Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!brutus.cs.uiuc.edu!psuvax1!gatech!mcnc!decvax!ima!esegue!johnl From: johnl@esegue.segue.boston.ma.us (John R. Levine) Newsgroups: comp.sys.ibm.pc Subject: Re: Memory in Compaq 386 Message-ID: <1989Sep3.175859.5068@esegue.segue.boston.ma.us> Date: 3 Sep 89 17:58:59 GMT References: <1538@draken.nada.kth.se> <6019@pt.cs.cmu.edu> <21803@cup.portal.com> Reply-To: johnl@esegue.segue.boston.ma.us (John R. Levine) Organization: Segue Software, Cambridge MA Lines: 28 In article <21803@cup.portal.com> Devin_E_Ben-Hur@cup.portal.com writes: >> In article <1538@draken.nada.kth.se> d88-eli@nada.kth.se (Erik Liljencrantz) >wr >> ites: >>> [CEMM] degrades the performance of the CPU (from 33Mhz to 31Mhz >>> according to Landmark). >> Not surprising, since you have to turn on Virtual-86 mode to be able to >> emulate EMS on the 386, which means that *every* memory access has to go >> through the on-chip memory managment unit, which it doesn't have to do in >> real mode. >Not quite. In virtual mode, every segment-register load takes extra cycles >for memory management, but normal memory accesses occur at the same speed >as real mode. ... It appears to be a combination of both. Virtual memory mapping takes no extra time so long as the mapped addresses are in the paging cache. Every paging cache miss causes two memory reads from the two-level page tables. You'd think that in virtual-86 mode segment register loads should be as fast as they are in real mode, since it is doing none of the descriptor table manipulation that it has to do in regular virtual mode, but as far as I can tell from reading the 386 Programmer's Reference, segment loads in virtual 86 mode run at the same speed as in protected mode. I suppose we're supposed to be grateful to have virtual 86 mode at all. -- John R. Levine, Segue Software, POB 349, Cambridge MA 02238, +1 617 492 3869 {ima|lotus}!esegue!johnl, johnl@ima.isc.com, Levine@YALE.something Massachusetts has 64 licensed drivers who are over 100 years old. -The Globe