Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!apple!bbn!rochester!pt.cs.cmu.edu!PLAY.MACH.CS.CMU.EDU!bsy From: bsy@PLAY.MACH.CS.CMU.EDU (Flexi-thumbs) Newsgroups: comp.arch Subject: Re: RT/PC Unaligned Accesses Keywords: lisp, tag bits Message-ID: <4636@pt.cs.cmu.edu> Date: 3 Apr 89 16:46:31 GMT References: <4618@pt.cs.cmu.edu> <16420@winchester.mips.COM> Organization: Cranberry Melon Lines: 14 One advantage of silently ignoring the low bits in unaligned accesses is that you can use the low order bits as free tag bits -- without runtime overhead when dereferencing the data if you don't care to type-check. This contrasts with the use of alignment traps for run time error checking. I don't know if the CMU RT Common Lisp actually does this (I'm not a Lisp hacker), but it would be a reasonable thing to do. -bsy -- Internet: bsy@cs.cmu.edu Bitnet: bsy%cs.cmu.edu%smtp@interbit CSnet: bsy%cs.cmu.edu@relay.cs.net Uucp: ...!seismo!cs.cmu.edu!bsy USPS: Bennet Yee, CS Dept, CMU, Pittsburgh, PA 15213-3890 Voice: (412) 268-7571 --