Embedded System Studying note 1/17

實作出 SD card 的 READ_SINGLE_BLOCK 的指令了
這個指令能從 SD card 讀出 512byte
終於能從 SD card 讀取資料了 :D

讀出 SD card 的 address 0 的內容
這個地方放的是鼎鼎大名的MBR (Master Boot Record), 主開機區塊


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對硬碟來說, x86 電腦開機會從 Cylinder=0, Header=0, Sector=1的地方拷貝512byte
到記憶體位址 0000:7C00 的地方執行, C/H/S = (0, 0, 1) 的這個區塊就是 MBR
我們會說 "硬碟如果開機磁區壞了, 就整個沒救了"

在磁碟片的時代, 我們經常說 "第 0軌壞掉的磁片, 跟報銷沒兩樣"
MBR 的角色是如此的重要, 值得花點時間研究一下 !!


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下面是我從 SD 卡讀出的 MBR 內容, 看不懂對吧? 看得懂才怪勒 XD
這堆是給 CPU吃的機械碼, 這一小段程式正是 bootloader
會去看 partiton table 的內容, 並且跳躍到不同 partition 的位置, 載入作業系統

下面的網頁有這段機械碼的反組譯, 有興趣的人可看看
http://www.geocities.com/thestarman3/asm/mbr/STDMBR.htm

很有趣的地方是標藍色的區塊, 原來拿一顆沒裝OS的硬碟開機
電腦會顯示 Missing operating system 是由 MBR 裡的程式跑出來的
很好玩吧 :)

                                                                                
Offset      0  1  2  3  4  5  6  7   8  9 10 11 12 13 14 15
                                                                                
00000000   FA 33 C0 8E D0 BC 00 7C  8B F4 50 07 50 1F FB FC   ??邾.|P.P.
00000016   BF 00 06 B9 00 01 F2 A5  EA 1D 06 00 00 BE BE 07   ?.?.禰?...噢.
00000032   B3 04 80 3C 80 74 0E 80  3C 00 75 1C 83 C6 10 FE   ?€<€t.€<.u..?
00000048   CB 75 EF CD 18 8B 14 8B  4C 02 8B EE 83 C6 10 FE   肙懰.?..?
00000064   CB 74 1A 80 3C 00 74 F4  BE 8B 06 AC 3C 00 74 0B   肕.€<.t藶??.t.
00000080   56 BB 07 00 B4 0E CD 10  5E EB F0 EB FE BF 05 00   V?.??^鋺錌?.
00000096   BB 00 7C B8 01 02 57 CD  13 5F 73 0C 33 C0 CD 13   ?|?.W?_s.3櫛.
00000112   4F 75 ED BE A3 06 EB D3  BE C2 06 BF FE 7D 81 3D   Ou篿?邆壇.錚}?
00000128   55 AA 75 C7 8B F5 EA 00  7C 00 00 49 6E 76 61 6C   U沿?纋.|..Inval
00000144   69 64 20 70 61 72 74 69  74 69 6F 6E 20 74 61 62   id partition tab
00000160   6C 65 00 45 72 72 6F 72  20 6C 6F 61 64 69 6E 67   le.Error loading
00000176   20 6F 70 65 72 61 74 69  6E 67 20 73 79 73 74 65    operating syste
00000192   6D 00 4D 69 73 73 69 6E  67 20 6F 70 65 72 61 74   m.Missing operat
00000208   69 6E 67 20 73 79 73 74  65 6D 00 00 00 00 00 00   ing system......
00000224   00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00   ................
00000240   00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00   ................
00000256   00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00   ................
00000272   00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00   ................
00000288   00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00   ................
00000304   00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00   ................
00000320   00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00   ................
00000336   00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00   ................
00000352   00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00   ................
00000368   00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00   ................
00000384   00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00   ................
00000400   00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00   ................
00000416   00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00   ................
00000432   00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 03   ................
00000448   3B 00 06 2E EE DE F7 00  00 00 09 F3 3C 00 00 00   ;...鍪?...?...
00000464   00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00   ................
00000480   00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00   ................
00000496   00 00 00 00 00 00 00 00  00 00 00 00 00 00 55 AA   ..............U

MBR 的結尾一定是 0x55, 0xAA, 當我從 SD card 讀出MBR以後
這兩個 byte 讓我相信我讀出來的不是垃圾

上圖上綠色的部分, 就是所謂的 partition table entry, 一個 entry 16 bytes.
我這張 SD 卡只有一個 partition, 在 0x55, 0xAA 以前, 還塞得下 3個 partition
這就是為什麼, 一顆硬碟只能放 4個 primary partition.


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下表是我這張 Transcend 2G SD 的 Partition Table 訊息

                                                                                
Offset  Description                             Size  Value
-----------------------------------------------------------------
0x00    Bootable flag                           1     0x00
0x01    First Partition Sector                  3     0x003B03
0x04    File System Descriptor                  1     0x06
0x05    Last Partition Sector                   3     0xDEEE2E
0x08    # of sectors between MBR to partition   4     0x000000F7
0x0C    # of sectors in the partition           4     0x003C3F09

一個 sector 等於 512byte, 這張卡一共有 0x3C3F09 = 3948297 sectors
容量為 3948297*512 = 2021528064 = 1927.879 MB, 約略等於 2G

然後他跟我說, 我可以到 Sector 0xF7 的位置, 那裡是這個 partition 的開頭
到這裡可以拿到 FAT table... 只要我能讀出內容, 我就能讀出檔案了