Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!tank!uxc!uxc.cso.uiuc.edu!mcdurb!aglew From: aglew@mcdurb.Urbana.Gould.COM Newsgroups: comp.arch Subject: Re: RT/PC Unaligned Accesses Message-ID: <28200296@mcdurb> Date: 6 Apr 89 21:03:00 GMT Lines: 32 Nf-ID: #R: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 Time yet again for expression of one of my favorite architectural "gimmicks": provide a mask that is implicitly ANDed with any {instruction,data} address before use -- this way you could place tag bits in the low bits, in the high bits, or in any bit you wanted (and not have to rely on features like "The 68000 only implements 24 address bits", etc.). Yes, this is a gimmick... Andy "Krazy" Glew aglew@urbana.mcd.mot.com uunet!uiucdcs!mcdurb!aglew Motorola Microcomputer Division, Champaign-Urbana Design Center 1101 E. University, Urbana, Illinois 61801, USA. My opinions are my own, and are not the opinions of my employer, or any other organisation. I indicate my company only so that the reader may account for any possible bias I may have towards our products.