Object based system

Abstract: This invention relates to a distributed object based system (10) comprising primary (P) and secondary (S) objects with provision for containership links between primary objects and objects which they contain and for weak links between objects. The invention concerns a weak link which enables a user indirectly to insert an object (or other item, eg, text) into a target object via an intermediate object weakly linked to the target object.

Description: The present invention relates to an object based system and relates particularly but not exclusively to a distributed object based system which is accessible using a windows interface. Definition of Terms The term "object based system" is used to mean a system comprising several software components or "objects". A software object has its own data and associated methods. Objects intercommunicate by sending "messages", ie, data, instructions, requests, etc, to one another, normally via object management software. An object is initially a data file stored on disc. If object management software wishes to pass a message to it, a process will be initiated which reads the data file as part of its initialisation. If an object is fully defined by its disc file and has no process associated with it, it is said to be "inactive". If an object has a process associated with it and is defined by the state of that process then it is said to be "active". Generally, an object can be regarded as a discrete entity, eg, it can individually be moved, copied, destroyed, etc. In this context, in the embodiment to be described, an object is something which has a unique identifier and which can be sent a message. The term "link" can have several different meanings in relation to objects in an object based system and an introduction to some of the possible meanings follows. More will be said about "containership links", other interobject links and "window links" in the specific description with reference to the drawings. When designing an object based system, as an optimisation feature it is possible to define a hierarchical classification system of class objects in which class objects lower down the hierarchy (sub-classes) "inherit" characteristics from their superclasses. An example from the field of biology is shown in Figure A. The Class Object Animals has certain data and methods associated with it which are generic to all Animal Objects. Its sub-classes, Class Object Vertebrates and Class Object Invertebrates inherit the characteristics of their superclass Animals and, in addition, each contains extra data and/or methods specific to their class and so on down the classification tree. Each of the links in Fig A is an "AKO" (a kind of) link, eg, a Mammal is a kind of vertebrate and a primate is a kind of mammal. When it is desired to create an instance of a class, eg, a particular Kangaroo called Hoppy, this can be done by taking a copy of a Kangaroo Class Object and specifying Hoppy's characteristics of interest, eg, height, weight or whatever. The Hoppy Object has an "ISA" (is a) link to the Kangaroo Class Object. Such a system reduces the amount of code needing to be written for new objects since objects automatically use generic code of superclasses (AKO Links) and relevant Class Objects (ISA Links). As well as ISA and AKO links there may be other kinds of object links designed into a system. One known type of link is a containership link (sometimes referred to as a "parentage" or "ownership" link) which largely determines the location of an object and there need to be rules governing what happens to an object when its container is moved, copied, destroyed, etc. It is with such further types of links that the present application is concerned rather than the basic AKO and ISA links mentioned above which may be inherent in the system design. In the embodiment to be described, these further links are bidirectional, ie, both of the objects involved have a record of the link. Yet another type of link is that between an object and a window viewing that object - a "window link". Furthermore, windows may also be objects and have inter object links with other windows. In this specification, when there is a risk of confusion between window objects and other objects the latter will be termed "semantic objects". Generally, however, the term "object" means "semantic object". As used herein, a "semantic object" or a "semantic part" of an object is an object or object part which has stored data and which is, by virtue of the stored data, persistent. A "presentation object" or "presentation part" is created to present to a user the data of a semantic object or semantic object part and has no persistence in the system on termination of the user session. The term "click" will be used in this specification to denote a selection made by the user using an input device, such as a mouse. A storage domain may be regarded as closely equivalent to a storage medium such as a hard disc or floppy disc in the sense that all objects in a given storage domain are on-line together or are off-line together. Consequently, a single machine may support a plurality of storage domains. Prior Art European Patent Application No. 0339221, also in the name of the Applicant, relates to an object based computer system designed for use by single users. Objects can be linked by parent-child links and a child object can be linked to more than one parent object. In EP 0339221 there is only one kind of inter-object link. A parent-child link can be used to support either solely the display of all or part of a child object in a window viewing a parent object of the child object or, in addition, also the passing of data to the parent object relating to changes made to the child object. The article by John A. Dysart, an inventor on EP 0339221, entitled "The NewWave Object Management Facility" also relates to a system of the type described in EP 0339221. International Patent Application No. WO 91/02307 is also in the name of the Applicant and was published after the filing of the present application. It relates to a distributed object-based system in which representation objects provide a level of abstraction in the displaying of objects. Invention The present invention is primarily concerned with an object based system having a plurality of user stations. Such a system may be provided by a single central processing device having a plurality of user stations coupled to it, or it may be provided by a distributed processing network consisting of a number of independent processing units each having a respective station associated with it. The invention is however also concerned with an object based system having a plurality of storage domains supported on a single station. According to the present invention we provide an object based computer system which is distributed across a plurality of discrete storage domains, wherein the objects in a storage domain are all on-line or all off-line together, the system comprising a plurality of system objects, means for activating objects and graphics display means for displaying representations of the objects to users, and means for recording the existence of containership links between objects, a containership link being from one object, the container, to another object, the contained object, the container containing the contained object, characterised in that the system sustains containership links so that an object has no more than one container, and by means for also recording the existence of weak links between objects, wherein the system is arranged so that when an object which contains other objects is destroyed the objects which it contains are also destroyed, whereas when either one of two objects which are weakly linked to each other are destroyed the other object is unlinked and continues to exist; wherein the system is capable of sustaining containership links between an object in a storage domain and other objects in the same storage domain and is capable of sustaining weak links between an object in a storage domain and other objects in a different storage domain and wherein the existence of a weak link between first and second objects causes an item moved or copied to the first object to be inserted into the second object. In this context, a link is a reference to another object which causes messages to be sent to the other object in certain predefined circumstances. Weak links between objects in the system can be of different types, ie, they can have different effects, according to the methods