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From: claudio@ethz.UUCP (Claudio Nieder)
Newsgroups: net.micro.amiga
Subject: Some remarks to AmigaDos.
Message-ID: <392@ethz.UUCP>
Date: Tue, 21-Oct-86 16:58:59 EDT
Article-I.D.: ethz.392
Posted: Tue Oct 21 16:58:59 1986
Date-Received: Thu, 23-Oct-86 23:02:58 EDT
Reply-To: claudio@ethz.UUCP (Claudio Nieder)
Organization: CS Department, ETH Zuerich, Switzerland
Lines: 52


[E t th s  ne i   ou ca . ]

We already had some discussions about AmigaDos. Here some new comments,
but this time NOT about the filesystem.

We tried to write a program dosspy which should monitor some
AmigaDos calls. We first used SetFunction to change the library pointer
to our routines and to get the address of the original routines.
This didn't work because SetFunction assumes that the code vectors are

			JMP abs.L

and returns you the last 4 byte of the 6 byte entry. AmigaDos has
another entry:

			MOVEQ.L #byte,D0
			BRA	libStart+$4E

A function dispatcher at libStart+4E will call the desired function
according to the value of byte.

After mastering this problem we tried our dosspy with some programs
and several dos commands. It worked with programs, but displayed
nothing with dos commands. It seems that commands (as rename)
doesn't call dos.library through the library vectors. To find out more
we disassembled rename. Here the first 3 instructions of rename:

			MOVEA.L	$164(A2),A4
			MOVEQ.L $C,D0
			JSR	(A5)

Seems that a program starts not only with A0 pointing to the arguments
and D0 indicating the number of the characters in the argument. At least
A2 and A5 are initialised with some magic values pointing probably to
the function dispatcher (A5) and to some global values (A2) of AmigaDos.

So if anybody ever had the idea to exchange the AmigaDos function by
own functions (e.g. try to implement a second filesystem which would
coexist with the present one and be transparent to the user) he will
have to rewrite all dos functions !

-- 
				claudio

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