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【我所認知的BIOS】—>Decompression

By LightSeed

2009-5-22

存在於BIOS的bin檔中的內容大多都是以模組的形式存在的。總所周知存在於裏面的模組都是有被壓縮的。這張我們就來探討一下關於模組的壓縮與解壓過程。這個章節裏同樣是針對於理解Awxxx公司的code。

1、LHA

1.1 LHA的背景

LHA是一種檔壓縮電腦軟體，也是此壓縮格式的名稱，其對應的副檔名有.lha及.lzh。發明人為日本的業餘程式設計師吉崎榮泰。擔任內科醫師的吉崎榮泰利用業餘時間，在奧村晴彥所發明的演算法的基礎上開發了名為LHarc的檔壓縮軟體及壓縮格式，1988年首次於網路上公開。1990年左右全面改寫程式，並改名為LHA。現在可能在別的國家用的不多了，但是在日本它仍是常見的壓縮格式之一。（來源於網路）

1.2 LZH的格式

要說明，LZH的header一共有三中級別。Level-0， Level-1和 Level-2。他們的數據格式如圖1。

讯享网 

讯享网

圖1 Level-0， Level-1和 Level-2的格式區別

Level-0， Level-1和 Level-2的格式中各個byte的對應數據如下麵的三個列表。

讯享网level-0

Offset   Length   Contents

讯享网  0      1 byte   Size of archived file header (h)

  1      1 byte   Header checksum

讯享网  2      5 bytes  Method ID

  7      4 bytes  Compressed size (n)

讯享网 11      4 bytes  Uncompressed size

 15      4 bytes  Original file date/time (Generic time stamp)

讯享网 19      1 byte   File attribute

 20      1 byte   Level (0x00)

讯享网 21      1 byte   Filename / path length in bytes (f)

 22     (f)bytes  Filename / path

讯享网 22+(f)  2 bytes  CRC-16 of original file

 24+(f) (n)bytes  Compressed data

讯享网 

讯享网level-1

Offset   Length   Contents

讯享网  0      1 byte   Size of archived file header (h)

  1      1 byte   Header checksum

讯享网  2      5 bytes  Method ID

  7      4 bytes  Compressed size (n)

讯享网 11      4 bytes  Uncompressed size

 15      4 bytes  Original file date/time (Generic time stamp)

讯享网 19      1 byte   0x20

 20      1 byte   Level (0x01)

讯享网 21      1 byte   Filename / path length in bytes (f)

 22     (f)bytes  Filename / path

讯享网 22+(f)  2 bytes  CRC-16 of original file

 24+(f)  1 byte   OS ID

讯享网 25+(f)  2 bytes  Next header size(x) (0 means no extension header)

[ // Extension headers

讯享网         1 byte   Extension type

     (x)-3 bytes  Extension fields

讯享网         2 bytes  Next header size(x) (0 means no next extension header)

]*

讯享网        (n)bytes  Compressed data

讯享网 

level-2

讯享网Offset   Length   Contents

  0      2 byte   Total size of archived file header (h)

讯享网  2      5 bytes  Method ID

  7      4 bytes  Compressed size (n)

讯享网 11      4 bytes  Uncompressed size

 15      4 bytes  Original file time stamp(UNIX type, seconds since 1970)

讯享网 19      1 byte   Reserved

 20      1 byte   Level (0x02)

讯享网 21      2 bytes  CRC-16 of original file

 23      1 byte   OS ID

讯享网 24      2 bytes  Next header size(x) (0 means no extension header)

[

讯享网         1 byte   Extension type

     (x)-3 bytes  Extension fields

讯享网         2 bytes  Next header size(x) (0 means no next extension header)