of the linked objects and the nature of the data sent over the link. The present invention enables the provision of a facility whereby a user can insert an item into a target object (the "second object" above) the location of which may not even be known by the user. Items other then objects may be inserted into target objects. Examples of such items are text or print commands. The ability to send and receive such items is governed by the methods of the objects in the system. Preferably, objects are capable of also sustaining weak links to other objects in the same storage domain. Therefore the target object may be remote or may be in the same storage domain as the first object. In the embodiment to be described, the system comprises primary objects and secondary objects and only primary objects are capable of containing other objects. A primary object is capable of being linked to a secondary object in the same storage domain by a containership link or a weak link. A primary object is capable of sustaining a weak link to a secondary object or to another primary object. A secondary object is capable of sustaining a weak link to a primary object or to another secondary object. Optionally, an object may be capable of sustaining a plurality of weak links. This feature can be used to advantage in a mailing system, eg, a user's Out Tray object could be linked to the In Tray objects of several other users. There may be provision for a user to select which links to use for a given transaction. A weak link may have a directional characteristic, eg, incoming or out-going or may be bidirectional. The directional characteristic of a weak link to a particular object is likely to be provided on a common sense basis eg, it may not make sense to have an indirect incoming link to an Out Tray Object. In a system according to the present invention, indirect insertion may entail moving or copying an item into the target object. Where there are multiple indirect links, copying must take place. In the embodiment to be described, a weak link between two objects has the effect that a user selecting one of the objects causes the other object automatically to be displayed. This feature permits a chain of links to be constructed so as to facilitate the creation of cross-references between work items by a user and the provision of specialised facilities for the user. Depending on the objects concerned, this property may be inseparable from the indirect insertion property or may be separately selectable by the user by the provision of different link options. Preferably, the system comprises means for storing, in a distributed manner, the identity of objects to which an object is linked together with an indication of the nature of each link. Alternatively (or as well) this information can be stored centrally for all objects rather than in association with each individual object. A particular embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a diagram showing the software components of the system; Figure 2 shows the System Window; Figure 3 shows a typical Task Window; Figure 4 shows a Task Window subsystem; Figure 5 shows a dynamic link library subsystem; Figure 6 illustrates links between objects; Figure 7 is a schematic representation of links between objects in a single storage domain; Figure 8 shows the link information for the objects of Figure 7; Figure 9 depicts message transfers in the system; Figure 9a shows the information held by an Object Manager; Figure 10a illustrates a Move operation performed by a user; Figure 10b depicts the objects involved in a Move operation; Figure 11 shows a secondary object being linked to a primary object by a user. Figure 12 illustrates a Copy operation performed by a user; Figure 12b depicts the objects involved in a Copy operation. Figures 13 to 33 are screen representations. The embodiment to be described is a distributed object based office system which has a user interface of the graphical, direct manipulation style. The system presents the user with objects on a Desktop and the user can change these objects, rearrange them, create new ones and move objects between containers on his own Desktop and other users' Desktops. The main components of the system will now be described. System Components Referring to Figure 1, the software components of a distributed object based system 10 according to the invention are shown. The system 10 may run on networked personal computers each provided with a hard disc, a flexible disc drive and a networking card. Each PC is loaded with the following software components: operating system software 12, MSDOS in this example; windows interface software 14, Microsoft Windows in this example; object management software (Object Manager (OM)) 16; distributed message switching software 18; local area networking software 20; windows software 22, in this case MS Windows Applications; semantic objects software 24. The OM 16 controls the sending and receiving of messages by objects and maintains a catalogue of objects which reside on the machine. In addition, the OM 16 activates an inactive object if a message is received for it and deactivates objects when system memory runs short. The OM 16 also acts as a library to provide primitive utility functions to objects and other processes. In this embodiment, the OM 16 is an MS Windows application, although it has no window and therefore cannot be viewed. The distributed message switching software (Distributed Message Switch (DMS)) 18 is a terminate and stay resident program and functions as a message router used by the OM 16 to send messages to the correct destinations. The DMS 18 will route messages to a local or remote OM as appropriate. In this embodiment, the local area networking software 20 comprises two terminate and stay resident programs - one according to IEEE 802.3 standard and the other providing IEEE 802.2 Class I and III services. The windows software 22 comprises a system window application (System Window) and window applications for displaying window objects (Task Windows). The System Window is a process which controls a user session. The objects software 24 comprises the semantic objects of the system 10. Each of the objects is a set of data tied to an MS Windows executable file. An object may be active or inactive as defined above. Every object in the entire distributed system has its own unique identifier. Each object identifier has a part indicating in which storage domain the object was created and a part unique within that storage domain. This identifier does not change if the object subsequently moves to a different storage domain. The objects in the system 10 are mobile and may be moved, copied and otherwise manipulated by any user irrespective of where in a physical sense the user and the objects are located. Such manipulation is achieved in a manner which is consistent for all object types. In other words, the network is transparent to a user of the system. An object, or part of an object, may be viewed by a plurality of Task Windows (belonging to one or more users) at the same time, and will have the same appearance in each Task Window. In this embodiment there are two types of semantic object - primary objects and secondary objects. All objects, except the Desktop, have a container. Generally, primary objects can contain other semantic objects and can be viewed by Task Windows in isolation from their containers. However, there are some primary objects which cannot contain other objects, eg, the ChessBoard and the Visible Data Object (VDO). Certain primary objects can also contain primitive information, ie, data, eg, text, image, etc. Primary objects have an icon which consists of a small picture representing the primary object and a title. The icon, or miniaturized version of the primary object is seen when the container of the primary object is viewed. Icons can be used to move or copy the associated primary object and double-clicking on an icon causes the Task Window in which the icon was seen to view the primary object. Examples of primary objects are a Folder, a Document, a Visible Data Object (VDO). The VDO acts as a store for data and its dimensions are set by the user. The structure of a VDO can be viewed by the user but the actual data stored in the VDO cannot be viewed directly. In this embodiment, secondary objects cannot contain other objects. They can only be viewed as part of their container object - they do not have miniaturised (iconic) states. They are used to supplement their container by providing an annotation or