C/GenBootSector/GetDrvNumOffset.c

/** @file

  Get Drv Num offset from Fat file system.

Copyright (c) 2006 - 2008, Intel Corporation. All rights reserved.<BR>
This program and the accompanying materials
are licensed and made available under the terms and conditions of the BSD License
which accompanies this distribution.  The full text of the license may be found at
http://opensource.org/licenses/bsd-license.php

THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.

**/

#include <stdio.h>
#include "FatFormat.h"

INTN
GetDrvNumOffset (
  IN VOID *BootSector
  )
{
  FAT_BPB_STRUCT  *FatBpb;
  UINTN           RootDirSectors;
  UINTN           FATSz;
  UINTN           TotSec;
  UINTN           DataSec;
  UINTN           CountOfClusters;

  FatBpb  = (FAT_BPB_STRUCT *) BootSector;

  //
  // Check FAT type algorithm from FAT spec
  //
  RootDirSectors = ((FatBpb->Fat12_16.BPB_RootEntCnt * sizeof(FAT_DIRECTORY_ENTRY)) +
                    (FatBpb->Fat12_16.BPB_BytsPerSec - 1)) / FatBpb->Fat12_16.BPB_BytsPerSec;

  if (FatBpb->Fat12_16.BPB_FATSz16 != 0) {
    FATSz = FatBpb->Fat12_16.BPB_FATSz16;
  } else {
    FATSz = FatBpb->Fat32.BPB_FATSz32;
  }
  if (FATSz == 0) {
    fprintf (stderr, "error E3003: FAT - BPB_FATSz16, BPB_FATSz32 - 0, expected: Non-Zero number\n");
    return -1;
  }

  if (FatBpb->Fat12_16.BPB_TotSec16 != 0) {
    TotSec = FatBpb->Fat12_16.BPB_TotSec16;
  } else {
    TotSec = FatBpb->Fat12_16.BPB_TotSec32;
  }
  if (TotSec == 0) {
    fprintf (stderr, "error E3003: FAT - BPB_TotSec16, BPB_TotSec32 - 0, expected: Non-Zero number\n");
    return -1;
  }

  DataSec = TotSec - (
                      FatBpb->Fat12_16.BPB_RsvdSecCnt +
                      FatBpb->Fat12_16.BPB_NumFATs * FATSz +
                      RootDirSectors
                     );

  CountOfClusters = DataSec / FatBpb->Fat12_16.BPB_SecPerClus;

  if (CountOfClusters < FAT_MAX_FAT16_CLUSTER) {
    return (INTN) ((UINTN) &FatBpb->Fat12_16.BS_DrvNum - (UINTN) FatBpb);
  } else {
    return (INTN) ((UINTN) &FatBpb->Fat32.BS_DrvNum - (UINTN) FatBpb);
  }
}