Abstract:
A system for providing interactive services in a subscription television system includes a system controller for controlling the interactive services, a video signal source for transmitting a video signal and a subscriber terminal for receiving a downstream transaction from the system controller and the video signal. The system controller comprises a memory for storing subscriber terminal address data and transaction data, a processor for generating a downstream transaction including subscriber terminal address data and transaction data, a transmitter for transmitting the downstream transaction, and a receiver for receiving an upstream transaction. The subscriber terminal includes memory for storing the downstream transaction, an on screen display generator for overlaying data in place of or superimposed on the video signal, data input circuitry for generating upstream data, and an upstream transmitter for transmitting the upstream data to the system controller. The downstream transaction may include screen data which defines at least one screen to be overlaid in place of or superimposed on the video signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a division of application Ser. No. 08/265,941, filed Jun. 27, 1994, now U.S. Pat. No. 5,754,940; which is a continuation of application Ser. No. 07/960,261, filed Oct. 13, 1992, now U.S. Pat. No. 5,355,480; which is a continuation of application Ser. No. 07/342,987, filed Apr. 20, 1989, now abandoned; which is a continuation of application Ser. No. 07/289,218, filed Dec. 23, 1988, now U.S. Pat. No. 4,987,486, and entitled “Automatic Inter-active Television Terminal Configuration” and is related to U.S. Application Serial Nos.: 07/340,642, entitled “Cable Television Transaction Terminal”; 07/340,731, entitled “Terminal Authorization Method”; 07/340,659, entitled “Terminal Polling Method”; 07/340,967, entitled “Interactive Television Terminal with Programmable Background Audio or Video”; and 07/340,660, entitled “Interactive Room Status/Time Information System”, filed concurrently herewith. 

   TECHNICAL FIELD 
   This invention relates generally to two-way interactive cable television systems in which entertainment services, information and data signals are provided over cable facilities from a system manager location to a user location, and more particularly to a method and apparatus for controlling the storage of data in local storage at the user location. 
   BACKGROUND OF THE INVENTION 
   Two-way interactive cable television systems are known for transmitting entertainment, information and data signals over a cable facility toward a plurality of users. Data may be transmitted and addressed to a particular subscriber over a separate data channel or over a so-called “in-band” data channel. In a downstream direction, addressed control data may represent services authorized to a particular terminal or control commands to that terminal. In an upstream direction from a terminal to the service provider or system manager location, control data may represent selections made by a user in response to a polling request or at the time of user selection. 
   In a hotel environment, the system manager delivers video signals, e.g., television programming, to user terminals located, for example, in various hotel rooms over cable lines, and the system manager also transmits digital messages to receivers at each room location. Similarly, information and data generated at each room are transmitted upstream to the system manager via the cable. 
   It is known in the prior art to provide an interactive terminal at a user location including a local processor for controlling access to a host computer located at the system manager location and a local data store connected to the local processor. Such terminals also normally consist of a display for displaying messages to the user or details of the transaction being conducted, and a keyboard or remote key pad comprising a number of keys by which the user can enter data and initiate functions. 
   Typically, a central data storage is provided at the system manager location for storing a complete data base, and a smaller sub-set of that data base is maintained in the local store at the user terminal. Because faster response times can be achieved if a requested transaction can be completed without the need for repeatedly accessing the central data base, configuration of the local store becomes an important design consideration. Prior methods of local storage management have been based upon last use or recency of use, and systems are also known in which those items in the data base most frequently requested by the user terminals are retained in the local store. These methods have not, however, been entirely satisfactory, and have not uniformly achieved the desired reduction in response time. Accordingly, there is a need for a more efficient technique for choosing data items to be stored in a local storage of an interactive terminal system. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an improved method of local storage management for an interactive terminal which ensures faster response times and which reduces the need for repeatedly accessing the central data base. 
   It is another object of the present invention to provide a method of local storage management for an interactive terminal which updates the local store in accordance with the relative priority of data entries. 
   It is still another object of the present invention to provide a method of local storage management which updates the local store in accordance with the system service requested by the user. 
   An interactive terminal, according to the present invention, comprises a processor, a local data store connected to the processor, a character generator responsive to processor control for generating a television signal output to an associated television receiver, a key pad or remote control for inputting information to the processor, and a power supply input. 
   In a hotel environment, the interactive terminals are located in individual hotel rooms and are coupled, via 2-way RF communications, to a central hotel location, designated as the system manager, where the terminals are accessed by polling for monitoring and servicing the guests&#39; requests. The local storage capacity of the interactive terminal is provided for storing data and a number of character screen commands, accessed from a central data base at the system manager, for use in generating prompting screens displayed on the television receiver. Thus, in use, a hotel guest may call for a variety of services, e.g., pay TV programs, room service and the like, by operating the input keys of the key pad to cause a prompting screen to be displayed on the television receiver. 
