Object models for computer aided design

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101 :!-.E r.a s he c for the I- . 􁪽􋵯 o ....·Jec ... i t s e Lf ) are Ocjec􁪽􋵳s are s:o:ec ir. cirec􁪽􋵣 c􁪽􋵣cess f:1Es wit􁪽􋴚 􁪽􋵡ash cooee c:􁪽􋵥ector:es. I􁪽􋵣 creer to rrin:rn:ze cisk access, tte :􁪽􋴚􁪽􋴚􁪽􋵳 stra􁪽􋵥e;y seerrs :0 be tc store each object contiq􁪽􋵯ous tlccks so t􁪽􋵡at only two heac rno􁪽􋵥e􁪽􋵥ents {orle fo! 􁪽􋵥ece5sary. 􁪽􋵨he􁪽􋵴 the object is up6a:ed, if the nurrber of t:CCk5 nee􁪽􋵥ec ha! c􁪽􋵡a􁪽􋵧ged, the old tlocks are ce-allccate6 a􁪽􋵤d new space is allocated. On che disk, objects are packed into contiguous t:ocks and all attribute lists and attribute pointer addresses are rrade relative to the beginning of the object header. 􁪽􋵨hen an object is transferred into the progra􁪽􋴚 􁪽􋵣crk􁪽􋵰pace, 􁪽􋵥e􁪽􋵯ory for the object header and each attribute is allo􁪽􋵡ated se􁪽􋴳3rately and all pointer􁪽􋴚 􁪽􋵲re relocated to wcr􁪽􋵓S􁪽􋴲2ce 􁪽􋵤ddresses. 􁪽􋵨hen the object is s':orec, =rocess is reversec anc the object i􁪽􋵡 again packed into a ccnti=uous se9rrent to be stored o􁪽􋵴 t􁪽􋵥e disk. Figure􁪽􋴴 4.1 a􁪽􋵤d 4.2 are fairly 􁪽􋵣ccurate pictures of the 􁪽􋵡ay objects look in rremory. The object header a􁪽􋵤d cttri􁪽􋵵ut􁪽􋴳3 are each cor.tiguous se?􁪽􋵥ents of rrerr.ory. EiI'lce ap;ear i􁪽􋵥 exactly one att􁪽􋵩ibut.c list, the first word of the attribute is reserved as t􁪽􋵥e list c c i n t e r • T!"":e r.e x t · .vc r c holds tr;e scalar type code arid t b e