The Topaz system as a whole is the work of many people at SRC. The guidelines for alertability in Section 3.3 of Part I resulted from discussions with Andrew Birrell and Roy Levin. The OS interface and its reference manual (Part II) profited from Andy Hisgen's careful reading and comments. Andrew Birrell and Michael Schroeder provided valuable advice on the overall design. John DeTreville, Cynthia Hibbard, Michael Schroeder, and Roger Needham provided comments that improved Part I of this report.