simplest way to kill your computer once and for all.

During the boot BIOS "installs" itself to the shadow
memory area usually from 000E0000 to 000FFFFF.
Older and smaller BIOSes used 000F0000 - 000FFFFF.
You can definitely read and write in Real Mode from and
to this area. This action WILL NOT change the BIOS and
the Flash and next reboot it will be lost.

So my question is "What do you really need" ?

 

by: OlivmaPosted on 2002-11-21 at 11:19:44ID: 7479885

Hi all,
I have the same request as gdstaff. I need to do a test to read/write/erase the content on the flash chip where BIOS is located. I have made a copy of the original BIOS so that I can use it to make my system return to normal situation.

How can I read/write this physical memory?

 

by: Alisher_NPosted on 2002-11-21 at 11:37:11ID: 7479973

why don't you use a flash utility for your motherboard ? ;-)

 

by: OlivmaPosted on 2002-11-22 at 16:15:49ID: 7485978

The problem is that the purpose of the test is to verify the read/write device driver functions I wrote for the Firmware hub (the boot device). It is more like to create such kind of flash utility (of course, much simpler one) to read/write the firmware hub.

There is another way to do the test - to build a PCI board which has ICH and firmware hub on it. So the test is more like a PCI device driver. But this way is too expensive and is not proved.

I am also concerned about what dimitry had mentioned. Maybe this kind of modification can not be done. But since the flash utility can do it, there should be some way to do it.

Any suggestions ^_^?

 

by: Alisher_NPosted on 2002-11-22 at 20:44:08ID: 7486412

now it is more clear, Olivma ;-), but there are many issues involved:

1) flash utilities work only under real DOS boot opposite to your driver, which is probably win oriented ?

2) as Dmitry pointed, you can read from BIOS flash area, but for writing there I think flash utility uses another approach than direct write (there are some specific ports to set address, data and write strobe or something like that) and this solely depends on MB design

3) if you want to test your board ROM, it would be easier to get ROM/Flash chip and test it in programmer device ;-)

4) if you want to test driver code by itself, you have to do it with completed board (all hardware including ROM/Flash is populated, tested and you know it is working)

this is because code to access physical locations in your board will be different depending on environment (PCI under real DOS, PCI under protected mode, ISA card under DOS ? etc etc)... if you don't have a board yet and trying to write some software for it at the same time - it just tends to be more coplicated ;-), because you never know where is the problem: is it your driver can not set some signal in some port or signal is there, but hardware can not understand it (bad soldjering joint ?) ;-)


 

by: OlivmaPosted on 2002-11-25 at 11:56:20ID: 7495470

You are right, Alisher. I will have to know the very detailed design of the existing MB to get the BIOS modified.
This project is much more complicated than I had expected:-( I will need more time on it. Thanks for your valuable advices :-)

20120131-EE-VQP-002

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