Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!rutgers!seismo!rochester!pt.cs.cmu.edu!k.cs.cmu.edu!dpm From: dpm@k.cs.cmu.edu.UUCP Newsgroups: comp.sys.m68k Subject: Re: move sr/move ccr: crock Message-ID: <1092@k.cs.cmu.edu> Date: Sat, 24-Jan-87 16:13:44 EST Article-I.D.: k.1092 Posted: Sat Jan 24 16:13:44 1987 Date-Received: Sat, 24-Jan-87 23:55:25 EST References: <809@imagen.UUCP> Organization: Carnegie-Mellon University, CS/RI Lines: 21 Keywords: MC680X0, virtual machine Summary: Why MOVE from SR is privileged The MOVE SR, instruction was made privileged in the later MC680X0 family members in order to support the "virtual machine" concept. If the proper trap handlers are installed, the operating system can "intercept" privileged instructions and effectively "fool" the user code into thinking it is running in supervisor mode. Among other things, this allows OS developers to debug new versions of the operating system while running under an older version. It is my understanding that the instruction was not privileged in the original MC68000 because of an oversight. ------------------------------------------------------------------------------ David P. Maynard Department of Electrical and Computer Engineering Carnegie Mellon University (412) ANT-FEED ARPA: dpm@faraday.ece.cmu.edu UUCP: {sun,seismo,...}!faraday.ece.cmu.edu!dpm ------------------------------------------------------------------------------