DOS文件系统-FAT32的理解

FAT32&BIO&DISK

众所周知存取数据最常见的是以文件方式。每当需要存取数据的时候，首先需要open and creat一个文件，随后read and write，完成后close，清空点格式化，查看文件敲ls/ll;这些看似简单的命令是如何到达disk（disk表示我可不知道命令、文件，我只认识数据）。

一个很明显的需求摆在眼前，我们需要在disk和文件直接建立映射关系。怎么建立？文件系统就是我们建立关系载体，bio就是的手段，disk就是目标。

1、 叫你一声你敢答应么

当我们在windows上插入一张从未使用过的盘片的时候，会出现什么状况？

——请格化

windows怎么知道你没被格式化呢？

——叫你一声没人应

       VxWorks面临一样的问题，我初次见到你怎么确定我们是否认识？叫你一声（读引导记录扇区），看你一眼（读文件系统保留区）, 问你一句（读空间分配状态和文件/目录存储关系区）。OK，我们看看VxWorks下FAT32是怎么识别盘片的。

       发read sector=0 length = 1 bio两次，识别第一个block中的某些字段（0~1BD引导代码、分区表），如果不匹配则初始化它为rawfs，如果匹配则从sector=X开始读大概几百兆（和盘片容量有关，下文解释）的数据，如果其中特定的字段是匹配的，再次读sector=X，这样就代表probe FAT32 成功了，接下来就可以对disk文件操作（cd/ls/creat/rm/copy）了。（经测试在读取数据的时候抹去当前page末的227字节数据也能匹配fs成功，说明文件系统不是比较其中的每次字节都匹配，只对其中特殊的字段作比较）

2、 文件系统即数据

DOS的外壳---引导记录扇区：

FAT32属于DOS文件系统的一种，那么FAT32的外壳就应该是DOSFS（测试观察dosfs和ntfs 的 第一个扇区的478~493不同）如下为lba = 0 的数据：

{33 c0 8e d0 bc 00 7c 8e c0 8e d8 be 00 7c bf 00 06 b9 00 02 fc f3 a4 50 68 1c 06 cb fb b9 04 00 
bd be 07 80 7e 00 00 7c 0b 0f 85 0e 01 83 c5 10 e2 f1 cd 18 88 56 00 55 c6 46 11 05 c6 46 10 00 
b4 41 bb aa 55 cd 13 5d 72 0f 81 fb 55 aa 75 09 f7 c1 01 00 74 03 fe 46 10 66 60 80 7e 10 00 74 
26 66 68 00 00 00 00 66 ff 76 08 68 00 00 68 00 7c 68 01 00 68 10 00 b4 42 8a 56 00 8b f4 cd 13 
9f 83 c4 10 9e eb 14 b8 01 02 bb 00 7c 8a 56 00 8a 76 01 8a 4e 02 8a 6e 03 cd 13 66 61 73 1c fe 
4e 11 75 0c 80 7e 00 80 0f 84 8a 00 b2 80 eb 84 55 32 e4 8a 56 00 cd 13 5d eb 9e 81 3e fe 7d 55 
aa 75 6e ff 76 00 e8 8d 00 75 17 fa b0 d1 e6 64 e8 83 00 b0 df e6 60 e8 7c 00 b0 ff e6 64 e8 75 
00 fb b8 00 bb cd 1a 66 23 c0 75 3b 66 81 fb 54 43 50 41 75 32 81 f9 02 01 72 2c 66 68 07 bb 00 
00 66 68 00 02 00 00 66 68 08 00 00 00 66 53 66 53 66 55 66 68 00 00 00 00 66 68 00 7c 00 00 66 
61 68 00 00 07 cd 1a 5a 32 f6 ea 00 7c 00 00 cd 18 a0 b7 07 eb 08 a0 b6 07 eb 03 a0 b5 07 32 e4 
05 00 07 8b f0 ac 3c 00 74 09 bb 07 00 b4 0e cd 10 eb f2 f4 eb fd 2b c9 e4 64 eb 00 24 02 e0 f8 
24 02 c3 49 6e 76 61 6c 69 64 20 70 61 72 74 69 74 69 6f 6e 20 74 61 62 6c 65 00 45 72 72 6f 72 
20 6c 6f 61 64 69 6e 67 20 6f 70 65 72 61 74 69 6e 67 20 73 79 73 74 65 6d 00 4d 69 73 73 69 6e 
67 20 6f 70 65 72 61 74 69 6e 67 20 73 79 73 74 65 6d 00 00 00 63 7b 9a 2d 0e df 60 00 00 }引导代码

