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計算檔案系統辨識總和檢查碼

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由 Windows 內部定義的 FILE_SYSTEM_RECOGNITION_STRUCTURE 結構，並由 檔案系統辨識 (FRS) 使用，包含必須正確計算總和檢查碼值，FRS 才能與指定的無法辨識檔案系統正常運作。 下列範例會完成此計算。

typedef struct _FILE_SYSTEM_RECOGNITION_STRUCTURE {
  UCHAR  Jmp[3];
  UCHAR  FsName[8];
  UCHAR  MustBeZero[5];
  ULONG  Identifier;
  USHORT Length;
  USHORT Checksum;
} FILE_SYSTEM_RECOGNITION_STRUCTURE, *PFILE_SYSTEM_RECOGNITION_STRUCTURE;

USHORT ComputeFileSystemInformationChecksum (
    __in PFILE_SYSTEM_RECOGNITION_STRUCTURE Fsrs
    )

/*++

Routine Description:

    This routine computes the file record checksum.

Arguments:

    Fsrs - Pointer to the record.

Return Value:

    The checksum result.

--*/

{
    USHORT Checksum = 0;
    USHORT i;
    PUCHAR Buffer = (PUCHAR)Fsrs;
    USHORT StartOffset;

    //
    //  Skip the jump instruction
    //

    StartOffset = FIELD_OFFSET(FILE_SYSTEM_RECOGNITION_STRUCTURE, FsName);
    
    for (i = StartOffset; i < Fsrs->Length; i++) {

        //
        //  Skip the checksum field itself, which is a USHORT.
        //

        if ((i == FIELD_OFFSET(FILE_SYSTEM_RECOGNITION_STRUCTURE, Checksum)) ||
            (i == FIELD_OFFSET(FILE_SYSTEM_RECOGNITION_STRUCTURE, Checksum)+1)) {

            continue;
        }

        Checksum = ((Checksum & 1) ? 0x8000 : 0) + (Checksum >> 1) + Buffer[i];
    }

    return Checksum;
}

檔案系統辨識

FILE_SYSTEM_RECOGNITION_STRUCTURE