[Vm-dev] ImageSegment, objects header and addresses
Mariano Martinez Peck
marianopeck at gmail.com
Tue Jul 27 14:32:29 UTC 2010
Hi guys. There is something I still don't understand about ImageSegment
hahaha. But each time there is less :)
There is a place in ImageSegment >> copyObj: oop toSegment: segmentWordArray
addr: lastSeg stopAt: stopAddr saveOopAt: oopPtr headerAt: hdrPtr
where it does this:
self forward: oop to: (lastSeg+BytesPerWord + extraSize -
segmentWordArray)
savingOopAt: oopPtr andHeaderAt: hdrPtr.
Basically, it forwards oop to another address which is actually the offset
inside the segment. And it backups the original oop and the hader, as you
can see here:
Interpreter >> forward: oop to: newOop savingOopAt: oopPtr andHeaderAt:
hdrPtr
"Make a new entry in the table of saved oops."
self longAt: oopPtr put: oop. "Save the oop"
self longAt: hdrPtr put: (self longAt: oop). "Save the old header
word"
"Put a forwarding pointer in the old object, flagged with forbidden
header type"
self longAt: oop put: newOop + HeaderTypeFree.
Now....the line I don't understand is this:
self longAt: oop put: newOop + HeaderTypeFree.
Because 'self longAt: oop'
will answer the object header of oop. And there, it stores a number (the
offset). Is this possible?
In addition, it not only stores the offset, by it plus the flag
HeaderTypeFree.
I really don't understand how you can plus an offset and the flag. What is
the result? how should I interpret that?
Thanks for any hint you can give me.
Mariano
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