   According to the invention, character screen commands are stored at the terminal according to an assigned priority. The stored screen commands also include pointers which are used by the terminal processor to prefetch other screens which follow the current screen in a decision tree structure manner. These screens are downloaded from the system manager for storage at the terminal so that they are available for display if and when called for. In operation, if a particular branch of a decision tree is not selected by the user, screens associated with that branch are deleted from terminal memory as needed to make room for new screens. The terminal memory is also controlled so that character screen commands are swapped in and out of memory based on their relative priority. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block schematic diagram of an interactive entertainment system in accordance with the present invention applied in a hotel or hospital system environment comprising automatically configurable two-way interactive terminals. 
       FIG. 1   a  is a block schematic diagram of system manager  12  of  FIG. 1 . 
       FIG. 2  is a block schematic diagram of a two-way interactive terminal according to the present invention comprising a microprocessor, a memory, a character generator, a key pad and a television signal output to an associated television receiver. 
       FIG. 3  is a schematic diagram showing the data types stored within the screen data area of RAM  217  of  FIG. 2 . 
       FIG. 4  is a schematic diagram showing the data structure of character screen commands stored in RAM  217  of  FIG. 2 . 
       FIG. 5  is a schematic diagram showing the data structure of the key template shown in  FIG. 4 . 
       FIG. 6  shows a number of exemplary display screens and associated key templates for the in-room checkout feature. 
       FIG. 7   a  is a block diagram showing the tree structure of a fan-out type feature. 
       FIG. 7   b  is a block diagram showing the tree structure of a serial chain type feature. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , there is shown a block schematic diagram of an interactive entertainment system in accordance with the present invention. The depicted embodiment relates to its application in a hotel or hospital system environment, however, the present invention is not so limited, and may be applied in any two-way interactive entertainment system such as a cable or satellite television entertainment system involving a distribution network comprising trunk lines and feeder cables of optical or coaxial cable. For example, satellite receiver and decoder  3 , cable receiver and descrambler  4 , video modulator  5 , addressable transmitter receiver  10 , locally generated video modulator  11 , and system manager  12  may all be located at the location of a cable television system head end. In this example, coaxial cable or optical fiber link  6  can connect the cable television head end to two-way interactive terminals  7 ,  8  or  9  of the entertainment system located at individual subscriber premises. 
   On the other hand, the embodiment of  FIG. 1  is especially exemplary of the application of the present invention in a hotel or hospital entertainment system. Transmission link  6 , then, may simply comprise a coaxial or optical fiber cable link between an entertainment control center and a plurality of terminals, for example, terminals  7 ,  8  or  9  located in rooms of the facility. 
   In particular, at the system control center, a satellite signal may be received at satellite antenna  2  and then received and decoded at satellite receiver decoder  3  for distribution via modulator  5  over a distribution cable  6  to the interactive terminals  7 ,  8  or  9  of the pre-sent invention. Additionally, a band of cable television channels received over cable facilities  1  are received and, if necessary, descrambled at cable receiver/descrambler  4 . The descrambled video signals are then modulated at modulator  5  for distribution over facilities  6  to interactive terminals  7 ,  8  or  9 . If desired, an optional scrambler may be provided for scrambling premium channel transmissions to interactive terminals  7 ,  8 , or  9 . Of course, terminal  7 ,  8 , or  9 , then, would comprise descrambler circuitry capable of descrambling the scrambled premium channel transmissions. Such circuitry is not shown in either  FIG. 1  or  2  but would be preferable in a system involving distribution of signals to remote subscriber locations. 
   According to the exemplary embodiment of  FIG. 1 , the hotel or hospital location may be connected to other hotels or hospitals via data link  14 . Over data link  14 , data may be received via modem  13  at system manager  12 . In this manner, the hotel or hospital system manager may maintain current status of all features and all interactive terminals of an entertainment system comprising a plurality of hotels. For example, entertainment schedules may be distributed over the data transmission link from a network control center where the entertainment schedule is composed. System manager  12  comprises a data processing unit and appropriate memory for storing status and features associated with all terminals in the system. System manager  12  also controls the generation of video channels at modulator  11 , if necessary, for transmission over facility  6  to the two-way interactive terminals  7 ,  8  or  9 . In particular, modulator  11  comprises the capability to generate signals for actuating the display of character screens at terminal locations responsive to the control of the system manager  12  in the event, for example, that terminals are unable to generate the character screens themselves. 
