Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!csd4.milw.wisc.edu!bbn!rochester!pt.cs.cmu.edu!b.gp.cs.cmu.edu!Ralf.Brown@B.GP.CS.CMU.EDU
From: Ralf.Brown@B.GP.CS.CMU.EDU
Newsgroups: comp.sys.ibm.pc
Subject: Re: Protected mode and PC/XT cards
Message-ID: <248e6761@ralf>
Date: 8 Jun 89 12:32:33 GMT
Sender: ralf@b.gp.cs.cmu.edu
Organization: Carnegie Mellon University School of Computer Science
Lines: 34
In-Reply-To: <1235@hiatus.dec.com>

In article <1235@hiatus.dec.com>, ballou@chalk.dec.com (Ken Ballou) writes:
}In article <30483@conexch.UUCP>, rob@conexch.UUCP (Robert Collins) writes:
}> So your question really becomes quite simple:  Does video (or any RAM)
}> below 1M work on a 286?
}
}Suppose I create a segment whose base address is 180000H.  Now, when I access
}this segment, address line A20 will carry a 1.  However, as you point out:
}
}>                          The answer is an obvious yes.  It doesn't
}> matter that you only have 20 address lines on your card when A20-A23
}> are 0 anyways.
}
}Now, specifically since the card only has 20 address lines, won't it ignore A20
}being 1?  If so, then it would see 8xxxxH on the low address lines.  Wouldn't it
}then say "Aha, you're talking to ME!" and try to respond?  So, as the original
}question put it, wouldn't you have video RAM appearing at 080000H, 180000H,
}280000H, and so on?

In order for the card to say "Aha, you're talking to ME!", not only must the
address be on the bus, but the MEMR or MEMW line must be pulled to active
(since the address could be an I/O port, in which case either IOR or IOW
is made active).  In the AT, the MEMR and MEMW lines on the original 8-bit
connector are only activated for memory accesses where the high four bits
(A20 to A23) are all zero.  If any of those lines are 1, then a second set
of MEMR and MEMW lines on the second, 16-bit, connector are activated instead.
Thus, an 8-bit card never sees addresses above 1M, since it never gets a
read or a write signal.  Kludgy, but it works....

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