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From: john@hpcvlo.HP.COM (John Eaton)
Newsgroups: comp.sys.ibm.pc
Subject: Re: IBM ROM Format???
Message-ID: <1610014@hpcvlo.HP.COM>
Date: Fri, 24-Apr-87 11:24:11 EDT
Article-I.D.: hpcvlo.1610014
Posted: Fri Apr 24 11:24:11 1987
Date-Received: Sun, 26-Apr-87 22:37:04 EDT
References: <4342@beta.UUCP>
Organization: Hewlett-Packard Co., Corvallis, OR, USA
Lines: 17

<<<<<
< It is easier than it looks. Just add ALL of the 8 bit locations together,
< modulo 256, and if the result is 0 then the ROM is valid (as far as the
< BIOS will be concerned).
<
Testing for the presence of a device or memory is always trickey because of 
the different ways that the system responds when nothing is present. Reading
a non existent memory location will usually return whatever data the bus
capacitance is holding at the time of the read. If the read is in a loop then
the same value can easily be read during each iteration of the loop. If the
loop count is a multiple of 256 then the sum of any constant value will be 
zero. Thats why IBM also uses the 55AA check to verify roms.



John Eaton
!hplabs!hp-pcd!john