目前，Reactos主要使用FAT的文件系统，因此下面就先来分析这个文件系统，以便了解这个文件系统的功能。这个文件系统的实现是在驱动程序FASTFAT.SYS里实现的，所以来分析这个驱动程序的源码，就可以了解FAT文件系统了。
FAT的文件系统是当作一个驱动程序来加载到系统里，因此它也有驱动程序的形式了，也就是它的入口点函数，还是DriverEntry函数，它的实现代码如下：
#001 NTSTATUS NTAPI
#002 DriverEntry(PDRIVER_OBJECT DriverObject,
#003 PUNICODE_STRING RegistryPath)
#004 /*
#005 * FUNCTION: Called by the system to initialize the driver
#006 * ARGUMENTS:
#007 * DriverObject = object describing this driver
#008 * RegistryPath = path to our configuration entries
#009 * RETURNS: Success or failure
#010 */
#011 {
#012 PDEVICE_OBJECT DeviceObject;
FAT作为设备名称。
#013 UNICODE_STRING DeviceName = RTL_CONSTANT_STRING(L"//Fat");
#014 NTSTATUS Status;
#015
创建文件系统设备对象。
#016 Status = IoCreateDevice(DriverObject,
#017 sizeof(VFAT_GLOBAL_DATA),
#018 &DeviceName,
#019 FILE_DEVICE_DISK_FILE_SYSTEM,
#020 0,
#021 FALSE,
#022 &DeviceObject);
#023
如果创建这个设备名称已经存在，就使用RosFat的名称来创建设备。
#024 if (Status == STATUS_OBJECT_NAME_EXISTS ||
#025 Status == STATUS_OBJECT_NAME_COLLISION)
#026 {
#027 /* Try an other name,if 'Fat' is already in use. 'Fat' is also used by fastfat.sys on W2K */
#028 RtlInitUnicodeString(&DeviceName,L"//RosFat");
#029 Status = IoCreateDevice(DriverObject,
#030 sizeof(VFAT_GLOBAL_DATA),
#031 &DeviceName,
#032 FILE_DEVICE_DISK_FILE_SYSTEM,
#033 0,
#034 FALSE,
#035 &DeviceObject);
#036 }
#037
#038
#039
#040 if (!NT_SUCCESS(Status))
#041 {
#042 return (Status);
#043 }
#044
获取FAT全局数据区的指针。
#045 VfatGlobalData = DeviceObject->DeviceExtension;
清空FAT全局数据区。
#046 RtlZeroMemory (VfatGlobalData,sizeof(VFAT_GLOBAL_DATA));
#047 VfatGlobalData->DriverObject = DriverObject;
#048 VfatGlobalData->DeviceObject = DeviceObject;
#049
设置设备是直接访问IO的方式。
#050 DeviceObject->Flags |= DO_DIRECT_IO;
设置驱动程序的IRP消息处理函数。
#051 DriverObject->MajorFunction[IRP_MJ_CLOSE] = VfatBuildRequest;
#052 DriverObject->MajorFunction[IRP_MJ_CREATE] = VfatBuildRequest;
#053 DriverObject->MajorFunction[IRP_MJ_READ] = VfatBuildRequest;
#054 DriverObject->MajorFunction[IRP_MJ_WRITE] = VfatBuildRequest;
#055 DriverObject->MajorFunction[IRP_MJ_FILE_SYSTEM_CONTROL] = VfatBuildRequest;
#056 DriverObject->MajorFunction[IRP_MJ_QUERY_INFORMATION] = VfatBuildRequest;
#057 DriverObject->MajorFunction[IRP_MJ_SET_INFORMATION] = VfatBuildRequest;
#058 DriverObject->MajorFunction[IRP_MJ_DIRECTORY_CONTROL] = VfatBuildRequest;
#059 DriverObject->MajorFunction[IRP_MJ_QUERY_VOLUME_INFORMATION] =
#060 VfatBuildRequest;
#061 DriverObject->MajorFunction[IRP_MJ_SET_VOLUME_INFORMATION] =
#062 VfatBuildRequest;
#063 DriverObject->MajorFunction[IRP_MJ_SHUTDOWN] = VfatShutdown;
#064 DriverObject->MajorFunction[IRP_MJ_LOCK_CONTROL] = VfatBuildRequest;
#065 DriverObject->MajorFunction[IRP_MJ_CLEANUP] = VfatBuildRequest;
#066 DriverObject->MajorFunction[IRP_MJ_FLUSH_BUFFERS] = VfatBuildRequest;
#067
#068 DriverObject->DriverUnload = NULL;
#069
缓存管理函数。
#070 /* Cache manager */
#071 VfatGlobalData->CacheMgrCallbacks.AcquireForLazyWrite = VfatAcquireForLazyWrite;
#072 VfatGlobalData->CacheMgrCallbacks.ReleaseFromLazyWrite = VfatReleaseFromLazyWrite;
#073 VfatGlobalData->CacheMgrCallbacks.AcquireForReadAhead = VfatAcquireForReadAhead;
#074 VfatGlobalData->CacheMgrCallbacks.ReleaseFromReadAhead = VfatReleaseFromReadAhead;
#075
#076 /* Fast I/O */
#077 VfatInitFastIoRoutines(&VfatGlobalData->FastIoDispatch);
#078 DriverObject->FastIoDispatch = &VfatGlobalData->FastIoDispatch;
#079
后备列表，以便提高速度。
#080 /* Private lists */
#081 ExInitializeNPagedLookasideList(&VfatGlobalData->FcbLookasideList,
#082 NULL,NULL,sizeof(VFATFCB),TAG_FCB,0);
#083 ExInitializeNPagedLookasideList(&VfatGlobalData->CcbLookasideList,
#084 NULL,sizeof(VFATCCB),TAG_CCB,0);
#085 ExInitializeNPagedLookasideList(&VfatGlobalData->IrpContextLookasideList,
#086 NULL,sizeof(VFAT_IRP_CONTEXT),TAG_IRP,0);
#087
初始化资源列表。
#088 ExInitializeResourceLite(&VfatGlobalData->VolumeListLock);
#089 InitializeListHead(&VfatGlobalData->VolumeListHead);
#090 IoRegisterFileSystem(DeviceObject);
#091 return(STATUS_SUCCESS);
#092}
