| OCR Text |
Show 6 lw ac lti eved hv intmrl11 cing <1 i<'vel of' it td irect ion i.e ... t hPsf' ohjPct n-·rer<? tt c'c'S are tll<-tp Jwd Llnuugh a c:on es pondence table to derive act ual object addresses. Tbere is <"1.uuLi tcr tPch nique, called "pointer swizzling" [16, 2:3] wbi c:h some systems employ. In t his scheme, t he object reference is changed to a direct memory pointer in-place. Depending on whethe r a new copy of the object is ma.de or not and whether pointer sw izz ling is done at thP time of loa.cl ing or later when needed , there a re a num ber of' poin ter swizz ling techni ques wh ich a.rc descri bed in [16]. 2.4.2 Object Identity Identi ty is t hat property of an object whi ch di st inguishes each object frorn all others [1 1]. An ident ifier attacbed to an object for t his purpose is called an object id(' tl l ifi c·r (OlD ). /\u oro can lw II S('d Lo refer to au object. Prov iding obj ects with an ide ntifier bas a rmmber or consequences. It means t bat an object bas an existence independent of its value. It also enables two objects to share a c:ornmon component [20]. Different systems provid e diffe rent degrees of support for OIDs. Some systems provide an OID to each and every object whi le some provide OIDs only to objects wb icb exist independently and these systems lJ ave an intern al way of referring to objects whi ch exi st as components of these ol>jeds. OIDs can be logical [1, 17, 2 1] or based on physical identifiers [15]. Phys ical OIDs co nt ain inforrnat ion about the object location on secondary storage within the1nselves whereas logical OIDs do not have such information. In the case or logical O!Ds, a co rrespondPnce table is needed for mappi ng OrDs into actual phys ical loca t ion s. As d res ult , two or mcm-· di sk accesses are likely to be performed to luad an ul >jcd. This in direction Lbro ugh tbe correspondence table, however, makes 1novi ng objects with logical O!Ds easier t han movi ng objects with phys ical OrDs [5]. |
