Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!hplabs!hpfcdc!hpislx!hplvli!boyne
From: boyne@hplvli.HP.COM (Art Boyne)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Protected mode and PC/XT cards
Message-ID: <360019@hplvli.HP.COM>
Date: 8 Jun 89 15:16:45 GMT
References: <158322@neabbs.UUCP>
Organization: Loveland Inst. Div
Lines: 26

ballou@chalk.dec.com (Ken Ballou) writes:
> 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?

No!  Phil Ngai, in his reponse, gave the correct, but terse answer.
Here is a more detailed version:

The XT has a single connector for plug-in cards.  Included among others
on this connector are A19-A0 and the \MEMR and \MEMW (memory read/write)
strobes.

On the AT, the \MEMR and \MEMW are renamed \SMEMR and \SMEMW, and the
second connector has A23-A20 and a new \MEMR and \MEMW.  On an access
to an address < 1M, all 24 address bits are driven (obviously, A23-A20
are all 0), and both \SMEMx and \SMEMx (x=R or W as appropriate) are
asserted.  Therefore both 8-bit and 16-bit cards will respond, 8-bit
cards looking at the \SMEMx strobe, 16-bit cards the \MEMx strobe.
On accesses to addresses >1M, only \MEMx is driven.  The 8-bit cards
therefore don't even know that an access is occurring, and don't respond;
only the 16-bit cards, which decode all 24 address bits, respond.

Therefore, no problems.

Art Boyne, boyne@hplvla.hp.com