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 <vatHeader> <baseHeader>
type: 1 <classHeader> <vatHeader> <baseHeader>
type: 0 <sizeHeader> <classHeader> <vatHeader> <baseHeader>
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, Rob