Path: csiph.com!usenet.pasdenom.info!weretis.net!feeder4.news.weretis.net!eternal-september.org!feeder.eternal-september.org!mx04.eternal-september.org!.POSTED!not-for-mail From: Stuart Newsgroups: comp.lang.java.programmer Subject: Re: Program in 32-bit, run on 64-bit OK? Date: Fri, 16 Nov 2012 00:08:58 +0100 Organization: A noiseless patient Spider Lines: 19 Message-ID: References: <50a3a6ea$0$290$14726298@news.sunsite.dk> <50a3d194$0$289$14726298@news.sunsite.dk> <1ime9q5hlhcfs.x8wnfo60hr9g$.dlg@40tude.net> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit Injection-Date: Thu, 15 Nov 2012 23:08:58 +0000 (UTC) Injection-Info: mx04.eternal-september.org; posting-host="21aac22f8896a48c2c25e23bfecada7e"; logging-data="3926"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1/g9Z/EvX0LclfdbsJLqOTvS3pZfmqC5b4=" User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:16.0) Gecko/20121026 Thunderbird/16.0.2 In-Reply-To: Cancel-Lock: sha1:GnIzBcT1M1zJnFEujBjdum6/MZE= Xref: csiph.com comp.lang.java.programmer:19760 On 11/15/12 sl@exabyte wrote: [snip] > And I suppose only hardware with 64-bit address bus can run 64-bit OS. Wrong. Typical 64 bit processors have either a 36 bit or a 40 bit physical address bus (40 bits are 1 TB, an almost incredible amount of RAM, wikipedia mentions that some AMD chips even offer a 48 bit memory bus). Note that bus width and register width do not have to have a one-to-one relationship. The early 286 processor had a register width of 16 bits but an address bus with 24 bits (4MB), so they had to invent this segmentation model in order to compute a 20bit address from two 16 bit registers. Twenty years later, the segmenation unit is still present at the Intel architecture while the numbers have been reversed: now the 36 or 40 bit physical address is computed from a 16 bit value and a 64 bit value. Sounds like overkill, and yes, it is. That's just for backwards compatibility for applications from 1985. Regards, Stuart