]*

讯享网        (n)bytes  Compressed data

針對於其中Method ID有對應的解釋，見表1

表1 LHA的壓縮方法ID代表的意義

"-lh0-"	No compression
"-lh1-"	4k sliding dictionary(max 60 bytes) + dynamic Huffman + fixed encoding of position
"-lh2-"	8k sliding dictionary(max 256 bytes) + dynamic Huffman (Obsoleted)
"-lh3-"	8k sliding dictionary(max 256 bytes) + static Huffman (Obsoleted)
"-lh4-"	4k sliding dictionary(max 256 bytes) + static Huffman + improved encoding of position and trees
"-lh5-"	8k sliding dictionary(max 256 bytes) + static Huffman + improved encoding of position and trees
"-lh6-"	32k sliding dictionary(max 256 bytes) + static Huffman + improved encoding of position and trees
"-lh7-"	64k sliding dictionary(max 256 bytes) + static Huffman + improved encoding of position and trees
"-lzs-"	2k sliding dictionary(max 17 bytes)
"-lz4-"	No compression
"-lz5-"	4k sliding dictionary(max 17 bytes)
"-lhd-"	Directory (no compressed data)

2、解壓縮詳解

2.1解壓縮的準備知識

2.1.1 Cbrom的壓縮

AWxxx的code整個燒入ROM的BIOS.bin檔其實大多都是由cbrom以模組的形式壓入的。比如說我手上板子的BIOS就一共有8個模組。見圖2

 

圖2  BIOS模組

由圖中可知，不管是System BIOS還是PCI rom都是以模組的形式壓入到bin檔裏的。並且在cbrom壓入的過程中，會有相應的一些格式。這個結構如表2。

表2 被cbrom壓入的模組header

Offset from 1st byte	Offset in Real Header	Contents
00h	N/A	The header length of the component. It depends on the file/component name.
01h	N/A	The header 8-bit checksum, not including the first 2 bytes (header length and header checksum byte).
02h - 06h	00h - 04h	LZH Method ID (ASCII string signature). In my BIOS it's "-lh5-" which means: 8k sliding dictionary(max 256 bytes) + static Huffman + improved encoding of position and trees.
07h - 0Ah	05h - 08h	compressed file/component size in little endian dword value, i.e. MSB at 0Ah and so forth
0Bh - 0Eh	09h - 0Ch	Uncompressed file/component size in little endian dword value, i.e. MSB at 0Eh and so forth
0Fh - 10h	0Dh - 0Eh	Decompression offset address in little endian word value, i.e. MSB at 10h and so forth. The component will be decompressed into this offset address (real mode addressing is in effect here).
11h - 12h	0Fh - 10h	Decompression segment address in little endian word value, i.e. MSB at 12h and so forth. The component will be decompressed into this segment address (real mode addressing is in effect here).
13h	11h	File attribute. My BIOS components contain 20h here, which is normally found in LZH level-1 compressed file.
14h	12h	Level. My BIOS components contain 01h here, which means it's a LZH level-1 compressed file.
15h	13h	component filename name length in byte.
16h - [15h+filename_len]	14h - [13h+filename_len]	component filename (ASCII string)
[16h+filename_len] - [17h+filename_len]	[14h+filename_len] - [15h+filename_len]	file/component CRC-16 in little endian word value, i.e. MSB at [HeaderSize - 2h] and so forth.
[18h+filename_len]	[16h+filename_len]	Operating System ID. In my BIOS it's always 20h (ASCII space character) which don't resemble any LZH OS ID known to me.
[19h+filename_len] - [1Ah+filename_len]	[17h+filename_len] - [18h+filename_len]	Next header size. In my BIOS it's always 0000h which means no extension header.

值得一提的是，表中畫藍線的地方。這是被壓入bin檔中，模組將要被解壓縮到哪里去的地址。由於Cbrom是Awxxx出的工具，在解壓縮的過程中也是與之默契配合的。（筆者：基本上由cbrom壓入的模組，在解壓位址中都是在4000段中，至於具體位置會有點變化。這個會和index有關係。）見截圖3，它是在板子上BIOS.bin的system BIOS被cbrom壓縮後的二進位截圖。

 

 

 

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