SMBIOS

最近花时间把之前学过的SMBIOS复习了一遍，做点笔记，以后若是记不清了还可以找回来看看。

首先，SMBIOS的基本概念

MBIOS (System Management BIOS)是主板或系统制造者以标准格式显示产品管理信息所需遵循的统一规范。

目的在于提供足够的信息给BIOS开发人员去实现必要的扩充，让他们产品的硬件和其他系统相关的信息能够正确地被用户定义的接口所判定。

SMBIOS也能提供足够的信息给管理设备的开发人员去开发通用的程序，能将SMBIOS的格式转换到他们选择的管理技术的格式。

然后，怎样访问SMBIOS information?

spec里边是这样说的：A table-based method, it provides the SMBIOS structures as a packed list of data referenced by a table entry point.

基于表结构的方法，表的内容是table entry point的数据，这个访问方法从SMBIOS2.1以后开始被推荐使用。要访问SMBIOS的内容，先要找到SMBIOS 的table entry point。

32位的entry point容许SMBIOS structure table位于4GB以下的物理空间。

在non-UEFI系统中要定位entry point structure可以通过在地址空间F000：0000 ~ F000：FFFF搜寻“_SM_” (5F 53 4D 5F), 找到后再确认其后16 byte的位置是不是“_DMI_”(5F 44 4D 49 5F).

在UEFI-based系统中，在EFI_CONFIGURATION_TABLE中找到SMBIOS_TABLE_GUID, 然后其关联的指针就是指向entry point structure。

下面来看看32位的entry point structure：

Offset

Name

Length

Description

00h

Anchor String

4 BYTEs

_SM_ , specified as four ASCII characters (5F 53 4D 5F). 

04h

Entry Point Structure Checksum

BYTE

Checksum of the Entry Point Structure (EPS).

05h

Entry Point Length

BYTE

Length of the Entry Point Structure, starting with the Anchor String field, in bytes, currently 1Fh.

06h

SMBIOS Major Version

BYTE

Major version of this specification implemented in the table structures.

07h

SMBIOS Minor Version

BYTE

Minor version of this specification implemented in the table structures.

08h

Maximum Structure Size

WORD

Size of the largest SMBIOS structure, in bytes, and encompasses the structure’s formatted area and text strings.

0Ah

Entry Point Revision

BYTE

EPS revision implemented in this structure and identifies the formatting of offsets 0Bh to 0Fh as follows:

       00h              Entry Point is based on SMBIOS 2.1 definition;

                           formatted area is reserved and set to all 00h.

       01h-FFh       Reserved for assignment by this specification.

0Bh – 0Fh

Formatted Area

5 BYTEs

Value present in the Entry Point Revision field defines the interpretation to be placed upon these 5 bytes.

10h

Intermediate Anchor String

5 BYTEs

_DMI_, specified as five ASCII characters (5F 44 4D 49 5F).

15h

Intermediate Checksum

BYTE

Checksum of Intermediate Entry Point Structure (IEPS).

16h

Structure Table Length

WORD

Total length of SMBIOS Structure Table, pointed to by the Structure Table Address, in bytes.

18h

Structure Table Address

DWORD

32-bit physical starting address of the read-only SMBIOS Structure Table, which can start at any 32-bit address This area contains all of the SMBIOS structures fully packed together.

1Ch

Number of SMBIOS Structures

WORD

Total number of structures present in the SMBIOS Structure Table.

1Eh

SMBIOS BCD Revision

BYTE

Indicates compliance with a revision of this specification.

根据entry point structure就可以知道SMBIOS的 structure table length, structure table address, number of SMBIOS structures等, 进而去访问address指定的各个SMBIOS structures（Types） 

64位的entry point与32位的大同小异，具体情况请参考spec.

最后就是各个SMBIOS structure 的定义了，总体来说会有一些共同点。

每个SMBIOS structure分为a formatted section和an optional unformed section.

The formatted section of each structures begins with a four-byte header.

Remaining data in the formatted section is determined by the structure type, as is the overall length of the formatted section.

The unformed section of the structure is used for passing variable data,such as text strings.

Each structure shall be terminated by a double-null(0000h).

每个字符串以null（00h）结束，单个string并没有长度的限制，但它会被计入structure table length

但是整个structure table的长度必须在Structure Table Length规定范围内

以下是four-byte header的format：

Offset

Name

Length

Description

00h

Type

BYTE

Specifies the type of structure. Types 0 through 127 (7Fh) are reserved for and defined by this specification. Types 128 through 256 (80h toFFh) are available for system- and OEM-specific information.

01h

Length

BYTE

Specifies the length of the formatted area of the structure, starting at the Type field. The length of the structure’s string-set is not included.

02h

Handle

WORD

Specifies the structure’s handle, a unique 16-bit number in the range 0 to 0FFFEh (for version 2.0) or 0 to 0FEFFh (for version 2.1 and later). The handle can be used with the Get SMBIOS Structure function to retrieve a specific structure; the handle numbers are not required to be contiguous. For version 2.1 and later, handle values in the range 0FF00h to 0FFFFh are reserved for use by this specification.

If the system configuration changes, a previously assigned handle might no longer exist. However, after a handle has been assigned by the BIOS, the BIOS cannot re-assign that handle number to another structure.

从SMBIOS version 2.3以来，兼容SMBIOS的应用被要求包含这些types：

BIOS Information (Type 0)

System Information (Type 1)

System Enclosure (Type 3)

Processor Information (Type 4)

Cache Information (Type 7)

System Slots (Type 9)

Physical Memory Array (Type 16)

Memory Device (Type 17)

Memory Array Mapped Address (Type 19)

System Boot Information (Type 32)