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From: phorgan@cup.portal.com
Newsgroups: comp.compilers
Subject: Disassembly
Keywords: assembler, debug
Message-ID: <9009091032.1.139@cup.portal.com>
Date: 9 Sep 90 17:32:55 GMT
Sender: compilers-sender@esegue.segue.boston.ma.us
Reply-To: phorgan@cup.portal.com
Organization: Compilers Central
Lines: 25
Approved: compilers@esegue.segue.boston.ma.us

The problem with disassembling arbitrary object code is that data bears a
disturbing resemblance to code at times:) Even when running through code
disassembling starting at known code, it's not always possible to
determine when code stops and data begins.  Then it's not possible to tell
when object code starts up again.  This is easy to see using most
dissassemblers; when you hit the data, the unknown op-code indicator
appears (typically ???), then random sequences of ??? and op-codes, then
when the code starts, often the disassembler has just guessed wrong and
includes the first byte or two of the 'real' op-code in a previous 'false'
one.  It might take a while to 're-synchronize' and start showing 'real'
op-codes.  The only time this isn't a problem would be with fixed single
length op-codes with an alignment requirement.  It is possible to reduce
the problem with an algorithm that looks ahead starting byte-by-byte and
sees which one generates a most successful string of instructions.  From a
'good starting byte', you could disassemble in reverse to find a previous
starting location.  Even this fails in many cases of self modifying code
or in cases where strange things are done like overlapped code.  (A trick
often done on code to be put in machines with limited ROM space).  If
you're familiar with coding practices for the processor though, heuristic
methods can be applied with some success.

Patrick Horgan                   phorgan@cup.portal.com
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