Thanks for the tips (all)...

Regarding the Slang Interpreter and the Interpreter Simulator, let me
just say: thank you! thank you! thank you!  What a nice piece of work!

- Stephen

> -----Original Message-----
> From: squeak-dev-admin at lists.squeakfoundation.org 
> [mailto:squeak-dev-admin at lists.squeakfoundation.org] On 
> Behalf Of Dan Ingalls
> Sent: Friday, August 24, 2001 1:15 AM
> To: squeak-dev at lists.squeakfoundation.org
> Subject: Re: Adding a word to Squeak's object header
> 
> 
> "Stephen Pair" <spair at advantive.com>  wrote...
> 
> >I'm trying to add 4 bytes to the object header (mainly for 
> educational 
> >purposes)...but I'm having a terrible time getting it work.  
> I've been 
> >able to use the SystemTracer to produce what I think is a clone with 
> >the extra header bytes inserted (and I've successfully 
> tested a regular 
> >clone), however, I can't get it to run successfully on a modified VM.
> >
> >I've tried adding the new header word before the oop (oop - 4) and 
> >sliding the class and size headers further back.  I've also tried 
> >increasing BaseHeaderSize to 8 (from 4) such that the new 
> word follows 
> >the normal base header.  Which is the best approach?  I've picked 
> >through almost all of the methods in ObjectMemory and Interpreter 
> >looking for hardcoded assumptions about the structure of the header. 
> >What about the plugins?  Should I be looking in there as 
> well for code 
> >that hardcodes object header offsets?
> >
> >I'm not familiar enough with the VM code to know what kind 
> of snafus I 
> >should be looking for.  I'm hoping someone who's been there 
> and done it 
> >before might have some tips.
> 
> Stephen -
> 
> You go, guy!
> 
> This is likely a tough one to do.  There are probably lots of 
> places that expect the base header to be where the oop 
> points, and it may be hard to track them all down.
> 
> If you increase BaseHeaderSize, I think you still want to 
> leave the base header at 0, and insert your new word at -4.
> 
> Another approach would be to force all headers to be 4 (count 
> 'em) words long -- like the current three-word headers plus 
> your additional word at oop-12.  Even before this, you may 
> want to get rid of compact classes, but I think they'll still 
> work with big headers (and I think the quick checks for 
> contexts expect they have compact class fields).   Hope this helps.
> 
> I assume you are running the simulator to try it out.  Check 
> the integrity of the entire memory between every bytecode at first.
> 
> Good luck.
> 
> 	- Dan
> -- 
> 
>