Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!amdahl!drivax!alexande From: alexande@drivax.UUCP Newsgroups: comp.sys.ibm.pc Subject: Re: New Dos from Microsoft Message-ID: <1362@drivax.UUCP> Date: Thu, 23-Apr-87 13:06:33 EST Article-I.D.: drivax.1362 Posted: Thu Apr 23 13:06:33 1987 Date-Received: Sat, 25-Apr-87 07:42:54 EST References: <705@imsvax.UUCP> <631@edge.UUCP> <1622@bnrmtv.UUCP> <225@homxb.UUCP> <141@edm.UUCP> Reply-To: alexande@drivax.UUCP (Mark Alexander) Organization: Bob-ist Temple of Monterey Lines: 32 Summary: Running real mode programs in protected mode In article <141@edm.UUCP> steve@edm.UUCP (Stephen Samuel) writes: > Also: I understand that Digital had some serious performance problems >getting concurrent PC-DOS running in protected mode. Can anyone >comment on this? 286 Protected mode isn't so much of a problem if you're running a program that was written specifically for protected mode (i.e. uses nice selectors, not physical segments). The performance problem comes in when you try to run real-mode programs (e.g. all existing DOS applications) in protected mode. Every time the DOS program loads a segment register, a protection exception occurs, because the value being loaded is a physical segment number, not a selector. So the OS has to jump through some very nasty (and slow) hoops to fool the DOS application into thinking it's running in real mode. DOS programs that were written for small model (few segment register loads), and which don't try to write directly to physical devices (like screen memory) can run pretty nicely in protected mode. The Lattice C compiler is a good example of a "nice" program. This performance problem is probably one reason why Microsoft chose to allow older DOS applications to run on their ADOS by switching back and forth between protected mode and real mode. DRI Concurrent DOS-286 stays in protected mode all the time. The 386, with its virtual 8086 mode, should fix a lot of these problems. -- Mark Alexander ...{hplabs,seismo,sun,ihnp4}amdahl!drivax!alexande "Omit needless words! Omit needless words! Omit needless words!" - W. Strunk