System Management BIOS

System Management BIOS (SMBIOS) is a standard developed by DMFT. The purpose of this standard is to allow the operating system to retrieve information about the PC.

On booting the SMBIOS will put a table somewhere in memory. By parsing this table it is possible to access information about the computer and its capabilities.

Locating the SMBIOS Entry Point Table

The SMBIOS Entry Point Table is located somewhere between the addresses 0xF0000 and 0xFFFFF. To find the specific location of the start of the table it is necessary to search that region of memory for the string "_SM_".

One example of how this can be done is demonstrated in the below code.

char *mem = (char *) 0xF0000;
while ((Bit32u) mem < 0x100000) {
	if (mem[0] == '_' && mem[1] == 'S' && mem[2] == 'M' && mem[3] == '_') {
		break;
	}
	mem++;
}

Now mem contains the address of the Entry Point Table. Some old systems may not have the SMBIOS. So...

if ((unsigned int) mem == 0x100000) {
	panic("No SMBIOS found!");
}

Parsing the Entry Point Table

The entry point table has the following structure

struct SMBIOSEntryPoint {
	char EntryPointString[4];    //This is _SM_
	uchar Checksum;              //This value summed with all the values of the table, should be 0 (overflow)
	uchar Length;                //Length of the Entry Point Table. Since version 2.1 of SMBIOS, this is 0x1F
	uchar MajorVersion;          //Major Version of SMBIOS
	uchar MinorVersion;          //Minor Version of SMBIOS
	ushort MaxStructureSize;     //Maximum size of a SMBIOS Structure (we will se later)
	uchar EntryPointRevision;    //...
	char FormattedArea[5];       //...
	char EntryPointString2[5];   //This is _DMI_
	uchar Checksum2;             //Checksum for values from EntryPointString2 to the end of table
	ushort TableLength;          //Length of the Table containing all the structures
	uint TableAddress;	     //Address of the Table
	ushort NumberOfStructures;   //Number of structures in the table
	uchar BCDRevision;           //Unused
};

TableAddress contains the address of the table that contains all the structures with information about the PC. All of the structures are located from [TableAddress] to [TableAddress + TableLength]. The structures are located directly adjacent to each other in memory, with a new structure beginning as soon as another one ends. Each structure is composed of a header, a structure specific table, and a string table.

The format of the header is as follows.

struct SMBIOSHeader {
	uchar Type;
	uchar Length;
	ushort Handle;
};

Located at TableAddress is a SMBIOS header. The value of Type indicates what element the structure contains information about. (0 = BIOS, 1 = System, 2 = Chassis, 3 = Processors, ...) Length indicates the size of header + data table. The strings are not included in the length.

Immediately after the end of the header is the data. At the end of the data table (Address + Length), the strings section starts. Each string is NULL terminated and is limited to 64 characters.

eg: the BIOS Struct (Type 0) is like this:

db 0 ; Indicates BIOS Structure Type            |
db 13h ; Length of information in bytes         | HEADER
dw ? ; Reserved for handle                      |

db 01h ; String 1 is the Vendor Name            |
db 02h ; String 2 is the BIOS version           |
dw 0E800h ; BIOS Starting Address               |
db 03h ; String 3 is the BIOS Build Date        | DATA
db 1 ; Size of BIOS ROM is 128K (64K * (1 + 1)) |
dq BIOS_Char ; BIOS Characteristics             |
db 0 ; BIOS Characteristics Extension Byte 1    |

db ‘System BIOS Vendor Name’,0 ;                |
db ‘4.04’,0 ;                                   | STRINGS
db ‘00/00/0000’,0 ;                             |

db 0 ; End of structure

At the end of the BIOS table, another table will start. The last structure has type 127.

External links

• SMBIOS Specification