   The generation of complete screens of characters for transmission to a terminal according to the present invention for display is not required because the terminal ( FIG. 2 ) comprises its own character generator  204 . Consequently, locally generated video modulator  11  is optional. Instead, system manager  12  generates commands to generate screens, and not the screens themselves, for transmission via addressable transmitter  10  to an interactive terminal according to the present invention. In a “hybrid” system comprising terminals having no character generator and terminals according to the present invention, locally generated video modulator  11  would be necessary. The screens generated either at the terminal or by way of modulator  11 , for example, may relate to the provision of information to guests or patients about hotel or hospital services, respectively. 
   Also according to  FIG. 1 , system manager  12  controls an addressable transmitter/receiver  10  for transmitting addressed communications which are uniquely addressed to terminals  7 ,  8  or  9  and receiving communications at random times or in response to polling requests of terminals. The addressable transmitter  10  according to  FIG. 1  may transmit addressed information on a separate data carrier, for example, at 108 megahertz and receive information transmitted in a so-called upstream direction from the terminals on another separate data channel at 25 megahertz. In an alternative embodiment, all downstream communications may be transmitted in band or within a particular television channel transmission, for example, within the horizontal or vertical intervals of transmitted video signals. Consequently, the system manager  12  accomplishes in band signaling by controlling data input into the video signals transmitted via either modulators  5  or  11  (not shown). A separate data carrier for each direction of transmission may be preferred due to the increased data carrying capacity. For upstream transmission, a telephone line or spread spectrum transmission may be employed as an alternative to a separate data channel. 
   Referring to  FIG. 1   a , system manager  12  particularly comprises a processor  100 , an associated memory, ROM, for storing control algorithms, a read/write memory, RAM, for storing a plurality of character screens, a display screen  101  and a keyboard  102  for screen design and modification. An optional printer  103  is also provided for printing out room checkout bills in a similar manner as would occur at a hotel front desk during checkout. 
   Besides specifically addressed data communications with the interactive terminals  7 ,  8  or  9 , the system manager may also address communications globally to all interactive terminals which communications may or may not require a terminal to respond. Such global communications, for example, may be addressed to a global address representing all terminals in the particular hotel system or to a group address representing a group of terminals within the system having a commonality of interest. 
   Referring to  FIG. 2 , there is shown a two-way interactive terminal according to the present invention. The terminal is coupled via facility  6  to video modulator  5 , addressable transmitter/receiver  10 , and locally generated video modulator  11 , if provided, as shown in  FIG. 1 . 
   In particular, a two-way interactive terminal according to the present invention comprises an up/down converter  201  for converting channels received over the facility  6  for display at an associated television receiver (not shown) or for transmitting data modulated to 25 megahertz by data transmitter  203  for transmission over facility  6 . Up/down converter  201  passes data on the separate data carrier at 108 megahertz for demodulation and reception at data receiver  203 . Received television entertainment signals are provided by up/down converter  201  to demodulator  202  which also provides automatic frequency control and gain control of up/down converter  201 . Demodulator  202  provides video via on screen character generator  204  to channel ¾ modulator  205 . In this manner, on screen character generation may appear superimposed upon an incoming video signal or displayed in the form of a teletext screen, for example, text on a plain colored background. Also at demodulator  202  the baseband audio channel is transmitted to audio output  211  or via channel modulator  205  to the television receiver at radio frequency. In addition, a baseband video output  213  may be provided from on screen character generator  204  at video output jack  213 . 
   The interactive terminal of the present invention further comprises a processor  206  for controlling data transmission and reception at data receiver/transmitter  203 . Processor  206  also controls character generation at character generator  204  via gate array  207 . Also via gate array  207 , the processor  206  controls a key pad  216  which may be directly coupled to the gate array or coupled via infrared or other remote control transmission link receiver  208 . Random access memory (RAM)  217 , provided with backup power by capacitor  218 , stores character screen commands, downloaded feature data and other data received over the data transmission link via data receiver  203  from the system manager responsive to processor  206  control. The processor  206  also has access to a nonvolatile random access memory  222  and access via gate array  207  to an outboard read only memory (ROM)  219 . Processor  206  receives remote control key commands from a remote control key pad via infrared or other remote control transmission receiver  208 . Processor  206  may also control the operation of a phase lock loop  209  and bandpass filter  210  for controlling operation of the up/down converter  201 . 
   Power is supplied via an alternating current input to power supply  215  which provides, for example, a plus 5 volt and a plus 12 volt DC input to various components of the terminal requiring such power. Furthermore, the alternating current power input to power supply  215  may be provided via a controlled switched outlet  214  to an associated device such as the television receiver associated with the terminal. The state of the switched outlet  214  is controlled via gate array  207  by processor  206 . For example, the switched outlet  214  may continuously provide power to its associated device or provide power only when an “on” button of key pad  216  or a remote control key pad is set to an on condition. 
