Object Format
Alan Kay
squeak-dev at lists.squeakfoundation.org
Tue Oct 15 03:51:50 UTC 2002
Who runs the Internet?
Cheers,
Alan
-----
At 7:07 PM -0400 10/13/02, PhiHo Hoang wrote:
>Who's flying the plane ?
>
>Cheers,
>
>PhiHo.
>
>----- Original Message -----
>From: "Alan Kay" <Alan.Kay at squeakland.org>
>To: <squeak-dev at lists.squeakfoundation.org>
>Sent: Sunday, October 13, 2002 7:51 PM
>Subject: Re: Object Format
>
>
>> Depends on who's flying the plane ...
>>
>> Cheers,
>>
>> Alan
>> -----
>>
>> At 6:47 PM -0400 10/13/02, PhiHo Hoang wrote:
>> >Hi Ian,
>> >
>> >> You might also find the following useful:
>> >>
>> >> http://www-sor.inria.fr/~piumarta/esug98/slides.ps.gz
>> >>
>> >> starting around page 33.
>> >
>> > Indeed, the whole document is very useful and interesting to read.
>> >
>> > Thanks a lot, Ian.
>> >
>> > BTW, were you teasing ?
>> >
>> > On page 17 you mentioned a 'violently stripped' thingy. ;-)
>> >
>> > Where can I find such a beauty ?
>> >
>> > Of course, I would love to lay my hand on an 'eXtremely violently
>> >stripped' one.
>> >
>> > The smaller the more fun. Size does matter ;-)
>> >
>> > Again, many thanks for the explanation and the document.
>> >
>> > Cheers,
>> >
>> > PhiHo.
>> >
>> > P.S: Now I also understand what it means by Blue Plane and Pink
>Plane.
>> > Just wondering what colorful plane is Squeak now riding.
>> > (Are Grey or Black also considered colors ? )
>> >
>> >
>> >----- Original Message -----
>> >From: "Ian Piumarta" <ian.piumarta at inria.fr>
>> >To: <phiho.hoang at rogers.com>
>> >Cc: <squeak-dev at lists.squeakfoundation.org>
>> >Sent: Sunday, October 13, 2002 4:47 PM
>> >Subject: Re: Object Format
>> >
>> >
>> >> On Sun, 13 Oct 2002, PhiHo Hoang wrote:
>> >> > - 4 bits object format
>> >> >
>> >> > What is this 4-bit 'object format' field.
>> >>
>> >> It tells you about what the instance contains.
>> >>
>> >> The top bit (bit 3) is 1 for byte objects, 0 for word/pointer objects.
>> >>
>> >> If the top bit is zero (words/pointers) then:
>> >> Bit 2 is 0 for pointer objects, 1 for words.
>> >> Bit 1 is set if there are indexable fields, 0 if there are none.
>> >> Bit 0 is set if there are fixed fields (named inst vars), otherwise 0.
>> >>
>> >> A "word" object (bit 2 set) that has neither indexable nor fixed
>fields
>> >> (bits 1 and 0 clear) contains weak references and may have both fixed
>and
>> >> indexable fields. (Unless the object contains only fixed fields then
>you
>> >> have to follow the class pointer and look in the class's
>"instanceSize"
>> >> field to find out how many fixed fields are in it.)
>> >>
>> >> If bit 3 is set (byte object) then bit 2 tells you whether it's a
>compiled
>> >> method (set means it's compiled method). Methods start with an extra
>> >> header word (look in class CompiledMethod to see what it contains)
>> >> followed by zero or more real pointers (the literals of the method).
>> >> After the pointers they turn into byte objects again. Bits 1 and 0
>(of
>> >> any byte object) are the number of bytes by which the size header (or
>size
>> >> field in the base header) is too large (since a bytes object might be
>0,
>> >> 1, 2 or 3 bytes short of an integral number of words long).
>> >>
>> >> To summarise, if we consider the format as an integer from 0 to 15, we
>> >> get:
>> >>
>> >> 0 0000 no fields
>> >> 1 0001 fixed fields only (all containing pointers)
>> >> 2 0010 indexable fields only (all containing pointers)
>> >> 3 0011 both fixed and indexable fields (all containing pointers)
>> >> 4 0100 both fixed and indexable weak fields (all containing
>pointers).
>> >> 5 0101 unused
>> >> 6 0110 indexable word fields only (no pointers)
>> >> 7 0111 unused
>> >> 8-11 10xx indexable byte fields only (no pointers)
>> >> 12-15 11xx compiled methods: # of literal oops specified in method
>header
>> >>
>> >> > Where can I find further detailed information about this 'object
>> >format'.
>> >>
>> >> Browse class ObjectMemory in the image, and the object access
>primitives
>> >> in class Interpreter.
>> >>
> > >> You might also find the following useful:
>> >>
>> >> http://www-sor.inria.fr/~piumarta/esug98/slides.ps.gz
>> >>
>> >> starting around page 33.
>> >>
>> >> Regards,
>> >> Ian
>> >>
>> >>
>> >>
>> >>
>> >>
>>
>>
>> --
>>
--
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