Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!caip!elbereth!rutgers!sri-spam!nike!cit-vax!ll-xn!adelie!axiom!linus!philabs!polaris!josh From: josh@polaris.UUCP (Josh Knight) Newsgroups: net.arch Subject: Re: paging and loading Message-ID: <752@polaris.UUCP> Date: Fri, 17-Oct-86 14:17:33 EDT Article-I.D.: polaris.752 Posted: Fri Oct 17 14:17:33 1986 Date-Received: Tue, 21-Oct-86 21:21:31 EDT References: <949@usl.UUCP> <5100148@ccvaxa> <7232@utzoo.UUCP> Reply-To: josh@polaris.UUCP (Josh Knight) Organization: IBM Research, Yorktown Heights, N.Y. Lines: 32 Keywords: paging, pages, memory, addressability Summary: Page addressing and memory addressing are not the same In article <7232@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes: >> ... The IBM 3090 has already shown an example of a >> system where it is faster to page synchronously rather than context switch, >> in which case your CPU *is* idle while paging - and these are likely to >> become more common. ... > >Ah, but what device are they paging from? If I remember it correctly -- >I'm not sure of this -- the 3090 is paging out of ordinary RAM, which they >would have preferred to make directly addressable except that they are >constrained by backward compatibility and not enough address bits. This >is a kludge, not something that is likely to become widespread. What IBM prefers to do is a difficult thing to determine (wish I could ;-). It is true that the expanded storage announced for the 3090 is "ordinary RAM". However, it is not necessarily true that one would prefer to have real memory instead of a fast page addressable store. The address bits required to maintain the latter (only page addresses are maintained) are fewer and one does not have to maintain whatever protection mechanisms (in the case of the IBM 370 architecture storage protect keys) are provided for "main storage". In addition, page stores could have different porting and provide access to multiple processors and/or I/O servers with less overhead. These latter features are not supported by the 3090 expanded storage, but other systems might choose page addressable stores over byte addressable stores for these reassons. Of course, I'm speaking only for myself, not my employer. -- Josh Knight, IBM T.J. Watson Research josh@ibm.com, josh@yktvmh.bitnet, ...!philabs!polaris!josh