paul's 8051 code library understanding the fat32 filesystem.htm

硬盘ide和fat文件系统开发详解

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<H2>Understanding FAT32 Filesystems</H2>This page is intended to help you 
understand how to access data on Microsoft FAT32 filesystems, commonly used on 
hard drives ranging in size from 500 megs to hundreds of gigabytes. FAT is a 
relatively simple and unsophisticated filesystem that is understood by nearly 
all operating systems, including Linux and MacOS, so it's usually a common 
choice for firmware-based projects that need to access hard drives. FAT16 and 
FAT12 are very similar and used on smaller disks. This page will concentrate on 
FAT32 only (to keep it simple), and briefly mention where these other two are 
different. 
<P>The <A 
href="javascript:if(confirm('http://www.microsoft.com/whdc/hwdev/hardware/fatgen.mspx%20%20\n\nThis%20file%20was%20not%20retrieved%20by%20Teleport%20Pro,%20because%20it%20is%20addressed%20on%20a%20domain%20or%20path%20outside%20the%20boundaries%20set%20for%20its%20Starting%20Address.%20%20\n\nDo%20you%20want%20to%20open%20it%20from%20the%20server?'))window.location='http://www.microsoft.com/whdc/hwdev/hardware/fatgen.mspx'" 
tppabs="http://www.microsoft.com/whdc/hwdev/hardware/fatgen.mspx">official FAT 
specification is available from Microsoft</A>, complete with a software "license 
agreement". Saddly, the document from Microsoft is hard to read if you do not 
already understand the FAT filesystem structure, and it lacks information about 
disk partitioning which also must be dealt with to properly use standard hard 
drives. While you may find the Microsoft spec useful, this page is meant to 
"stand alone"... and you can simply read it without suffering through 3 pages of 
legalese! 
<P>However, this page will intentionally "gloss over" many small details and 
omit many of the finer points, in an attempt to keep it simple and easy to read 
for anyone faced with learning FAT32 without any previous exposure. 
<H2>Where To Start... How About At The Beginning?</H2>The first sector of the 
drive is called the Master Boot Record (MBR). You can read it with LBA = 0. For 
any new projects, you should not even worry about accessing a drive in CHS mode, 
as LBA just numbers the sectors sequentially starting at zero, which is much 
simpler. All IDE drives support accessing sectors using LBA. Also, all IDE 
drives use sectors that are 512 bytes. Recent Microsoft operating systems refer 
to using larger sectors, but the drives still use 512 bytes per sector and MS is 
just treating multiple sectors as if they were one sector. The remainder of this 
page will only refer to LBA address of 512 byte sectors. 
<P>The first 446 bytes of the MBR are code that boots the computer. This is 
followed by a 64 byte partition table, and the last two bytes are always 0x55 
and 0xAA. You should always check these last two bytes, as a simple "sanity 
check" that the MBR is ok. 
<P>
<TABLE width=262 align=center>
  <TBODY>
  <TR>
    <TD><IMG height=300 alt="MBR Diagrag" 
      src="Paul's 8051 Code Library Understanding the FAT32 Filesystem.files/mbr.gif" 
      width=250 
      tppabs="http://www.pjrc.com/tech/8051/ide/mbr.gif"><BR><SMALL><B>Figure 
      1</B>: MBR (first sector) layout</SMALL></TD></TR></TBODY></TABLE>
<P>The MBR can only represent four partitions. A technique called "extended" 
partitioning is used to allow more than four, and often times it is used when 
there are more than two partitions. All we're going to say about extended 
partitions is that they appear in this table just like a normal partition, and 
their first sector has another partition table that describes the partitions 
within its space. But for the sake of simply getting some code to work, we're 
going to not worry about extended partitions (and repartition and reformat any 
drive that has them....) The most common scenario is only one partition using 
the whole drive, with partitions 2, 3 and 4 blank. 
<P>Each partition description is just 16 bytes, and the good news is that you 
can usually just ignore most of them. The fifth byte is a <I>Type Code</I> that 
tells what type of filesystem is supposed to be contained within the partition, 
and the ninth through twelth bytes indicate the <I>LBA Begin</I> address where 
that partition begins on the disk. 
