Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!srcsip!orion!rogers
From: rogers@SRC.Honeywell.COM (Brynn Rogers)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Phoenix BIOS 3.07 (5/27/87) for 80286
Message-ID: <31087@srcsip.UUCP>
Date: 13 Sep 89 20:55:38 GMT
References: <398@wjh12.harvard.edu> <9523@cbnews.ATT.COM>
Sender: news@src.honeywell.COM
Reply-To: rogers@src.honeywell.com (Brynn Rogers)
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Organization: Honeywell Systems & Research Center
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In article <9523@cbnews.ATT.COM> cbema!las@cbnews.ATT.COM (Larry A. Shurr,20650,cb,9a110,6148605851) writes:
>In article <398@wjh12.harvard.edu> djb@wjh12.UUCP (David J. Birnbaum) writes:
>}While snooping around in high RAM I noticed a peculiarity.  My CompuAdd
>}286 has a Phoenix ROM BIOS, version 3.07, dated 5/27/87.  Doing a debug
>}dump of f000:8000 shows that the BIOS begins at this address (at least
>}the display begins with PPhhooeenniixx, etc.).  But dumping e000:8000
>}(note: different segment) shows the identical information.  In fact,
>}dumping several random addresses between e000:8000 and e000:ffff produced
>}exactly the same display as dumping the same offsets in the f000 segment.

>Actually, it is probably not "really there."  Instead they're probably just
>the same place.  What I mean is that the address decoding circuitry is
>probably mapping e000 and f000 to the same place - note that there is only
>a one bit difference between e (1110) and f (1111).  This may have been done
>on purpose, though not necessarily to achieve the affect of having e000 and
>f000 both refer to the ROM space. 

Actually it is because there is a 32kx8 rom in a socket for a 64kx8 and
one address line (A15 on the chip, A16 on the bus) is just plain ignored by
the 32k chip. The reason that A16 is run to the socket is to allow for the
ROM bigger chips, which in IBM systems include basic in ROM.
  Nobody uses the e000 segment because if they did, it wouldn't work in a
IBM, and IBM compatability (No matter how stupid IBM is) is the name of
the game.




 Brynn Rogers    Honeywell S&RC        rogers@src.honeywell.com
 work 612-782-7577 home 874-7737