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From: resnicks@netcom.COM (Steve Resnick)
Newsgroups: comp.sys.ibm.pc.misc,comp.sys.ibm.pc.hardware,comp.os.msdos.programmer
Subject: Re: Memory above 640K being redundant (or something like that)
Message-ID: <28292@netcom.COM>
Date: 14 Mar 91 19:35:31 GMT
References: <27437@uflorida.cis.ufl.EDU>
Organization: Me? Organized? Ha!
Lines: 35

In article <27437@uflorida.cis.ufl.EDU> jdb@reef.cis.ufl.edu (Brian K. W. Hook) writes:
>
>Okay, I have been running saying that the VGA memory area is at A000,
>right?  And this is above 640K....but what about systems with 512K?  Where
>do all the ROM extensions and stuff go?  Are they on the cards?  If they
>are, is that memory above 640K really there as far as applications are
>concerned?  Couldn't you recover it for use with applications?
>

Devices and ROM extensions still go into adresses >A000. Even if you had 128K 
in the machine (I used to have a 128K PC) the hardware addresses are
pretty much the same. There are some devices which will put themselves in
various addresses depending on jumpers and such, but no device should attempt
to map itself to an address < A000. As far as the ROM goes, the 8086
at reset jumps to address FFFF:0000. The 286/386/486 go to a higher memory
address, but at boot time, the upper address lines are masked off, and the
reset vector still works out to be FFFF:0000. This is "hard-coded" to the
chip itself....


Cheers!
Steve




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