pagcnt

    OpenVMS usage:longword_unsigned
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    Number of pagelets to add to the current end of the program
    or control region. The pagcnt argument is a longword value
    containing this number.

    On Alpha and Integrity server systems, the specified value is
    rounded up to an even multiple of the CPU-specific page size.

 retadr

    OpenVMS usage:address_range
    type:         longword (unsigned)
    access:       write only
    mechanism:    by reference
    Starting and ending process virtual addresses of the pages that
    $EXPREG has actually added. The retadr argument is the address of
    a 2-longword array containing, in order, the starting and ending
    process virtual addresses.

 acmode

    OpenVMS usage:access_mode
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    Access mode to be associated with the newly added pages. The
    acmode argument is a longword containing the access mode.

    The most privileged access mode used is the access mode of the
    caller.

    The newly added pages are given the following protection:
    (1)  read and write access for access modes equal to or more
    privileged than the access mode used in the call, and (2)  no
    access for access modes less privileged than that used in the
    call.

 region

    OpenVMS usage:longword_unsigned
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    Number specifying which program region is to be expanded. The
    region argument is a longword value. A value of 0 (the default)
    specifies that the program region (P0 region) is to be expanded.
    A value of 1 specifies that the control region (P1 region) is to
    be expanded.