ACP_DATACHECK

   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.