[Vm-dev] extending every header by a word

Rob Withers reefedjib at yahoo.com
Thu Nov 1 15:33:50 UTC 2007

I am looking into what changes will be required for me to add a word to 
every header.  I am hoping someone can give me a 5 minute overview to 
confirm/extend what I have learned from A Tour of the Squeak OE.

I see on page 20 of the Tour of the Sq OE the layout of an object in memory, 
with headers.  The first decision is where does the new vatHeader go, and 
since it will go on every header, I think I will put it just before the 
baseHeader, such that headers look like:

type: 3

type: 1

type: 0

So looking into the code, I see the BaseHeaderSize running around all over 
the place, but please correct me, this is just to account for the fact that 
an oop points to the first header word, so it gets sized and stripped in 
various situations.  I am hoping this should not be affected by a header 
size increase.

I have just started looking, but I see that I will need to change the 
#classHeader:, #sizeHeader, add #vatHeader:, and anywhere that writes to the 
classHeader and sizeHeader, like: 
#allocate:headerSize:h1:h2:h3:doFill:with:.  This is by no means exhaustive. 
Is there anything in particular you can point me to consider or warn me of?

many thanks,

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