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To: =?iso-8859-1?Q?=DEor=F0ur?= Ivarsson <totii@est.is>
cc: "Jamil J. Weatherbee" <jamil@trojanhorse.ml.org>,
        Mike Smith <mike@smith.net.au>, hackers@FreeBSD.ORG
Subject: Re: 7400 gates effected by probe routine 
In-reply-to: Your message of "Sun, 02 Nov 1997 14:01:45 -0000."
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Date: Mon, 03 Nov 1997 11:04:35 +1030
From: Mike Smith <mike@smith.net.au>
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> I am not sure at the moment but I have been working with Rocwell 6522
> PIA and I think it should be possible to probe 8255 in same manner as 
> 6522, send byte to output ports and read back, if they are defined as
> output you should get the same back out from them. I think you can
> also with the control register.

a) The 8255 defaults to all-ports-input at reset.
b) It is unacceptable to re-mode the 8255 and then write random garbage 
   into its output registers, as this may have undesirable or fatal 
   effects on attached hardware.

> Do you have the 8255 datasheet ? it is available from Intel website

I have (now) in front of me the Intel Peripheral Components databook.  
This confirms that it is not possible to probe the 8255 in any useful 
fashion without knowing what is connected on the other side.

mike