more complex representation as will be explained. The types of object will now be more fully described, firstly window objects and secondly semantic objects. Window Objects Window objects also have the basic features of data and associated methods together with a unique identifier and the ability to receive and respond to messages from other objects. However, they differ from semantic objects in that they are transient, ie, not stored to disc whereas semantic objects are so that they persist even when the relevant machine is switched off. Window objects obtain their data from the System Window and the semantic object which they are viewing rather than from disc. There are three types of window objects, namely, the System Window, Task Windows and Secondary Windows. Each Window Object has its own unique object identifier. This identifier has an element identifying the storage domain on which the window was created and an element which is unique within that storage domain. The System Window is started automatically by the OM 16 and remains in memory while the system 10 is running, controlling the creation, destruction, opening and closing of Task Windows. The System Window also handles a user logging in and out of the system 10. Figure 2 shows the System Window 30 which covers the whole screen and acts as a background once a user has logged into the system. There is an "Activities" Menu bar 31 at the top of the System Window. The System Window 30 simply gives the user the option of entering the system (NOVA) or exiting. Each user has up to seven Task Windows at their disposal. The user can control the size and position of a Task Window as well as choosing which object the Task Window is to view. Figure 3 shows a typical Task Window 32. The title 33 of the Task Window 32 shows the name (Desktop) of the object which the Task Window 32 is viewing and the name of the machine (RS/20) on which that object resides. The Task Window 32 has a menu bar 34 which has a "GOTO" pull-down menu and may have other pull-down menus depending on the type of object being viewed. In the example of Figure 3, the Desktop object is seen to contain three objects - Colleagues, Supplies and Bin. A Task Window may be either visible (open) or invisible (closed). The System Window 30 controls the opening and closing of Task Windows and the user can close a Task Window by double-clicking its "system box" 35. In this way a user may have several tasks running in the background and switch rapidly between them. A Task Window can only view one primary object at a time. By clicking on an icon or button (see section entitled "Semantic Objects") the Task Window can be switched to view a different object. Each user has up to seven Task Windows at their disposal. The user can view up to seven primary objects at the same time by creating the required number of Task Windows and navigating these to chosen objects. The System Window stores data for each Task Window that it controls, namely: a) Task Window Object ID b) Window Handle (an internal identifier used for the windows software 22) c) Border Colour d) Open/Closed Flag e) Window Icon Flashing Flag f) Object ID of object being viewed by Task Window; g) Icon and title of viewed object. To create a Task Window 32, a user selects the "Create" item from the "Activities" menu. The System Window 30 updates its internal window information and sends a Window Create message to the new Task Window 32 containing the above information. The System Window 30 disables all user input before sending this message and re-enables it on receiving a Window Create Done reply message from the new Task Window. There is a Task Window subsystem which handles all operations forming part of the viewing mechanism such as moving and sizing the window, shutting the window, changing the object viewed, etc. Operations which are specific to the object to be viewed are handled by a dynamic link library suitable for that class of object. The dynamic link library is unloaded when the Task Window is no longer viewing that object. Each Task Window stores the Object ID of the primary object which it is viewing and information about which part of that object is being viewed. The Task Window also stores the name of the relevant dynamic link library for the object which is being viewed. There can be more than one Task Window viewing an object at any one time so that, in a distributed system, a plurality of users can view the same object at the same time. Figure 4 depicts a Task Window subsystem 36 comprising a main program 37 for activating Task Windows and a supporting library 38. The subsystem 36 is linked to one of a set of dynamic link libraries 39 for different classes of object. For example, the set 39 includes dynamic link libraries for VDO, Folder and ChessBoard primary objects referenced 39a, b and c respectively. The dynamic link library 39 caches information enabling the repainting of the relevant presentation object, or part of it as appropriate, without the need for communication with the semantic object. During operation, the Task Window sends generic information to the Task Window subsystem 36 and sends information specific to the particular presentation object to the relevant dynamic link library 39. A Task Window 32 can display any type of semantic object and it is a Primary Window in the sense that it can be linked to any primary object type, eg, Folder, VDO, etc. If the primary object which a Task Window is viewing contains a secondary object, eg, a Bar Chart, the Task Window creates a Secondary Window and links it to the secondary object as will be described. These Secondary Windows are created automatically when a user views a container of one or more secondary objects and are also destroyed automatically when the user moves the Task Window from the container. Task Windows are linked to semantic objects by a GOTO operation. This will often involve a GOAWAY operation. The first step in the GOTO operation is for the Task Window to send an AddViewer message to the desired object. This object will store the Task Window ID and reply with a Here Contents message. The first part of the Here Contents message provides information on the object ID, class, icon and the relevant dynamic link library 39. This causes the subsystem 36 to load the relevant dynamic link library 39. The remainder of the Here Contents message is private to the object and is passed to the dynamic link library 39. If the object to be viewed contains any secondary objects, the Task Window creates a Secondary Window for each secondary object. A Secondary Window is a child window used to display and interact with a secondary object on the surface of a primary object which contains it. Secondary Windows differ from Task Windows in that they are created to view one object and are destroyed when that view is no longer needed whereas Task Windows are used to view many objects in succession. When a dynamic link library 39 for a primary object initially receives a Here Contents message, that will include a list of contained secondary objects. Referring to Figure 5, the dynamic link library 39 uses a support library 40 to provide it with utility functions including the creation and registration of Secondary Windows for any contained secondary objects. The support library 40 stores the ID of the contained secondary objects and their position in the primary object. An Add Viewer message is sent to each such secondary object. In due course each such Secondary Window will receive its own Here Contents message including the name of the secondary object dynamic link library 41, for it to use. This secondary object dynamic link library is then loaded and all messages for the Secondary Window are passed to a window procedure in the secondary object dynamic link library 41. The GOAWAY operation starts by the Task Window sending a Remove Viewer message to the viewed object. If the Task Window has created Secondary Windows, this message is also broadcast to them causing them to destroy themselves. All objects which receive the Remove Viewer message update their list of windows and reply with a Viewer Removed message. The Task Window knows how many replies to expect and, when these are received the GOAWAY operation is complete. The Task Window may then view another object, or shutdown or destroy itself. To destroy a Task Window, the user selects the Destroy item from the Activities menu 31. The mouse can then be moved and clicked anywhere on the screen, but all other user input is temporarily disabled. If the mouse is clicked