Path: utzoo!yunexus!maccs!cs4g6ag From: cs4g6ag@maccs.dcss.mcmaster.ca (Stephen M. Dunn) Newsgroups: comp.sys.ibm.pc Subject: Re: 80286 protected mode, What on earth is a Summary: info on the various 286/386 modes Keywords: 80286, protected mode Message-ID: <2511D9FB.11588@maccs.dcss.mcmaster.ca> Date: 16 Sep 89 05:28:27 GMT Article-I.D.: maccs.2511D9FB.11588 References: <1219@marlin.NOSC.MIL> <411@castle.ed.ac.uk> Reply-To: cs4g6ag@maccs.dcss.mcmaster.ca (Stephen M. Dunn) Organization: McMaster University, Hamilton, Ontario Lines: 56 In article <411@castle.ed.ac.uk> erck12@castle.ed.ac.uk (Gnome) writes: $In article <1219@marlin.NOSC.MIL> jbjones@marlin.nosc.mil.UUCP (John B. Jones) writes: $-What is protected mode? Is there an unprotected mode? It sounds like $-some programming term, but what's it for? $-Thanks in advance for turning some lights on. $ As far as I know (and most of this knowledge is 2nd hand) the 80286 can $operate in two modes, protected and unprotected. The former of these is the real $80286, while the latter, which is used by DOS, makes it act like a 8086/88. The $term protected comes from the protected memory scheme available in that mode. The 286 does in fact have two modes - "real" mode (which makes it behave like an 8086/8088 and is used under DOS on AT-class machines) and "protected" mode, which provides the memory support for multi-tasking. This mode, as pointed out by "Gnome", provides the hardware support for protecting one process' private memory from other processes; this protection is pretty well necessary in order to implement a good multi-tasking OS. $This is needed by multi-tasking operating systems to stop one process $corrupting anothers memory. OS2 uses protected mode, except when the DOS box is $active. Yup. The 286 provides four levels of privilege, with (of course) the pro- vision of a method of switching between them. Generally, OS/2 applications run in the lowest mode, while the various components of the operating system itself run in the other modes. Anyone who has taken a university "introduction to operating systems" course will be familiar with the fundamentals of this. $If you think all this is complicated the 80386 has four modes! It has $the two suported by the 80286, true 80386 mode, and multi-8086 mode (which $simulates an individual 8086 in each 1M of the memory map - nice!). I hope the $lights are at least glowing now. This is one of the great problems with OS/2 - the 386 has the ability to multi-task DOS applications, but the 286 doesn't. Since OS/2 assumes it has a 286 (and runs a 386 in 286 protected mode), DOS applications will either run in the foreground or not at all. BTW, those who are familiar with DOS 4 will have seen in the manual a nifty little driver that uses extended memory (that memory outside the 0-640K range) to actually be expanded memory on 386 systems. This driver uses the advanced memory-mapping ability of the 386 to actualy make this memory appear to be in the 8086's 0-1M addressing range (as opposed to programs like PC magazine's EMM40.SYS, which uses extended memory to emulate expanded; this program assumes that you are running on a 286 and actually eats up 64K of your conventional mem- ory, because what it does it copies the data between conventional and extended memory each time you do a page swap ... this will make sense to EMS programmers and will probably sound like complete gibberish to others - write me if you'd like a few more details) -- Stephen M. Dunn cs4g6ag@maccs.McMaster.CA ********************************************************************** = "\nI'm only an undergraduate!!!\n"; "VM is like an orgasm: the less you have to fake, the better." - S.C.