The file access block (FAB) defines file characteristics, file
    access, and certain run-time options. It also indicates whether
    other control blocks are associated with the file.

    The following table lists the symbolic offset, the size in bytes,
    the FDL equivalent, and a brief description of each FAB field.

    Field Offset   Size  FDL Equivalent          Description

    FAB$B_         1     None                    File access modes
    ACMODES
    FAB$L_ALQ      4     FILE ALLOCATION         Allocation quantity
                                                 (blocks)
    FAB$B_BID      1     None                    Block identifier
    FAB$B_BKS      1     FILE BUCKET_SIZE        Bucket size
    FAB$B_BLN      1     None                    Block length
    FAB$W_BLS      2     FILE MT_BLOCK_SIZE      Magnetic tape block
                                                 size
    FAB$V_CHAN_    -     None                    Channel access mode
    MODE                                         protection
    FAB$L_CTX      4     FILE CONTEXT            Context
    FAB$W_DEQ      2     FILE EXTENSION          Default file
                                                 extension quantity
    FAB$L_DEV      4     None                    Device
                                                 characteristics
    FAB$L_DNA      4     FILE DEFAULT_NAME       Default file
                                                 specification string
                                                 address
    FAB$B_DNS      1     FILE DEFAULT_NAME       Default file
                                                 specification string
                                                 size
    FAB$B_FAC      1     ACCESS                  File access
    FAB$L_FNA      4     FILE NAME               File specification
                                                 string address
    FAB$B_FNS      1     FILE NAME               File specification
                                                 string size
    FAB$L_FOP      4     FILE                    File-processing
                                                 options
    FAB$B_FSZ      1     RECORD CONTROL_FIELD_   Fixed-length control
                         SIZE                    area size
    FAB$W_GBC      2     FILE GLOBAL_BUFFER_     Global buffer count
                         COUNT
    FAB$W_IFI      2     None                    Internal file
                                                 identifier
    FAB$B_         1     None                    Journal flags status
    JOURNAL
    FAB$V_LNM_     -     None                    Logical name
    MODE                                         translation access
                                                 mode
    FAB$L_MRN      4     FILE MAX_RECORD_        Maximum record
                         NUMBER                  number
    FAB$W_MRS      2     RECORD SIZE             Maximum record size
    FAB$L_NAM      4     None                    Name (NAM) or long
                                                 name (NAML) block
                                                 address
    FAB$B_ORG      1     FILE ORGANIZATION       File organization
    FAB$B_RAT      1     RECORD                  Record attributes
    FAB$B_RFM      1     RECORD FORMAT           Record format
    FAB$B_RTV      1     FILE WINDOW_SIZE        Retrieval window
                                                 size
    FAB$L_SDC      4     None                    Secondary device
                                                 characteristics
    FAB$B_SHR      1     SHARING                 File sharing
    FAB$L_STS      4     None                    Completion status
                                                 code
    FAB$L_STV      4     None                    Status values
    FAB$L_XAB      4     None                    Extended attribute
                                                 block address