efn

    OpenVMS usage:ef_number
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    Number of any event flag in the cluster whose status is to be
    returned. The efn argument is a longword containing this number;
    however, $READEF uses only the low-order byte. Specifying an
    event flag within a cluster requests that $READEF return the
    status of all event flags in that cluster.

    There are two local event flag clusters, which are local to the
    process: cluster 0 and cluster 1. Cluster 0 contains event flag
    numbers 0 to 31, and cluster 1 contains event flag numbers 32 to
    63.

    There are two common event flag clusters: cluster 2 and cluster
    3. Cluster 2 contains event flag numbers 64 to 95, and cluster 3
    contains event flag numbers 96 to 127.

 state

    OpenVMS usage:mask_longword
    type:         longword (unsigned)
    access:       write only
    mechanism:    by 32- or 64-bit reference (Alpha and Integrity
                  servers)
    State of all event flags in the specified cluster. The state
    argument is the 32- or 64-bit address (on Alpha and Integrity
    server systems) of a longword into which $READEF writes the state
    (set or clear) of the 32 event flags in the cluster.