DDJ, December 1995: Examining the Windows 95 Layered File System
by Mark Russinovich and Bryce Cogswell

Listing Three

typedef struct _DCB_COMMON {
    ULONG   DCB_physical_dcb;       // DCB for physical device
    ULONG   DCB_expansion_length;   // total length of IOP extension filled
    // Link fields follow
    PVOID   DCB_ptr_cd;             // pointer to calldown list
    ULONG   DCB_next_dcb;           // pointer to next DCB  
    ULONG   DCB_next_logical_dcb;   // pointer to next logical dcb
                    // for physical device
    BYTE    DCB_drive_lttr_equiv;   // drive number (A: = 0, etc.)
                    // set up by iosserv during logical
                    // device associate processing.
    BYTE    DCB_unit_number;    // either physical drive number
                    // (sequential drive number or'd
                    // with 80h) or unit number within
                    // tsd. set up by iosbid for disk
                    // physical dcb's. set up by tsdpart
                    // for disk logical dcb's. set up by
                    // tsdaer for cdrom physical dcb's.
    USHORT  DCB_TSD_Flags;      // Flags for TSD
    // Volume Tracking fields follow
    ULONG   DCB_vrp_ptr;            // pointer to VRP for this DCB
    ULONG   DCB_dmd_flags;          // demand bits of the topmost layer
    ULONG   DCB_device_flags;       // was BDD_Flags
    ULONG   DCB_device_flags2;      // second set of general purpose flags
    ULONG   DCB_Partition_Start;    // partition start sector
    ULONG   DCB_track_table_ptr;    // pointer for the track table buffer
                    // for ioctls
    ULONG   DCB_bds_ptr;            // DOS BDS corresp. to this DCB
                    // (logical DCB's only)
    ULONG   DCB_Reserved1;          // reserved
    ULONG   DCB_Reserved2;          // reserved
    BYTE    DCB_apparent_blk_shift; // log of apparent_blk_size
    BYTE    DCB_partition_type;     // partition type
    USHORT  DCB_sig;                // padding and signature
    BYTE    DCB_device_type;        // Device Type
    ULONG   DCB_Exclusive_VM;       // exclusive access handle to device
    UCHAR   DCB_disk_bpb_flags;     // bpb flags see defines below
    UCHAR   DCB_cAssoc;             // count of logical drives
                    // associated with this logical DCB
    UCHAR   DCB_Sstor_Host;     // indicates a sstor host volume
    USHORT  DCB_user_drvlet;    // the userdriveletter settings
    USHORT  DCB_Reserved3;      // reserved
    ULONG   DCB_Reserved4;      // reserved
} DCB_COMMON, *PDCB_COMMON;

typedef struct _DCB {
    DCB_COMMON DCB_cmn;
    ULONG   DCB_max_xfer_len;         // maximum transfer length
    // Actual geometry data follows
    ULONG   DCB_actual_sector_cnt[2]; // number of sectors as seen below 
                    // the tsd.            
    ULONG   DCB_actual_blk_size;      // actual block size of the device 
                    // as seen below the tsd.      
    ULONG   DCB_actual_head_cnt;      // number of heads as seen below   
                    // the tsd.            
    ULONG   DCB_actual_cyl_cnt;       // number of cylinders as seen     
                    // below the tsd.          
    ULONG   DCB_actual_spt;           // number of sectors per track as  
                    // seen below the tsd.         
    PVOID   DCB_next_ddb_dcb;   // link to next DCB on DDB chain
    PVOID   DCB_dev_node;       // pointer to dev node for this device
    BYTE    DCB_bus_type;       // Type of BUS
    BYTE    DCB_bus_number;     // channel (cable) within adapter  
    UCHAR   DCB_queue_freeze;   // queue freeze depth counter
    UCHAR   DCB_max_sg_elements;// max # s/g elements
    UCHAR   DCB_io_pend_count;  // number of requests pending for
                    // this DCB (Vol Track Layer use)
    UCHAR   DCB_lock_count;     // depth counter for LOCK MEDIA
    // SCSI fields follow
    USHORT  DCB_SCSI_VSD_FLAGS; // Flags for SRB builder       
    BYTE    DCB_scsi_target_id; // SCSI target ID
    BYTE    DCB_scsi_lun;       // SCSI logical unit number 
    BYTE    DCB_scsi_hba;       // adapter number relative to port drv.
    BYTE    DCB_max_sense_data_len; // Maximum sense Length        
    USHORT  DCB_srb_ext_size;       // miniport srb extension length   
    BYTE    DCB_inquiry_flags[8];   // Device Inquiry Flags        
    BYTE    DCB_vendor_id[8];       // Vendor ID string        
    BYTE    DCB_product_id[16];     // Product ID string           
    BYTE    DCB_rev_level[4];       // Product revision level      
    BYTE    DCB_port_name[8];
    UCHAR   DCB_current_unit;       // used to emulate mltpl log. devices
                    // with a single physical device
    ULONG   DCB_blocked_iop;    // pointer to requests for an inactive
                    // volume
    ULONG   DCB_vol_unlock_timer; // unlock timer handle
    UCHAR   DCB_access_timer;     // measures time between accesses
    UCHAR   DCB_Vol_Flags;        // Flags for Volume Tracking         
    BYTE    DCB_q_algo;           // queuing algorithm index 
    BYTE    DCB_unit_on_ctl;      // relative device number on ctlr
    ULONG   DCB_Port_Specific;    // bytes for PORT DRIVER use       
    ULONG   DCB_spindown_timer;   // timer for drive spin down     
    DCB_BLOCKDEV    DCB_bdd;
} DCB, *PDCB;

// define the device control block (dcb) for logical disk devices
typedef struct _LOG_DCB {
    DCB_COMMON DCB_cmn;
} LOG_DCB, *PLOG_DCB;