Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!oliveb!amiga!kodiak
From: kodiak@amiga.UUCP (Robert R. Burns)
Newsgroups: comp.sys.amiga.tech
Subject: Re: SDB and tracing through the ROMs
Keywords: SDB, BCPL, breakpoint
Message-ID: <3228@amiga.UUCP>
Date: 30 Dec 88 21:16:38 GMT
References: <1988Dec27.223345.29513@ziebmef.uucp>
Reply-To: kodiak@tooter.UUCP (Robert Burns)
Organization: Commodore-Amiga Inc, Los Gatos CA
Lines: 35

In article (Colin Plumb) writes:
)00fff1b0: 0000 0000	ori.b	#0,d0
)00fff1b4: 0000 038d	ori.b	#-73,d0
)00fff1b8: 2401     	move.l	d1,d2
)(more reasonable-looking code)
)...
)BCPL compiler, anyone?  (1.3 kickstart, BTW.)
)...
)For the technically oriented, I patched rn->RestartSeg to hold 0x7f2d,
)0x1fcb4 in the APTR world, and at that address is found:
)
)0001fcb4: 003f fc6c		ori.b	$6c,usp
)0001fcb8: 4e71	4e71		nop nop
)0001fcbc: 4ef9 00ff f1b4	fmp	fff1b4

Well, that *fmp* instruction will get you every time! :-)

Seriously:
1.  You're looking at a BCPL segment global table.  The 0000 0000 is indeed
    a NULL next pointer, and the 0000 038d is the size of the segment.
    You have now entered the twilight zone.  The reason you're crashing is
    because BCPL segments are different than any other kind of segment, and
    the lie about either the next segment pointer, or (more likely) the size
    of this one caused the crash.  Moreover, execution does not start at the
    first code in the first segment.  I honestly don't want to go into more
    detail lest folks start using the mechanisms involved, because they are
    structurally flawed and will break if too many people use them.  Hope
    you're not the "knowledge for knowledge's sake" type :-).
2.  This mechanism (if you get it to work) will not be supported for 1.4,
    but I see Steve has already said that.
-- 
Bob Burns, amiga!kodiak                   _
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