Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!gem.mps.ohio-state.edu!ginosko!uunet!mcsun!hp4nl!philapd!ssp1!roelof From: roelof@idca.tds.PHILIPS.nl (R. Vuurboom) Newsgroups: comp.arch Subject: Re: Instruction (dis)continuation Summary: Oh what a tangled web we weave... Message-ID: <254@ssp1.idca.tds.philips.nl> Date: 11 Sep 89 17:48:00 GMT References: <241@ssp1.idca.tds.philips.nl> <34228@grapevine.uucp> Organization: Philips Telecommunication and Data Systems, The Netherlands Lines: 35 In article <34228@grapevine.uucp> johnz@grapevine.uucp (John Zolnowsky ext. 33230) writes: > >Although virtual memory was desired for the 68000, it was deemed too costly >to provide an instruct restart model. This required many more temporary > >After the 68000 went to market, the instruction continuation model was >conceived. This is best understood as an interrupt at the microcode level. > >Presumably, in later processors from the family, the provision of extra >hardware to reduce instruction cycle counts also leads to a reduction >in the indeterminancy of the values of registers. This makes the cost >of instruction restart more tractable. > So what you're saying is that motorola first thought that virtual memory support could only be supported through instruction restart later on they conceived the concept of instruction continuation. The point is still this: why go to instruction restart seeing that (1) you _can't_ provide virtual (memory-mapped) i/o with instruction restart (this is the virtual machine part that can't be supported) (2) you can have big problems with memory-mapped i/o based on reads Instruction restart appears to be _less_ powerful than instruction continuation. Just look at the algorithm recently defined here to prevent instruction restart from doing multiple read accesses. -- wiskunde: Dutch for mathematics. Literally: Knowledge of certainty wis: certainty kunde: Knowledge Roelof Vuurboom SSP/V3 Philips TDS Apeldoorn, The Netherlands +31 55 432226 domain: roelof@idca.tds.philips.nl uucp: ...!mcvax!philapd!roelof