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From: stever@Octopus.COM (Steve Resnick )
Newsgroups: comp.sys.ibm.pc.programmer,comp.sys.ibm.pc
Subject: Re: Caches and Extended Memory
Message-ID: <1990Mar9.223329.9818@Octopus.COM>
Date: 9 Mar 90 22:33:29 GMT
References: <9003091838.AA21834@decwrl.dec.com>
Reply-To: stever@octopus.UUCP (Steve Resnick )
Organization: Octopus Enterprises, Cupertino CA
Lines: 21

In article <9003091838.AA21834@decwrl.dec.com> stoppani@slough.enet.dec.com (Pete Stoppani) writes:
>My understanding is that memory above 1Meg (except for the first
>64k above 1Meg) is accessible only when a 268/386/486 is in protected mode.
>How does a hard disk cache driver use extended memory on a system running
>MS-DOS?  Does it switch to protected mode?  Do the machines run in
>protected mode all the time?  Neither seems right...What am I missing?

To use extended memory beyond FFFF:FFFF you MUST place the machine in protected
mode. I am unsure of the technique used, but it goes something like this. 
You set a flag/IO port/register (dunno which) which flags the BIOS that you've
switched into protected mode intentionally. Load a dummy LDT, IDT and GDT
do your memory work, then write to a IO port which resets the CPU. This is the
only way out of protected mode on a 286 is to force the CPU to do a reset.
The flag that you left when you entered protected mode tells the BIOS during 
POST that it was all a bad dream and the machine is not to be rebooted. It's
sort of kludgy but it works. I believe this is how int 15 func 87H (move 
extended memory block) and the VDISK driver works... Hope this helps...
Steve

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