ACP_SYSACC sets the number of directory file control blocks
    (FCBs) that are cached for disks mounted with the /SYSTEM
    qualifier. Each directory FCB contains a 16-byte array containing
    the first letter of the last entry in each block of the directory
    (or group of blocks if the directory exceeds 16 blocks). Since
    entries in a directory are alphabetical, the cached FCB provides
    quick access to a required directory block. This parameter value
    should be roughly equivalent to the number of directories that
    are in use concurrently on each system volume. It might be
    overridden on a per-volume basis with the /ACCESSED qualifier
    to the DCL command MOUNT. The value should be kept low in systems
    with small physical memory and little file activity, because
    the FCBs require a significant amount of space in the nonpaged
    dynamic pool.

    Too small a value causes excessive XQP I/O operations, while too
    large a value causes excessive physical memory to be consumed by
    the FCB caches.

    ACP_SYSACC is an AUTOGEN and DYNAMIC parameter.