Corba and Ram

8/3/2019 Corba and Ram

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CO RBA



The Common Object Request Broker Architecture (CORBA) isa standard def ine d by t he Obj ec t Manag eme nt Group (OMG) t hate nabl e s so f twar e compon e nts writte n in m ultiple com put e r languag e s and running on multiple comput e rs to work tog e the r

(i.e ., it supports m ultiple plat f or m s ).

CORBA e nabl e s s e parat e piece s o f so f twar e writte n in di ffer e ntlanguag e s and running on di ffer e nt comput e rs to work wit h e a ch othe r like a singl e appli cation or s e t o f s e rvice s. Mor e sp ec if ically,

CORBA is a mech anis m in so f twar e f or nor malizing t he me thod-call s em anti cs b e twee n appli cation obj ec ts r e siding e ithe r in t he sa me addr e ss spa ce (appli cation) or r em ote addr e ss spa ce (sa me host, or r em ote host on a n e twork). V e rsion 1.0 was r e le as e d inOc tob e r 1991. CORBA us e s an inte r f a ce def inition languag e (IDL)to sp ec if y the inte r f a ce s w h ich obj ec ts pr e s e nt to t he out e r world.



CORBA t he n sp ec if ie s a mapping f rom IDL to asp ec if ic im pleme ntation languag e like C++ or Java . Standard m appings ex istf or Ada , C, C++ , Lisp ,Ruby , S m alltalk , Java , COBOL , PL/I and Pyt hon . The r e ar e also non-standard m appings f or P e rl, VisualBasi c , Erlang , and Tc l im pleme nte d by obj ec tr eq ue st brok e rs (ORBs) writt e n f or t hos e languag e s. Curr e ntly t he OMG is working on ane w IDL to C++11 languag e m apping.



The CO RBA specification dictates there shall be anO RB through which an application would interact withother objects. In practice, the application simplyinitializes the O RB, and accesses an internal Object

Adapter , which maintains things like referencecounting, object (and reference) instantiationpolicies, and object lifetime policies. The O bjectAdapter is used to register instances of the generated code classes . Generated code classes are the result ofcompiling the user IDL code, which translates the

high-level interface definition into an O S- andlanguage-specific class base for use by the userapplication. This step is necessary in order to enforceCO RBA semantics and provide a clean user process forinterfacing with the CO RBA infrastructure.



This figure illustrates the high-level paradigm for remoteinterprocess communications using CO RBA. Issues notaddressed here, yet accounted for in the CO RBAspecification, include data typing, exceptions, networkprotocols, communication timeouts, etc. For example:Normally the server side has the Portable O bjectAdapter (PO A) that redirects calls either to thelocal servants or (to balance the load) to the other servers.Also, both server and client parts often have interceptorsthat are described below. Issues CO RBA (and thus thisfigure) does not address, but that all distributed systems

must address, include object lifetimes, redundancy/fail-over, naming semantics (beyond a simple name), memorymanagement, dynamic load balancing, separation of modelbetween display/data/control semantics, etc.



RAM



Random-access memory (RAM) is a form of computer datastorage . Today, it takes the form of integrated circuits that allowstored data to be accessed in any order with a worst caseperformance of constant time . Strictly speaking, modern typesof DRAMare therefore not random access, as data is read inbursts, although the name DRAM/ RAM has stuck. However, manytypes of SRAM, RO M, O TP, and NO R flash are still randomaccess even in a strict sense. RAM is often associatedwith volatile types of memory (such as DRAM memory modules ),where its stored information is lost if the power is removed .Many other types of non-volatile memory are RAM as well,including most types of RO Mand a type of flash

memory calledN

OR-Flash . The first RAM modules to come intoth e market were created in 1951 and were sold until the late1960s and early 1970s.O ther memory devices (magnetic tapes, disks) can access thestorage data only in a predetermined order, because ofmechanical design limitations.



An early type of widespread w ritable random-access memorywas magnetic core memory , developed from 1955 to 1975, andsubsequently used in most computers up until the developmentand adoption of the static and dynamic integrated RAM circuits inthe late 1960s and early 1970s. Before this, computersused relays , delay line/delay memory, or various kinds of vacuumtube arrangements to implement "main" memory functions (i.e.,hundreds or thousands of bits), some of which were randomaccess , some not. Drum memory could be expanded at low costbut retrieval of non-sequential memory items requiredknowledge of the physical layout of the drum to optimize speed.Latches built out of vacuum tube triodes, and later, out of

discrete transistors , were used for smaller and faster memoriessuch as random-access register banks and registers. Prior to thedevelopment of integrated R O M circuits, permanent (or read-only ) random-access memory was often constructedusing semiconductor diode matrices driven by address decoders ,or specially wound core memory planes.



The two main forms of modern RAM are staticRAM(SRAM) and dynamic RAM (DRAM). In static RAM,a bit of data is stored using the state of a flip-flop .This form of RAM is more expensive to produce, but isgenerally faster and requires less power than DRAMand, in modern computers, is often used as cachememory for the CPU. DRAM stores a bit of data using atransistor and capacitor pair, which together comprisea memory cell. The capacitor holds a high or lowcharge (1 or 0, respectively), and the transistor acts

as a switch that lets the control circuitry on the chipread the capacitor's state of charge or change it. Asthis form of memory is less expensive to produce thanstatic RAM, it is the predominant form of computermemory used in modern computers.



Both static and dynamic RAM are considered v olatile , astheir state is lost or reset when power is removed from thesystem. By contrast, Read-only memory (RO M) stores databy permanently enabling or disabling selected transistors,such that the memory cannot be altered. Writeablevariants of R O M (such as EEPRO M and flash memory ) shareproperties of both R O M and RAM, enabling datato persist without power and to be updated withoutrequiring special equipment. These persistent forms ofsemiconductor R O M include USB flash drives, memory cardsfor cameras and portable devices, etc. As of 2007, NAND

flash has begun to replace older forms of persistentstorage, such as magnetic disks and tapes , while NO Rflash is being used in place of R O M in netbooks and ruggedcomputers , since it is capable of true random access,allowing direct code execution.



ECC memory (which can be either SRAM or DRAM)includes special circuitry to detect and/or correctrandom faults (memory errors) in the stored data,

using parity bits or error correction code .In general, the term RAMrefers solely to solid-state memory devices (either DRAM or SRAM), andmore specifically the main memory in most

computers. In optical storage, the term DVD-RAMis somewhat of a misnomer since, like CD-RW, a rewriteable DVD must be erased before itcan be rewritten.



Many computer systems have a memory hierarchy consistingof C PU registers , on-die SRAMcaches,external caches , DRAM, paging systems, and virtualmemory or swap space on a hard drive. This entire pool ofmemory may be referred to as "RAM" by many developers, eventhough the various subsystems can have very different accesstimes , violating the original concept behind the randomaccess term in RAM. Even within a hierarchy level such as DRAM,the specific row, column, bank, rank , channel,or interleave organization of the components make the accesstime variab le, although not to the extent that rotating storagemedia or a tape is variable. The overall goal of using a memory

hierarchy is to obtain the higher possible average accessperformance while minimizing the total cost of the entirememory system (generally, the memory hierarchy follows theaccess time with the fast C PU registers at the top and the slowhard drive at the bottom).



In addition to serving as temporary storageand working space for the operating systemand its applications, RAM is used in numerousother ways.



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Corba and Ram

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