{

00 -----是否可引导，0不可，0x80可引导

20 21 00 ----分区起始chs

07----分区类型，（不重要，由分区引导决定）

fe ff ff-----分区chs结束地址

00 08 00 00----分区lba地址0x00000800=X(上文中提到的)

00 08 eb 01----分区大小扇区数

}分区表1

下同

{

80

fe ff ff

0c

fe ff ff 

00 10 eb 01 

00 50 60 02

}分区表2

{

00

feff ff 

07

fe ff ff 

00 60 4b 04 

00 b8 7c 6b 

}分区表3

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 

{55 aa}签名

DOS的外壳FAT的核----FAT保留区：

从上面分区信息可以看到分区起始LBA地址为0x800，这个地址就是FAT保留区，下文为0x800数据：

eb 3e 90 ---汇编指令，跳转到引导代码处

56 58 35 44 4f 53 33 32---文件系统标志

00 02---每个扇区字节数（0x0200）

40---每个block扇区数

20 00 ---保留扇区数

02---FAT表个数

00 00 ---FAT32为0

00 00 ---扇区总数，超过长度存放于0x20~0x23

f8 ---介质描述符

00 00---FAT32为0

d0 9d ---每个此道扇区数

18 00---磁头数

00 08 00 00 ---分区前已用扇区数

00 10 c8 6f ---文件系统扇区总数

41 7e 03 00---每个FAT表大小

00 00 ---标记

00 00---版本号

02 00 00 00----根目录起始block号

01 00 --FSINFO所在扇区号，通常为1号扇区

06 00---备份引导扇区

00 00 00 00 00 00 00 00 00 00 00 00 

80 ---bios int 13h设备号

00---

29---扩展引导标识，如果猴年三个值有效则为0x29

00 01 98 07---卷序列号

00 47 01 bd d0 32 b8 45 32 00 00---卷标 

46 41 54 33 32 20 20 20 ---文件系统格式

{

00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 aa 
52 52 61 41 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 72 72 41 61 4c 04 bf 01 ff ff ff ff 00 00 00 00 00 00 00 00 00 00 00 00 00 00 

}引导代码错误信息

{55 aa}签名 

DOS的外壳FAT的核---FAT表：

FAT表描述block分配状态及表明文件和目录的下一个block号。

FAT32每个block地址使用4个字节记录在FAT表中，FAT表中所有的字节位置意4字节对齐，对所有 的划分后的位置由0进行地址编号，0号 1号地址被系统保留并存储特殊的标识内容，从2号地址每个地址对应的数据区的block号当文件系统被创建时，也就是格式化的时候分配给FAT区域的空间将被清空，在FAT1与FAT2的0号表项与1号表项写入特定的值在2号表项写入结束标识。

eg： 看0x820扇区数据：

|——0号—|   |——1号—|   |——2号—|

f8   ff   ff   ff   ff   ff   ff   f7  ff   ff   ff   0f  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 

如果某个block未被使用对应的FAT表项4字节为0。

当某个block已被使用对应的FAT表项的4字节为该文件的下一关存储位置的block号。

如果该文件结束于该block，则在它的FAT表项中记录一关文件结束标记0x0fffffff。

如果该扇区为坏扇区记录0x0ffffff7，FAT32的0号表项为0xf8ffff0f。

文件系统建立新文件的时候，如果建立的新文件只占用一个block，则在其所占的FAT表项会被写入结束标记。

DOS的外壳FAT的核---数据区：

根目录一般位于2号block（位于数据区的2号block），用于存放文件基本信息（包括文件名、文件建立时间、修改时间、文件数据等）。

根目录在文件系统创建时创建，并将结束符写入2号block对应的FAT表项。

目录项：文件名部分不足8个字符，用0x20补齐。

3、系统调用的来龙去脉

4、 格式化的神秘面纱

1、 获取盘片在位状态

2、 获取盘片容量和blocksize

3、 获取xbd 设备

4、 读sector 0 两次，比较两次读取的数据是否一致

5、 写文件系统引导代码及分区表数据到sector 0、写FAT保留区、写FAT表项。

6、 向盘片发送flush命令

7、 Read sector 0