over a Task Window, the System Window sends a Window Destroy message to that Task Window which replies with a Window Destroy Done message. The Window Destroy message causes the Task Window to initiate a GOAWAY operation as described above. Figure 6 illustrates the links between semantic objects and between window objects as well as windows communication links (shown dotted) between windows and the objects which they are viewing. Container links between system objects are marked C and weak links between primary and secondary objects are marked W. Window communication links and links between windows are transient and disappear when the user's machine is turned off whereas the links between system objects are persistent. In Figure 6 there are three Task Windows viewing the Desktop, Folder and Document respectively. The Task Window viewing the Document has two Secondary Windows viewing the Bar Chart and the Pie Chart which have weak (updating) links to the VDO. No window is viewing the VDO directly. The data in a Visible Data Object (VDO) is not directly viewable but the structure of the data is viewable. Icons representing contained primary objects are shown as boxes in the Task Windows. Instead of opening new Task Windows it is possible to view the desktop, Folder and Document in succession with a single Task Window by clicking on the relevant icons. Semantic Objects As mentioned above, there are primary and secondary objects in the system 10. In the system 10 there are two distinct types of object linkages: containment links and weak links. The containment link largely determines where an object is physically located as explained previously and can only be present between two primary objects located in the same storage domain or a primary object and a secondary object located in the same storage domain. An object can only be contained by one other object - if its container is destroyed, so will be the object. In this embodiment weak links can exist between a primary object and a secondary object. Weak links can be established between objects located in different storage domains and do not affect the containment of an object. There are two main types of secondary object: the Button and the Representation. A Button itself has three possible functions: firstly it may provide text or graphic annotation for its container; secondly it may permit traversal to a primary object to which it is linked and thirdly it may permit indirect insertion of an object (or other item) into a target object. A Representation may be linked to a part of a VDO (Visible Data Object) and it will display the information held by that part of the VDO in a form which depends on the nature of the Representation, eg, a Pie Chart, Bar Chart or Table. Changes made to the data in the VDO will be reflected in the Representations linked to the VDO and the data held by the VDO can be altered by altering the data shown by a Representation linked to the VDO. Further details of the link between a VDO and Representation objects are disclosed in Applicant's copending International Patent Application No. WO 91/02307 entitled "Object Based System". Figure 7 shows schematically a set of possible links between objects in a single storage domain 42. Each user has a Desktop object: Desktop 44 which contains all of the other objects in the storage domain 42. The Desktop 44 is the only object which has no container and it may not be moved, copied or destroyed. Figure 7 shows the Desktop 44 containing two Folder objects: Folder A and Folder B as indicated by (double-line) containment links. Folder A contains a document object: Document A. Folder B contains a Visible Data Object: VDO B which has (single-line) weak links to Representation objects: a Bar Chart and a Table. Folder B also contains a primary object, Document B, which contains the Table and Bar Chart Representation objects. Each object stores information identifying the objects to which it is linked and the nature of the links. Figure 8 shows some of the information which is stored in respect of each of the objects shown in Figure 7. In addition, each primary object stores the position of the objects which it contains. Contained objects do not store information on their position in the object containing them. Primary objects also cache data defining their icon. Regarding weak links, the secondary object stores information about which part of the primary object it is linked to whereas the primary object does not store such link location information. Secondary objects cannot contain other objects. There are some limitations on the container properties of primary objects: document objects can only contain secondary objects and certain primary objects, eg, Chessboard, VDO cannot contain other objects. Different weak links can have different effects depending on the methods of the linked objects and the nature of the data sent over the link. For example, a weak link between a button and a primary object may be a traversal link set up so that when the user clicks on the button, the Task Window viewing the primary object containing the button switches to view the primary object weakly linked to the button. In contrast, a weak link between a VDO and a Representation object may be a two-way updating link so that changes in the VDO are reflected in the Representation object and vice-versa. It is also possible to have a traversal-type of weak link between a VDO and a Representation object so that clicking on the Representation object causes traversal to the VDO. It is also possible to have a weak link between two secondary objects to establish a "traversal chain". For example, linking a first button in Folder A to a second button in Folder B which second button is linked to Folder C. Clicking on the first button will cause the Task Window viewing Folder A to switch to Folder C. This feature enables more complex cross-referencing between user items. According to the present invention a weak link may also enable indirect insertion of an object or other item into a target object as will be further explained in the section entitled "Linking". Object Messaging Objects intercommunicate by sending messages to one another. All messages are routed through the object manager (OM) local to the object transmitting the message. Figure 9 depicts two storage domains SD1 and SD2 on a network. In each storage domain there is an OM and a transport controller TC (incorporating the DMS 18 and Local area networking software 20 of Figure 1) for sending and receiving messages over the network. Semantic Objects (A, B, C, etc) each have their own message queue Q for incoming and outgoing messages. Each OM also has a message queue Q OM and a table T containing information about objects in that storage domain. Referring to Figure 9A, the table T of each OM records the general object type, ie, whether window object or semantic object and the specific object type such as Folder, Button, etc,. The table T also stores the object ID's and flags F 1 , F 2 and F 3 indicating respectively whether a semantic object is active and whether it is in the process of closing down (ie, has a QUIT message been posted) or deregistering (ie, has a Deregister message been posted). Each OM also has a cache C containing the object ID's and current locations of objects to which messages have recently been sent. Outgoing messages are sent from the message queues of objects A, B, C in storage domain SD1 to the message queue Q OM of OM 1. The target of a message is specified in each message as an object ID. Each object ID comprises a part identifying the storage domain in which that object was created and a part which is unique in that storage domain. On receiving a message, the OM1 checks its table T to see whether the target of the message is in its own storage domain SD1. If so, the OM1 sends the message to the message queue Q of that target object. If the target object is not local, say the target is object E on storage domain SD2, the OM1 checks its cache C1. If the target object is not listed in the cache C1, the OM broadcasts a search message onto the network, via the transport controller TC1, identifying the target object. In response, all OM's on the network search their tables and the one containing the target object in its table replies. The message is then sent over the network from OM1 to OM2 in the storage domain SD2 (in this case) via transport controllers TC1 and TC2. The message is then sent from the message queue Q OM2 of OM2 to the message Q of the target object E. If a target object