LinuxAsmTools
LinuxAsmTools - HowTo read CMOS memory
Reading BIOS memory with assembler mini-HOWTO
jeff owens, jeff@linuxasmtools.net
v1.01, 06 April 2009
How to read BIOS area with example program.
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Table of Contents
1. Disclaimer
2. Introduction
3. Where is physical memory
4. How do we find the BIOS
5. Running the Example program
6. Compiling the example program
7. Finding more information
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1. Disclaimer
The following document is offered in good faith as comprising only
safe programming and procedures. No responsibility is accepted by the
author for any loss or damage caused in any way to any person or
equipment, as a direct or indirect consequence of following these
instructions.
2. Introduction
The most recent version of this document can always be found at
https://thlorenz.com/linuxasmtools-net
The BIOS (boot code) is contained in ROM (Read Only Memory)
and includes startup code plus configuration menus. After
a system boots programs are provided virtual memory and
ignores the BIOS memory area. This program finds the BIOS
in physical memory and displays information stored by
manufacturer.
The sample program is built from two files: bios.asm and
bios.inc.
3. Where is physical memory
To get at physical memory we need to read the device
/dev/mem. This device also can use the seek function
or it can be mapped with mmap. We will use the open,
seek, read functions to search for the BIOS area.
Before starting it is a good idea to check our status
as follows:
call sys_getuid ;asmlib call to get our UID
or eax,eax ;check if root
jz we_root ;jmp if root
Next, we open the physical memory device with:
mov ebx,mem_dev
call block_open_read
The opened device handle is returned in eax, we
use it to seek to address 0e0000h. At this point
we will begin our search.
mov [mem_handle],eax
mov ebx,eax ;handle to ebx
mov ecx,0e0000h ;seek offset
mov [current_seek],ecx
call block_seek
4. How do we find the BIOS
We are now at address 0e0000h in physical memory.
Now, we want to search for the string "_DMI_" which
indicates the top of BIOS tables.
dmi_search:
mov ecx,buffer
mov edx,16 ;size of read
call block_read ;read 16 bytes
cmp [ecx],dword '_DMI'
je got_dmi
add [current_seek],dword 16
dec ebp ;timeout
jnz dmi_search
If we have a standard plug-and-play BIOS, it
will have a standard format. We can scan the
tables and extract the manufacturer and BIOS
author.
5. Running the example program.
If we currently have root status, the example program
can be started with:
./bios
It will show date values read from bios. Normally
we will not be root, so we might us sudo as follows:
sudo ./bios
6. Compiling the example program.
The example program can be compiled with the
following tools:
nasm - assembler
asmlib - library of assembler functions
The easy way to do this is to install, asmide
and start it as follows:
asmide bios.asm
asmide will provide a menu, with compile and
debug options.
To manually compile the source use the following:
nasm -g -f elf bios.asm
ld bios.o -static -o bios /usr/lib/asmlib.a
7. Finding more information
Standards are at:
http://www.dmtf.org/standards/smbios
