| OCR Text |
Show POSs which a re descr ibed in t hi s chapte r. The POSs that were chosen for evalu at ion a re a ll publi c-domain research systems. They are ObServer, Exodus Storage .\l<ttl<t.get. <Lttd Poslgr<· s. Tlwse <LI<' L-t.itly sLal> le <Litd provicl<-' a s uUi ciettL va ri ety uf f<'<L1ttt <'s for Litis st ttd .v. '!' he cumrnou ground for usiug them is t hat they a ll treat an object as a stream of bytes attached to an OID . 4.1 Support for OIDs /\fLer mod ification s to Alph a_] 's storage mcwagernent, an object is a byte st ream witlt an OlD attach ed to it. To generate OIDs, a counter is kept with each objectgraph and new 0 IDs are generated by increment in g t his counter. In order to present <L byte stream to the POS for sto rage, all pointers in RAM must be accounted for, as they point to arbitrary memory locations. This is partially achieved by "pointer swizz ling," whereby pointers to a ll other objects a re replaced by their OlDs (these Ol])s are repl aced by the act ual vir t tt al addresses of the objects wlwn the objects ct t<·' llJ <t.<il'd itt JHi tnary JlH'Jilory.). TIJt~ r< ' <-t.re , l1owevn, other poin te rs wh ich a re not L)(·' ing swizz led. T hese are point<'~ r s to funclanwntal data types and character str ings. FundanHc~ntal data types and character st rings are regarded as part of an obj ect and therefore poin te rs to t hem are not assigned any OID. To accommodate them in the byte st ream represent ing t he object contents , th ese data types must lw cont iguous with the ohject beiug stored. Th is can on ly be done by copying the object. Tbe size of' the final object includ es the surn of the sizes of the targets of <til such pointers. These changes have resulted in un iform treatment to various types of pointers usee! by Alpha_] an d , conseq uentl y, in code simp li fication. The new bi nary file now has ouly three dist in ct data items, namely, file headers, obj ect descriptors , and u!Jject::; . TlH' object nag is llOW r·ull owe<i hy the object's OlD , its tag number (IIlli |