moves while a message is being sent to it, the posting process begins again. A message may specify more than one target object in which case each of the target objects is located in the manner described above and the message is despatched to all of them. The addressing mechanism for a message depends on whether the target is a semantic object, a Task Window or a Secondary Window. All messages received by a semantic object are for that object, messages received by a window object may be for that window object or for a Secondary Window of that Task Window. Messages contain an indicator as to whether they are for a semantic object or a window object. Certain messages have special significance because these signify that an object is moving to another storage domain or is closing down. Such messages are shown blacked in on Figure 9 and will be referred to later when describing the DeRegister and Destroy operations. How semantic objects can be manipulated and the effect of containment and weak links between semantic objects will now be described. Object Manipulations System Objects can be created, destroyed, moved, copied, linked and unlinked. These and other object manipulations will now be described in more detail. The messages involved in each manipulation are shown in Appendix A. Parameters contained in a message are shown in parentheses after the name of the message. Create A user logging in to the system 10 for the first time automatically acquires a Desktop (Figure 3) which contains certain objects: a Bin, a Colleagues Folder and a Supplies Cupboard. The Supplies Cupboard contains templates for each possible type of semantic object, which templates can be copied but not moved from the Supplies Cupboard. A copy of a template object is an ordinary object which can be moved, copied, destroyed, etc. To create a new object, an existing primary object sends a Copy message to the Supplies Cupboard containing the identity of the existing object and the sub-class of object to be created. This causes a copy of the relevant template in the Supplies Cupboard to be made and the new object sends a Copy Done message to its container together with its object identifier which it obtains from OM 16. The new object is entered in the records of the OM16 and the records of its container object. The Desktop is a special case - as it has no container it sends its Copy Done message to the System Window instead. The copy operation is described in more detail subsequently. Destroy When a user wishes to end a session the System Window sends a Flush message to the Bin. In response, the Bin sends Destroy Yourself messages to the objects it contains and the objects reply with a Destroy Done message. When the Bin has received Destroy Done messages from all of the objects which it contains, the Bin sends a Flush Complete message to the System Window. Generally, on receipt of a Destroy Yourself message an object (Object 1 in Appendix A) does two things before quitting: i) it broadcasts an Unlink message to all of the objects to which it is weakly linked. If Object 1 is a primary object it may be weakly linked to one or more secondary objects and sends a Hi to Lo Unlink message to these secondary objects. If Object 1 is a secondary object it may only be weakly linked to one primary object and sends a Lo to Hi Unlink message. There is no reply to the Unlink message; ii) if it is a primary object, it destroys all of the objects (Object 2 in Appendix A) which it contains by sending each of them a Destroy Yourself message and waiting for a Destroy Done reply. Once actions 1) and 2) above are completed, the object sends a request for deregistration to the OM and deregistration proceeds as described below. Just before quitting, the object sends a Destroy Done message to its parent. Register/Deregister An object only exists in the system if it is registered with the OM 16 of the storage domain in which it is located. Newly migrated objects are registered by their container objects. Once registered, an object will remain so until it is destroyed or moved to another storage domain. Deregistration occurs when an object is destroyed or moved to another machine. The deregistration process involves the object sending a request for deregistration (a message blacked in in Figure 9 as distinct from the other messages shown) after which the OM will refuse to accept any more messages for the object and will put a Deregistered message at the end of the queue of messages for the object. When the earlier messages in the message queue have been sent, the Deregistered message is sent to the object. The entry for the object in the table T of the OM is then deleted. When the object receives the Deregistered message it closes down. Activate Semantic Objects are either active or inactive. An object is active if it has a task assigned to it in which case its state is stored in RAM and it may use the processor of its host machine. When inactive, an object exists only as a disc file. On receiving a message for an active object, the OM passes the message to the task (ie, methods) associated with the object. On receiving a message for an inactive object, the object must be activated and this involves procedure calls. The OM creates a process for the object using a procedure to start an instance of the correct type of object. This process does two things - firstly it calls the OM Activate (Task) procedure to inform the OM of which queue to send the incoming message. The task also calls a Get My Object ID procedure in the OM which returns the object ID. The task then reads the object's state into RAM. The object is now active. Deactivate Deactivation of objects occurs in two ways and also involves procedure calls. The object may request its own deactivation by calling a Deactivate Request procedure in the OM and the OM then sends the object a QUIT message. Alternatively the OM may send an object a QUIT message so as to control the number of active objects on a machine according to the resources available. An object which receives a QUIT message saves its state to disk, calls a Deactivate Acknowledge procedure in the OM and quits its process. Move The degree of complexity of the operation of moving an object depends on whether the object has children and on its target location. Figure 10a illustrates the action of a user moving a folder object M from a source window (SW) on a first machine (M/C 1) to a target window (TW) on a second machine (M/C 2). Figure 10b illustrates the objects involved in the move operation, namely, the source window (SW), the target window (TW), a source object (S), a target object (T), the moving object (M) and child objects C 1 and C 2 of the moving object M. The move operation begins when a user moves an icon representing a primary object, or a secondary object, from one window to another, eg, by dragging these items across the screen using a mouse. The validity of attempting to send a message initiating a move operation to the target object is checked by the presentation system and the target object decides whether such a move is acceptable and the operation is aborted if the attempted move is not valid, eg, if the user is trying to move an object which cannot be moved, such as a Supplies Cupboard object. The source window SW obtains from the target window TW the object ID of the target object and the position within the target object to which the moving object M is to move. The source window SW sends a Move Object message to the source object S. If the source object S and the target object T are in fact the same object, the user has simply adjusted the position of the moving object M within its current container and the source window S updates its tables and informs its windows by sending a Move Local message. All windows which receive this message update their tables and repaint the moving object M. If S ≠ T, so as to avoid potential problems caused by interleaving of two or more move/copy operations, a lock is put on the relevant machine so that no more move/copy operations can be initiated until the current operation can be completed. This is done by S sending a Move Copy Lock message to the OM 16. If this call fails the move operation must be aborted and S sends a Can't Move message to SW which causes a warning message to be displayed to the user. If the call succeeds, S sends a Move Yourself message to M. If M is an object which is not allowed to be moved, it sends a Won't Move message to S which causes a warning message to be displayed to the user in SW. If M can move, it sends a