<P>
<TABLE width=480 align=center>
  <TBODY>
  <TR>
    <TD><IMG height=42 alt="Partition Entry" 
      src="Paul's 8051 Code Library Understanding the FAT32 Filesystem.files/partition.gif" 
      width=469 
      tppabs="http://www.pjrc.com/tech/8051/ide/partition.gif"><BR><SMALL><B>Figure 
      2</B>: 16-byte partition entry</SMALL></TD></TR></TBODY></TABLE>
<P>Normally you only need to check the <I>Type Code</I> of each entry, looking 
for either 0x0B or 0x0C (the two that are used for FAT32), and then read the 
<I>LBA Begin</I> to learn where the FAT32 filesystem is located on the disk. 
<P>TODO: add a table of known type codes, with the FAT32 ones colored 
<P>The <I>Number of Sectors</I> field can be checked to make sure you do not 
access (particularly write) beyond the end of the space that is allocated for 
the parition. However, the FAT32 filesystem itself contains information about 
its size, so this <I>Number of Sectors</I> field is redundant. Several of 
Microsoft's operating systems ignore it and instead rely on the size information 
embedded within the first sector of the filesystem. (yes, I have experimentally 
verified this, though unintentionally :) Linux checks the <I>Number of 
Sectors</I> field and properly prevents access beyond the allocated space. Most 
firmware will probably ignore it. 
<P>The Boot Flag, CHS Begin, and CHS End fields should be ignored. 
<H2>FAT32 Volume ID... Yet Another First Sector</H2>The first step to reading 
the FAT32 filesystem is the read its first sector, called the Volume ID. The 
Volume ID is read using the LBA Begin address found from the partition table. 
From this sector, you will extract information that tells you everything you 
need to know about the physical layout of the FAT32 filesystem. 
<P>Microsoft's specification lists many variables, and the FAT32 Volume ID is 
slightly different than the older ones used for FAT16 and FAT12. Fortunately, 
most of the information is not needed for simple code. Only four variables are 
required, and three others should be checked to make sure they have the expected 
values. 
<P>
<TABLE width=262 align=center>
  <TBODY>
  <TR>
    <TD><IMG height=300 alt="Volume ID Diagram" 
      src="Paul's 8051 Code Library Understanding the FAT32 Filesystem.files/volume_id.gif" 
      width=250 
      tppabs="http://www.pjrc.com/tech/8051/ide/volume_id.gif"><BR><SMALL><B>Figure 
      4</B>: FAT32 Volume ID, critical fields</SMALL></TD></TR></TBODY></TABLE>
<P>
<TABLE cellSpacing=0 cellPadding=4 align=center border=1>
  <TBODY>
  <TR>
    <TH>Field</TH>
    <TH>Microsoft's Name</TH>
    <TH>Offset</TH>
    <TH>Size</TH>
    <TH>Value</TH></TR>
  <TR>
    <TD>Bytes Per Sector</TD>
    <TD>BPB_BytsPerSec</TD>
    <TD>0x0B</TD>
    <TD align=right>16 Bits</TD>
    <TD>Always 512 Bytes</TD></TR>
  <TR>
    <TD>Sectors Per Cluster</TD>
    <TD>BPB_SecPerClus</TD>
    <TD>0x0D</TD>
    <TD align=right>8 Bits</TD>
    <TD>1,2,4,8,16,32,64,128</TD></TR>
  <TR>
    <TD>Number of Reserved Sectors</TD>
    <TD>BPB_RsvdSecCnt</TD>
    <TD>0x0E</TD>
    <TD align=right>16 Bits</TD>
    <TD>Usually 0x20</TD></TR>
  <TR>
    <TD>Number of FATs</TD>
    <TD>BPB_NumFATs</TD>
    <TD>0x10</TD>
    <TD align=right>8 Bits</TD>
    <TD>Always 2</TD></TR>
  <TR>
    <TD>Sectors Per FAT</TD>
    <TD>BPB_FATSz32</TD>
    <TD>0x24</TD>
    <TD>32 Bits</TD>
    <TD align=right>Depends on disk size</TD></TR>
  <TR>
    <TD>Root Directory First Cluster</TD>
    <TD>BPB_RootClus</TD>
    <TD>0x2C</TD>
    <TD align=right>32 Bits</TD>
    <TD>Usually 0x00000002</TD></TR>
  <TR>
    <TD>Signature</TD>
    <TD>(none)</TD>
    <TD>0x1FE</TD>