Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!gem.mps.ohio-state.edu!apple!baum
From: baum@Apple.COM (Allen J. Baum)
Newsgroups: comp.arch
Subject: Re: Self-modifying code
Message-ID: <35710@apple.Apple.COM>
Date: 16 Oct 89 21:29:47 GMT
Organization: Apple Computer, Inc.
Lines: 39

Reply-To: baum@apple.UUCP (Allen Baum)
Distribution: net
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[]
>In article <274@ssp1.idca.tds.philips.nl> roelof@idca.tds.PHILIPS.nl (R. Vuurboom) writes:
>
>I came up with the following little routine:
>
>		save real registers/condition codes 
>		load registers/condition codes from emulated registers
>
>		nop
>		nop
>		nop
>		nop
>
>		save registers and condition codes to emulated registers
>		load real register/condition code values
>
>
>By loading the instruction I wanted to trace in the area with nop's
>and calling the routine I could let the Z80 "calculate" the condition codes 
>and the changed registers for me. The next step was to calculate the
>new pc and load the following instruction to be traced.
>
>Of course, this was pretty primitive. It wouldn't work for branches
>(but I could determine if a branch would take place and set the 
>emulated pc accordingly and continue from there). Nor would it
>work for io instructions :-) but they didn't occur often anyway.

The original Apple II monitor ROM had a single step/trace routine that
did exactly that. I think for branches I just replaced the offset,
so it always branched to a fixed place, and updated the emulated PC
with the offset if it did.


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