Remove Object message to S. This causes S to remove M from its object tables and to send Remove Object messages to all of the windows viewing S. These windows then remove M from their tables and repaint. If M is already on the same machine as T the move operation is relatively simple. M sends an Insert Local message to T which causes T to insert M into its object tables and send Insert messages to the windows viewing T which will draw M. M then sends a New Parent message to all its viewers to inform them of its change of containership and sends a Move Complete message to S. S then triggers release of the Move Copy Lock. If M and T are on different machines, M must deregister so that the OM 16 on its current machine stops accepting messages for it. However, if M has children, it needs to be able to monitor Move Complete messages from its children therefore M re-registers itself using a new object ID. M then sends an Insert message to T containing a serialized form of M allowing T to write the data for M to disk and to register M on the new machine. At this point there are two copies of M: the original (Old M ) which now has a new object ID, and the moved (New M ) which is using the correct object ID for M. It is Old M which controls the move operation. T then inserts New M into its tables and sends Insert (New M ) messages to its viewers which causes the windows to repaint accordingly. Old M then sends a New Node Prompt message to New M to prompt it to update its state and inform its viewers of its new location. Old M also sends a New Parent message to all viewers of New M. If M has no children it now sends a Move Complete message to S and deregisters and destroys itself. If M has children it sends a Send Yourself message to one of its children C. C checks to see if it is already on the same machine as New M. If so it replies with a Move Complete message; otherwise it deregisters itself. When deregistered, C sends a Register Me message containing a serialized form of C to New M which causes New M to write C's data to disk and to register C on its machine. If C has children of its own, it sends a Send Yourself message to one of them and waits for the Move Complete reply. Note that, if C has children of its own it needs to go through the process of deregistering and re-registering under a new object ID as described above with reference to M so that there exist two objects, Old C and New C for a period,. Old C sends one of its children a Send Yourself message and waits for the Move Complete reply. This is a recursive operation and eventually Old C will have moved all of its children. It then sends a Move Complete message to Old M and destroys itself. Old M then deletes C from its list of children and sends a Send Yourself message to another child. When Old M has moved all of its children it sends a Move Complete message to S and destroys itself. S then triggers release of the Move Copy Lock and the operation is complete. Thus once an object has been moved to a different storage domain, it acquires a new parent in the new storage domain. The child (and further descendant) objects of the moved object are moved with their parent/ancestor. As mentioned previously, the Move operation violates the convention that an object and its container are in the same storage domain because the container object moves before the objects which it contains. An alternative approach would be to transfer an object together with all of the objects which it contains, ie, all of these objects being off-line at the same time, although this approach has practical disadvantages. Linking The user is able to link a secondary object (S in Appendix A) to a primary object (P in Appendix A) in the course of a user session. Figure 11 shows a screen with a view of part of a folder 60 containing a Visible Data Object (VDO) 62 (a primary object) and a Bar Chart (a secondary object). Each secondary object has a border 66 which appears when the input device, eg, mouse cursor, is positioned over it. The border has a link box 68, an unlink box 70, and two size boxes 72 and 74. The Bar Chart 64 is linked to the VDO 62 by clicking the mouse over the link box 68 and dragging the mouse to the VDO and releasing the mouse. This causes a "Chain" 76 to appear on the screen. On releasing the mouse, the Secondary Window (SW) viewing the Bar Chart 64 queries the Task Window (TW) viewing the VDO 62 whether the VDO will accept a link from a Bar Chart. A negative response would cause the operation to be aborted. However, in this case the link is acceptable and a LINK (Target Object) message is sent by the Secondary Window to the Bar Chart. If the Bar Chart 64 is already linked, it sends an UNLINK message to the object (O) to which it is currently linked. The Bar Chart 64 then updates its link data and sends a LINK message to the VDO. The VDO then updates its link data to include the Bar Chart. In the particular case of a VDO linking to a Representation object, the link must be to a specific part of the VDO. Therefore, the Bar Chart 64 sends a Get VDO Info message to the VDO 62. This causes the VDO to send a description of itself to the Bar Chart. The secondary window SW then displays a dialogue to enable the user to select the part of the VDO to which the Bar Chart is to be linked. The effect of the Bar Chart 64 being linked to the VDO 62 is that any changes in the relevant part of the VDO are automatically reflected in the Bar Chart. Also, the data in the VDO can be changed by making alterations to the Bar Chart. Other Representation Objects such as Pie Charts and Tables can be linked to a VDO in a similar manner. The Bar Chart and the VDO may be in different storage domains in a distributed system. Another type of secondary object is a Button. Buttons serve to decorate primary objects. There are Graphic Buttons and Text Buttons. The user can draw on a Graphic Button or can paste a picture into a Graphic Button. Text can be written onto a Text Button or moved or copied to a Text Button. Buttons also have another use. They can be linked to a primary object (generally as described above in relation to the VDO and Bar Chart) so that when a user selects a Button this causes the system to switch to viewing the primary object to which the Button is linked. Generally, if a primary object P is weakly linked to a secondary object S, the following rules apply: If P or S is moved - P will remain weakly linked to S; If P is copied - the copy of P will not be linked to S; If S is copied - P will be weakly linked to both S and the copy of S; If P or S is destroyed - the one remaining is unlinked. In addition, in a system according to the present invention, linking a Button to a primary object may provide a facility for a user to send an object to a remote target object the location of which may not be known by the user. The user simply "drops" the object to be sent onto a Button which is weakly linked to the target object which initiates a Move operation to move the object to the target object. For example, a Button could be linked to the In Tray folder object of another user. It is possible that for security reasons, a user may not be permitted to view the contents of a particular object. However, the user could still move objects to the secure object using the above mechanism. It is envisaged that two secondary objects may be weakly linked together and that the links may have a directional characteristic, ie, incoming or outgoing. This opens up the possibility of a chain of links, eg, a first Button in a first Folder is linked to a second Button in a second Folder which is also linked to a third Button in a third Folder. The second and third Buttons may be in the In Tray folders of different users so that a message sent from a first user to a second user is automatically forwarded to a third user. Automatic backup is another possible use of this facility. It is also envisaged that items other than objects may be sent using this facility, eg, text or print commands. An example of sending text using this facility would be to insert an address from one document, eg, an address list into another. In automatic document editing applications a Button could be linked to precise points in a text so that character strings can be inserted from a directory of user names and addresses, for example, by dropping the text items on the relevant Buttons. Likewise two primary objects could be weakly linked together eg, to utilise the ability automatically to send text between objects. For example entries in a document primary object which is