ExFAT boot sector checksum簡單介紹

在測試各種卡插拔測試的時候，遇到這樣的一個error：

ExFAT: Invalid boot sector checksum.  Was 0x12345678, should be 0x87654321.

這個error會導致系統彈出提示框

“You need to format the disk in driver J: before you can use it. Do you want to format it?”

於是很好奇什麼是boot sector checksum，是否真的是boot sector checksum錯誤導致這樣的error。

        要了解ExFat中的boot sector checksum，首先需要了解MBR，因爲只有正確解析了MBR這個sector，纔可以找到ExFAT boot sector的位置。MBR的細節可以參考：http://en.wikipedia.org/wiki/Master_boot_record#PTE

這裏我們只看Classical Generic的版本，參考下圖：

Structure of a classical generic MBR

Address		Description		Size (bytes)
Hex	Dec
+000h	+0	Bootstrap code area		446
+1BEh	+446	Partition entry #1	Partition table (for primary partitions)	16
+1CEh	+462	Partition entry #2		16
+1DEh	+478	Partition entry #3		16
+1EEh	+494	Partition entry #4		16
+1FEh	+510	55h	Boot signature[a]	2
+1FFh	+511	AAh
Total size: 446 + 4×16 + 2				512

可以看到其中的boot signature是 0x55AA，partion entry從0x1BE開始，最多支持4個partition entry。其中的每個partion entry的格式如下（16 byte）：

Offset
(bytes)

Field
length

Description

+0h

1 byte

status / physical drive (bit 7 set: active / bootable, old MBRs only accept 80h), 00h: inactive,01h–7Fh: invalid)^[a]

+1h

3 bytes

CHS address of first absolute sector in partition.^[b] The format is described by 3 bytes, see the next 3 rows.

+1h

1 byte

h7–0								head[c]
x	x	x	x	x	x	x	x

+2h

1 byte

c9–8		s5–0						sector in bits 5–0; bits 7–6 are high bits of cylinder[c]
x	x	x	x	x	x	x	x

+3h

1 byte

c7–0								bits 7–0 of cylinder[c]
x	x	x	x	x	x	x	x

+4h

1 byte

Partition type^[14]

+5h

3 byte

CHS address of last absolute sector in partition.^[b] The format is described by 3 bytes, see the next 3 rows.

+5h

1 byte

h7–0								head[c]
x	x	x	x	x	x	x	x

+6h

1 byte

c9–8		s5–0						sector in bits 5–0; bits 7–6 are high bits of cylinder[c]
x	x	x	x	x	x	x	x

+7h

1 byte

c7–0								bits 7–0 of cylinder
x	x	x	x	x	x	x	x

+8h

4 bytes

LBA of first absolute sector in the partition^[d]

+Ch

4 bytes

Number of sectors in partition^[d]

Notes:

^ a Originally, status values other than 00h and80h were invalid, but modern MBRs treat bit 7 as active flag and use this entry to store the physical boot unit (see below).

^ b Starting Sector fields are limited to 1023+1 cylinders, 255+1 heads, and 63 sectors; Ending Sector fields have the same limitations.

^ c The range for sector is 1 through 63; the range for cylinder is 0 through 1023; the range for head is 0 through 255 inclusive.^[15]

^ d Used by OSes in certain circumstances; in such cases the CHS addresses are ignored.^[16]

對於我們的各種卡片來講，我們只需要關心 LBA of first absolute sector in the partition ，也就是partition開始的絕對地址。有了這個地址，我們就可以找到ExFAT文件系統所對應的boot sector。這裏舉個例子，參考下面的MBR：

從中可以獲取這張卡只有一個partition，他開始的絕對地址是0x2000 sector。接下來就是要從0x2000 sector來解析ExFAT的boot sector了。

ExFat是從FAT12、FAT16、FAT32演變而來，主要增加了一下改進：

支持更大的文件、更大的卷

原生支持unicode文件名

更大的boot空間，支持更多的boot code

更好的速度、性能

對時區的支持

對OEM參數的支持

    接下來我們的重點還是首先了解一下ExFAT boot sector的組成，可以參考：http://www.ntfs.com/exfat-volume-layout.htm

Volume Layout

Offset, sectors	Size, sectors	Block	Comments
Main Boot Region
0	1	Boot Sector
1	8	Extended Boot Sectors
9	1	OEM Parameters
10	1	Reserved
11	1	Boot Checksum
Backup Boot Region
12	1	Boot Sector
13	8	Extended Boot Sectors
21	1	OEM Parameters
22	1	Reserved
23	1	Boot Checksum
FAT Region
24	FatOffset - 24	FAT Alignment	Boot Sectors contain FatOffset
FatOffset	FatLength	First FAT	Boot Sectors contain FatOffset and FatLength
FatOffset + FatLength	FatLength	Second FAT	For TexFAT only
Data Region
FatOffset + FatLength * NumberOfFats	ClusterHeapOffset – (FatOffset + FatLength * NumberOfFats)	Cluster Heap Alignment
ClusterHeapOffset	ClusterCount * 2^SectorsPerClusterShift	Cluster Heap
ClusterHeapOffset + ClusterCount * 2^SectorsPerClusterShift	VolumeLength – (ClusterHeapOffset + ClusterCount * 2^SectorsPerClusterShift)	Excess Space

從這個表中可以看到我們所需要找的 boot sector checksum 位於第12個sector，它是對前11個sector所做的checksum。我們可以從上面的鏈接中找到boot sector checksum的計算方法，參考下面：

exFAT Boot Checksum

This sector contains a repeating 32-bit checksum of the previous 11 sectors. The checksum calculation excludes VolumeFlags and PercentInUse fields in Boot Sector (bytes 106, 107, 112). The checksum is repeated until the end of the sector. The number of repetitions depends on the size of the sector.

  UNIT32 BootChecksum(constunsignedchardata[], intbytes)
{
UINT32 checksum   =   0; 
for (inti = 0; i < bytes; i++)
{
if (i == 106 || i == 107 || i == 112)
continue;
checksum = (checksum<<31) | (checksum>> 1) + data[i];
}
returnchecksum;

有了上面的partition開始地址、boot sector checksum計算方法，我們就可以讀取並check boot sector checksum是否正確。checksum這個sector以4個字節爲計算結果，一直重複這個計算結果。例如：

...

第12個sector開始地址是 11*512 = 5632 = 0x1600

上述的boot sector checksum爲 0x8B165B51

通過手動將這個數字改變，並重新寫入相同的位置，重新認卡，會發現彈出上述的對話框，提示卡需要format，同時在debug log中顯示boot sector checksum error。

以上就是對ExFAT boot sector checksum error所做的實驗。

ExFAT對比FAT32可以參考上述鏈接，主要區別如下：

exFAT vs. FAT32 Comparison

Feature	FAT32	exFAT
Maximum Volume Size	8 TB*	128 PB
Maximum File Size	4 GB	16 EB
Maximum Cluster Size	32 KB **	32 MB
Maximum Cluster Count	228	232
Maximum File Name Length	255	255
Date/Time resolution	2 s	10 ms
MBR Partition Type Identifier	0x0B, 0x0C	0x07

* Windows cannot format FAT32 volumes bigger than 32GB, though itsupports larger volumes created by third party implementations; 16 TB is the maximum volume size if formatted with 64KB cluster

** According to Microsoft KB184006 clusters cannot be 64KB or larger, though some third party implementations support up to 64KB.