ACP_DATACHECK controls the consistency checks that are performed
    on internal file system metadata such as file headers.

    ACP_DATACHECK is a bit mask. The following table shows the bits
    that are defined currently:

    Bit        Description

    0          Set this bit to perform consistency checks on read
               operations.

               When this bit is set, the IO$M_DATACHECK function
               modifier is automatically set on all subsequent IO$_
               READLBLK operations that read file system metadata (see
               the OpenVMS I/O User's Reference Manual).

    1          Set this bit to perform consistency checks on write
               operations.

               When this bit is set, the IO$M_DATACHECK function
               modifier is automatically set on all subsequent IO$_
               WRITELBLK operations that read file system metadata
               (see the OpenVMS I/O User's Reference Manual).

    2          Set this bit to perform read-after-write consistency
               checks.

               This is similar to setting bit 1, except that in this
               case the file system does the checks, not the lower
               level device or disk driver.

               Note that read-after-write consistency checks are not
               allowed on deferred writes. Deferred writes are turned
               off if this bit is set.

    3          reserved for VSI use only; must be zero.

    4          reserved for VSI use only; must be zero.

    5 and 6    These two bits control the checks that are performed on
               reads and writes of directory blocks. You can select
               one of four different levels:
                                                 By
                                      Select     Setting
                                      This       Bit 6     And Bit 5
               To Check That...       Level...   to...     to...

               The block is a valid   0          0         0
               directory block
               (reads only)

               The block is a valid   1          0         1
               directory block
               (reads and writes)

               The block is a valid   2          1         0
               directory block
               and contains valid
               entries (reads and
               writes)

               The block is a valid   3          1         1
               directory block
               and contains valid
               entries in correct
               alphanumeric order
               (reads and writes)

               When you set the SYSTEM_CHECK system parameter to 1,
               you enable level 3 checking of directory blocks.

               Write errors result in BUGCHECK and crash your system;
               read errors exit with error status SS$_BADDIRECTORY.

    7          reserved for VSI use only; must be zero.