VMS Help MACRO, VAX MACRO Assembler, Directives, .MASK  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Reserves a word for a register save mask for a transfer vector.

      .MASK symbol[,expression]

    symbol

    A symbol defined in an .ENTRY directive.

    expression

    A register save mask.

    If .MASK does not contain an expression, the  assembler  directs
    the Linker to copy the register save mask specified in .ENTRY to
    the word reserved by .MASK.

    If .MASK contains an expression, the assembler directs the Linker
    to combine  this  expression  with  the  register save mask
    specified in .ENTRY and store the result in  the  word  reserved
    by .MASK.  The Linker  performs an inclusive OR operation to
    combine the mask in the entry point  and  the  value  of  the
    expression.   Consequently,  a register  specified in either .ENTRY
    or .MASK will be included in the combined mask.