presented as a form to the user could automatically be sent to another primary object functioning as a database. Another example is that parts of two document objects could be linked so that text from one document automatically gets sent to the other, eg, in compiling a departmental activity report from individual reports. A "Link" operation may be provided on the Task Window menu bar which, in the case of primary objects, could cause a link box to appear around the primary object or could cause the user to be prompted to identify the or each object she or he wishes to link to the primary object as well as the directional characteristic of each link. There may be provision for multiple weak links from or to an object permitting indirect insertion, for example in an electronic mailing system. This entails inserting copies of an object or other item into target objects. Unlink To unlink a secondary object, the user selects its unlink box 70. This causes the secondary window SW viewing the secondary object to send an UNLINK message to the object. The secondary object clears its link data and sends an UNLINK message to the primary object to which it is linked. The primary object then amends its list of linked objects. In the event of a primary object to which a secondary object is linked being destroyed, the secondary object receives an UNLINK message and clears its link data. If a Representation, such as a bar chart, is unlinked, it will retain the values and labels which it last received from the VDO to which it was linked. These values can still be changed by a user but such changes will no longer update the VDO. Copy The copy operation is somewhat similar to the move operation, but simpler. To initiate a copy operation, a user drags an icon or a secondary object from one window to another or to a different position in the same window. Figure 12a shows a folder icon 80 representing a folder object C which is contained by a source object S (eg, the Desktop) on a first machine (MC1) and being viewed by a source window SW, being copied to a target object T which is being viewed by a target window TW. Figure 12b depicts these objects. If the attempted copy operation is valid, the source window SW obtains from the target window TW the ID of the target object T and an insert position within that target object T. The source window SW then sends a Copy Object message to S. S then calls the OM to place a Move Copy Lock. If this call fails the operation will be aborted and S will send a Can't Copy warning message to the user. Otherwise, S sends a Copy Yourself message to C. If C is a type of object which cannot be copied, eg, the Bin, it sends a Won't Copy message to S and S in turn sends a Won't Copy message to SW which displays a warning message to the user. Otherwise, C sends an Insert (C 1 ) message to T which comprises as data a serialized form of C. If C is a secondary object, it sends a Link Yourself message to C 1 to prompt C 1 to link itself to the same primary object to which C is linked On receiving the Insert message, T registers C 1 and inserts C 1 into its tables and sends a corresponding Insert message to all of its viewers These windows then insert C 1 into their tables and repaint accordingly. If C has no children, it sends a Copy Complete message to S which then triggers release of the Move Copy Lock. If C has children, it sends a Copy Yourself message to one of these The child object acts just as C did and eventually will reply with a Copy Complete message C does this for all of its children and finally sends a Copy Complete message to S. S triggers release of the Move Copy Lock and the copy operation is complete. The ability to link primary objects and secondary objects as described above permits a chain of links to be constructed so as to facilitate cross-referencing between work items and also enables special facilities to be provided. These and other features of the invention will now be illustrated with reference to screen representations. User Session This section does not describe a complete user session but serves rather to illustrate the main object manipulations from a user's viewpoint. Referring to Figure 13, a user (Eric) having just entered the system (ASTRA) via the System Window 80 sees the contents of his Desktop through a first Task Window 81. There is a Window Selection column 82 on the right hand side of the System Window 80. The Window Selection column 82 is part of the System Window and is designed to save screen space by enabling a user to minimise (or iconise) a Task Window as shown in the two examples referenced 84 and 85 in Figure 13 and then to restore the Task Window by clicking on the relevant icon. The System Window 80 has an "Activities" menu bar 86 (as seen previously in Figure 2) for controlling windows. The Task Window 81 has a menu bar 88 offering four menus: "Goto", "Configure", "Presence", "Create". In this example, the Desktop contains three objects; "Bin", "Supplies Cupboard" and "PJet Work" therefore three icons can be seen. All three of these objects are primary objects of the folder type although the Bin and the Supplies Cupboard have special properties. Objects can be destroyed by dropping them on the Bin, the contents of which are deleted on log-off. Templates for new objects to be created are stored in the Supplies Cupboard and a user can add further templates. Referring to Figure 14, on selecting the Activities menu bar 86, a menu 87 having the following three options is displayed: "Create", "Destroy" and "Shutdown". Selecting Shutdown causes the user to log-off. If the user wishes to open a new Task Window on, say, the PJet Work folder, the user selects Create from the menu 87 followed by clicking on the PJet Work icon. This causes the creation of a second Task Window 90 which initially is viewing the Desktop and can be navigated to view the contents of the PJet Work folder as shown in Figure 15. The PJet Work folder contains three objects: two documents, namely Drawings and PJ Log and a folder PJ Temp. Selecting Destroy and then clicking on a Task Window causes that Task Window to be destroyed. As well as creating a new Task Window to view a different object, it is possible to change the focus of an existing Task Window (as explained previously). To do this, the user selects the GOTO option from the menu bar 88 resulting in a menu 91 being displayed as shown in Figure 16 having four options: "Last", "Recent", "Parent" and "Navigator". Selection of these options have the following consequences: Last: causes the system to back track by one window; Recent: causes a Window History area (not shown) to be displayed showing the icons of objects recently viewed; Parent: changes the focus of the Task Window to view the parent of the object presently being viewed; Navigator: a specialist function that will not be described here. The "Configure" option on the menu bar 88 enables the GoTo menu to be edited. To create a new object, the user selects the Create option from the menu bar 88 which brings up a menu 92 detailing the possible objects which can be created as shown in Figure 17. This list corresponds to the templates which are stored in the Supplies Cupboard. On selection of one of these (the Chessboard in this case) the cursor changes into the object selected and the user can "deposit" the newly created object "Chess" at will. In this example, the user deposits the Chess Object in the PJet Work Folder as illustrated in Figure 18. Referring to Figure 19, the user's screen now has two Task Windows 100 and 102. Task Window 100 is viewing the folder object PJet Work which contains the documents. Drawings and PJ Log and a folder PJ Temp. The Task Window 102 is viewing the Desktop which contains the Bin (and the folder object PJet Work?). If the user wishes to move the document PJ Log from the folder PJ Work into the Desktop, the user selects the document causing its icon to darken as shown in Figure 20 and "drags" PJ Log from Task Window 100 to Task Window 102 and then releases PJ Log. Figure 21 shows that PJ Log is now in the Desktop and is no longer contained by PJet Work. Referring to Figure 22 if the user wishes to copy the document Drawings into the Desktop, the user again selects the document Drawings, this time for copying, drags it into the Task Window 102 as indicated in Figure 22 and deposits it in the Task Window 102 as shown in Figure 23. A copy of the document Drawings then resides in the Desktop and the original document remains in the folder PJet Work. It is necessary to differentiate a Move