52 lines
2.1 KiB
Markdown
52 lines
2.1 KiB
Markdown
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## FAT32 Volume Boot Record Format
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Windows 10 checks for the following (https://github.com/microsoft/Windows-driver-samples/blob/16022c593787aef040b8a37ef697316e07f1322e/filesys/fastfat/fsctrl.c#L2546):
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* [offset=0x000, length=3] First byte must be: 0xE9, 0xEB, 0x49
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* [offset=0x00B, length=2] BytesPerSector must be: 128, 256, 512, 1024, 2048, or 4096
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* [offset=0x00D, length=1] SectorsPerCluster must be: 1, 2, 4, 8, 16, 32, 64, or 128
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* [offset=0x00E, length=2] ReservedSectors must be > 0
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* [offset=0x010, length=1] NumberOfFATs must be > 0
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* Either [offset=0x013, length=2] Sectors or [offset=0x020, length=4] LargeSectors must be > 0
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* [offset=0x016, length=2] SectorsPerFat > 0 or ([offset=0x024, length=4] LargeSectorsPerFAT > 0 and [offset=0x02A, length=2] FsVersion = 0)
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* [offset=0x015, length=1] MediaType must be: 0xF0, 0xF8, 0xF9, 0xFB, 0xFD, 0xFE, 0xFF
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* Either [offset=0x016, length=2] SectorPerFAT = 0 or RootEntries > 0
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* Either [offset=0x016, length=2] SectorPerFAT > 0 or MirrorDisabled
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Important Fields:
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* 0x000 - 0x002 : Boot jump code {0xEB 0xFE 0x00}
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* 0x003 - 0x00A : OEM Name
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* 0x00B - 0x05A : BPB and Extended BPB
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* 0x05B : CPU #
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* 0x05C - 0x05D : Adjustment spot for checksum (?)
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* 0x05E - 0x05F : Unused
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* 0x060 - 0x1FC : Boot sector code area
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* 0x1FD : Physical drive number (?)
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* 0x1FE - 0x1FF : Signature: 0x55 0xAA
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For Foenix/MCP: 16-bit big-endian checksum of entire sector must equal 0x2560
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Checksum: `sum = ((sum & 1) << 15) + (sum >> 1) + *w++;`
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### Boot Process from Floppy and SDC
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1. Load VBR into memory at 0x00000400
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2. Compute checksum and verify it comes to 0x2560
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3. Check that 0x05B has the right CPU ID
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4. Call to 0x00000460 as a subroutine
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## Master Boot Record Format
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* 0x000 - 0x1BD : Boot code area (approximate end point)
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* 0x1BE - 0x1FD : Partition data
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* 0x1FE - 0x1FF : Signature: 0x55 0xAA
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For Foenix/MCP: 16-bit big-endian checksum of entire sector must equal 0x2560
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### Boot Process for PATA drive
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1. Load MBR into memory at 0x00000400
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2. Compute checksum and verify it comes to 0x2560
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3. Check that 0x05B has the right CPU ID
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4. Call to 0x00000400 as a subroutine
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