Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!elroy.jpl.nasa.gov!decwrl!ucbvax!tut.cis.ohio-state.edu!sei.cmu.edu!firth From: firth@sei.cmu.edu (Robert Firth) Newsgroups: comp.arch Subject: Re: Segmented Architectures ( formerly Re: 48-bit computers) Message-ID: <23614@as0c.sei.cmu.edu> Date: 4 Apr 91 13:40:39 GMT References: <1360009@aspen.IAG.HP.COM> <1PGAOP7@xds13.ferranti.com> Reply-To: firth@sei.cmu.edu (Robert Firth) Organization: Software Engineering Institute, Pittsburgh, PA Lines: 25 In article <1PGAOP7@xds13.ferranti.com> peter@ficc.ferranti.com (Peter da Silva) writes: >But that's what you just described: you only have 32 bits of flat address >space in a 48 bit machine. Sigh. I have seen this decade after decade, generation after generation. It seems to be a working rule among the builders of segmented machines that the most flat space anyone will ever need is 4 bits more than the current market leader. That's how we went from 12 bits to 16; the same arguments I heard from the builders of the 20-bit machine in 1974 are lying in my mailbox explaining why 32 bits (after all, 2 more than the VAX!) is enough. Here's an analogy. You live in a three-bedroom house. To get to two bnedrooms, you climb the interior staircase. To get to the third, you go outside and climb a ladder on the North wall. It's time to trade up. You look at a five-bedroom house. Three bedrooms open off the interior staircase; the other two are reached by a ladder on the North wall. The builder says "Look, you have three directly accessible bedrooms, which is 50% more that your current home, what more could you ever need?" You explain that what matters is not the absolute number of bedrooms, it is rather that, however many there are, they all be directly accessible by a simple and uniform route. He shakes his head in bewilderment. As do I.