operation from a Copy operation and this can be done in several ways. One way is for a Move operation to require the user to click on the object which he/she has decided to move and then to drag the object to the new location. When a mouse input device is being used, clicking implies pressing the mouse button when the cursor is located over the desired object and dragging means then moving the mouse whilst the button is held down. The Copy operation in contrast could require the user to click on the desired object and then to press a designated key, such as the Alt key, and to drag the object to the new location. Figure 24 depicts a screen in which there are two Task Windows, Task Window 100 viewing PJet work and another Task Window 104 viewing a Folder object. The Folder object contains a Button object. Figure 24 shows that the user has selected the Button object which causes a Secondary Window 106 surrounding the Button object to become visible. The Secondary Window 106 contains several specialised selector regions including a link box 108. By clicking on the link box 108 and dragging to the Drawings document in PJet Work, the user links the Button object to the Drawings document and a "chain" 109 is temporarily visible. The Button is now linked to the Drawings document and can be labelled as such as shown in Figure 25. Subsequently, selecting the Button will cause the system automatically to traverse to view the document Drawings so that the screen changes from that shown in Figure 26 to that shown in Figure 27. Referring to Figure 28, the user is viewing a VDO in a Task Window 110 which shows the dimensions and indices of the VDO but not the actual data. Figures 29 and 30 each show a Bar Chart secondary object 111 linked to the VDO being viewed in a Secondary Window 112 in the Task Window 110 and a Table secondary object 113 being viewed in a secondary Window 114 in another Task Window 115 which is also linked to the VDO. Figures 29 and 30 show how altering an entry in the Table (for Admin Department Travel) causes a corresponding change in the Bar Chart owing to a change in the data held by the VDO. Figure 31 shows a Task Window 116 viewing the PJet Work folder and a Task Window 118 viewing a Folder object on the same machine. A Button 120 in the PJet Work folder is weakly linked to the Folder object. By selecting the Drawings document in PJet Work, causing it to darken as shown in Figure 32, and dragging it to the Button 120 and dropping it on the Button, the Drawings document is automatically moved to the Folder object and the result of the transaction is shown in Figure 33. Although in this example the Task Window 118 is shown, it is not necessary for a user to be viewing the target object of an indirect insertion. APPENDIX A FROM TO MESSAGE Create Container Supplies Cupboard Object Create (Container ID, Subclass) New Object OM Get Object ID OM New Object Here Object ID New Object Container Create Done (New Object ID) Destroy Object 1 Object 2 Destroy Yourself linked object Object 1 Hi 2 Lo Unlink (Object 1 ID) or linked object Object 1 Lo 2 Hi Unlink (Object 1 ID) Object 1 Contained Object Destroy Yourself Contained Object Object 1 Destroy Done Object 1 OM Deregister Request OM Object 1 Deregister Object 1 Object 2 Destroy Done Register Container OM Register Object (Object ID, Class) Deregister Object OM Deregister Request OM Object Deregistered Activate Task OM Get My Object ID Task OM Activate (Object ID, h Task) Deactivate Object OM Deactivate Request OM Object QUIT Object OM Deactivate Acknowledge Move SW TW Get Move Info TW SW Here Move Info (ID, target position) SW S Move object (M ID, T ID, Target Position) If S = T: S Viewers of S Move Local (M ID, new position) If S ≠ T: If S ≠ T : S OM Move Copy Lock (S SW Can't Move) S M Move Yourself (M S Won't Move) M S Remove Object (M ID) S Viewers of S Remove Object (M ID) If M on same M/C as T: M T Insert Local T Viewers of T Insert M Viewers of M New Parent M S Move Complete S OM Release Move Copy Lock If T not on same M/C as M: (old) M OM Deregister Request (old) M OM Get Object ID (old) M T Insert (M) T Viewers of T Insert (New M) (old) M New M New Node Prompt (old) M New M viewers New Parent (T ID) M childless: (old) M S Move Complete M has children (old) M C Send Yourself C on same M/C as New M C of M Old M Move Complete C New M Register Me [C] C has children Old C . Child Object of C Send Yourself Child Object of C Old C Move Complete Old C Old M Move Complete Old M S Move Complete S OM Release Move Copy Lock Link SW S Link (Target Object) If S already linked: S O Unlink S P Link (S VDO Get VDO Info VDO S Here VDO Info) Unlink SW S Unlink S P Unlink (P S Unlink) Copy SW TW Get Copy Info TW SW Here Copy Info (T ID, position in T) SW S Copy Object (C ID, T ID, insert position) S OM Move Copy Lock (S SW Can't Copy) S C Copy Yourself (C S Won't Copy) (S SW Won't Copy) C T Insert (C') C = secondary object C C' Link Yourself (Primary Object ID) T T's viewers Insert C has no children: S C Copy Complete S OM Release Move Copy Lock C has children: C Child of C Copy Yourself Child of C C Copy Complete

Claims: An object based computer system (10) which is distributed across a plurality of discrete storage domains, wherein the objects in a storage domain are all on-line or all off-line together, the system comprising a plurality of system objects (24), means for activating objects and graphics display means for displaying representations of the objects to users, and means (16) for recording the existence of containership links between objects, a containership link being from one object, the container, to another object, the contained object, the container containing the contained object, characterised in that the system sustains containership links so that an object has no more than one container, and by means (16) for also recording the existence of weak links between objects, wherein the system is arranged so that when an object which contains other objects is destroyed the objects which it contains are also destroyed, whereas when either one of two objects which are weakly linked to each other are destroyed the other object is unlinked and continues to exist; wherein the system is capable of sustaining containership links between an object in a storage domain and other objects in the same storage domain and is capable of sustaining weak links between an object in a storage domain and other objects in a different storage domain and wherein the existence of a weak link between first and second objects causes an item moved or copied to the first object to be inserted into the second object. A system (10) according to claim 1 wherein objects (24) are capable of also sustaining weak links to other objects in the same storage domain. A system (10) according to claim 1 or claim 2 comprising primary objects and secondary objects wherein only primary objects are capable of containing other objects. A system (10) according to claim 3 wherein a primary object is capable of being linked to a secondary object in the same storage domain by a containership link or a weak link. A system (10) according to claim 3 or claim 4 wherein a primary object is capable of sustaining a weak link to a secondary object or to another primary object. A system (10) according to claim 3 or claim 4 wherein a secondary object is capable of sustaining a weak link to a primary object or to another secondary object. A system (10) according to any preceding claim wherein an object is capable of sustaining a plurality of weak links. A system (10) according to any preceding claim wherein a weak link has a directional characteristic. A system (10) according to any preceding claim wherein said insertion entails moving said item into the second object. A system (10) according to any one of claims 1 to 8 wherein said insertion entails copying said item into the second object. A system (10) according to any preceding claim wherein a weak link between two objects has the effect that a user selecting one of the objects causes the other object automatically to be displayed. A system (10) according to any preceding claim comprising means for storing in a distributed manner the identity of objects to which an object is linked together with an indication of the nature of each link.

Inventors: DUGGAN HUGH, MOREL WILLIAM, DUGGAN, HUGH, MOREL, WILLIAM, DUGGAN, HUGH, C/O HEWLETT PACKARD LIMITED, MOREL, WILLIAM, C/O HEWLETT PACKARD LIMITED
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1990
Main classification: G06F9
IPC codes: G06F3, G06F9