   Values inside of boxes or associated with boxes are exemplary of memory sizes, clock rates, or component types. For example, clock  220  for clocking the microprocessor operation may operate at 14.318 megahertz. The clock signal is divided down by gate array  207  for operation of the microprocessor  206  which may be a Motorola MC 6803U4 or for clocking other processes of the terminal. The on screen character generator  204 , for example, may be a Mitsubishi M50455 component. RAM  217  may comprise 16 kilobytes of memory and ROM  219  may comprise 32 kilobytes of memory. 
   Referring now to  FIGS. 2-8 , the method for controlling storage of screen data in RAM  217  of the interactive terminal will now be described. Referring first to  FIGS. 2 and 3 , in the preferred embodiment, RAM  217  comprises a screen data area which is divided into a plurality of data blocks. Six data blocks are shown in  FIG. 3 , however, in the preferred embodiment, RAM  217  is divided into forty seven (47) such blocks. Each data block is comprised of 292 characters, and may contain any one of three types of data, i.e., header data, communications data and screen data. Data in each data block is uniquely identified by a screen number, which is stored at the beginning of the data block, and this screen number also identifies the data “type”. In the preferred embodiment, header data have a screen number of 127, screen data have screen numbers in the range of 128-1023, and communications data have screen numbers over 1023. 
   A Priority List is also stored in RAM  217  and consists of a plurality of pointers to the RAM locations at which the data blocks are stored. These pointers are arranged in order based on the relative priority of data stored within the screen data area, and the Priority List is updated whenever the contents of the screen area are changed. In the preferred embodiment, the Priority List contains forty seven entities which indicate, in priority order, the locations in RAM  217  at which data blocks are stored. Once a data block is stored in RAM  217 , its location is never altered, but the data block may be updated with new information or replaced entirely, as will be explained. 
   Referring again to  FIG. 3 , each of the data types stored in RAM  217  is assigned a relative priority which is reflected in the organization of the Priority List. Header data is of the highest priority. Next is communications data followed by screen data. 
   Screen data may be any one of three types—“Special”, “Feature Start” and “Other”, and is further prioritized according to “type”. Screen data is also prioritized within each type according to an assigned priority number in the range from 0 to 7, with 0 being the highest priority and 7 being the lowest priority. 
   A Special screen is of higher priority than either of the other two screen types, and follows communications data in the priority order. Special screens include, for example, a screen associated with “Power Up”, a “Stand By” screen, and various other screens which are critical to the operation of the user terminal. 
   A Feature Start screen is next in the priority order. The Feature Start screen is the first screen of each feature, e.g., room service, in-room checkout, to be displayed to the user when the feature is selected. A channel map stored in RAM  217  correlates key pad entered channel numbers with either an EIA channel frequency to which the television receiver is to be tuned or with a unique screen number. The channel map thus correlates each of the Feature Start screens with a predetermined user entered channel number so that the Feature Start screen of a particular feature is displayed whenever the user enters the predetermined channel number via key pad  217  ( FIG. 2 ). For example, in  FIG. 6 , channel  20 , entered via key pad  217 , is associated with Feature Start screen  490  of the in-room checkout feature. Other channel keyboard entries activate the terminal to tune to an EIA (Electronic Industries Association) channel frequency for receiving a color television signal. 
   The third screen type is “Other”, and “Other” screens rank last in the priority order. These screens are, thus, the first to be analyzed for possible replacement to make room for other screens of higher priority or associated with the feature currently being viewed by the user, as will be explained. Once the “Other” screen area is exhausted, i.e., when no “Other” screen remain in memory which can be removed to make room for incoming screens, the other data types, i.e., Feature Start screens, Special screens, communications data, and header data, are also analyzed in reverse priority order, for possible deletion. 
   Focusing more specifically on the screen data, the data structure of the character screen commands stored in RAM  217  is shown in  FIG. 4 . Briefly, as shown in the figure, the screen data consist of a screen number in the range of 128-1023 which is used, as explained above, to uniquely identify the particular screen. Also associated with each screen is a priority number in the range from 0-7, with 0 being the highest priority and 7 being the lowest priority; a cannibal bit, which aids in swapping screen commands in and out of memory, as will be explained; a type field which indicates the screen type, i.e., Special, Feature Start, or Other; a channel number indicating the channel of any background video or audio associated with the screen; an audio field for indicating whether background audio is being used; a feature number field for identifying the feature scenario, i.e., room service, in-room checkout, etc., of which the screen is a part; a time out field for specifying the maximum viewing time of the screen; a key template map for associating a particular key of the key pad with a particular operation to be performed by processor  206 ; a text field which contains the actual text to be outputted on the television receiver; and an attribute field for defining the screen display characteristics, e.g., the color and intensity of the background, the size of the characters, the character blink rate and the like. 
   The key template structure is shown in  FIG. 5 , and specifies for a particular key of the key pad the action to be taken, e.g., prefetch a screen, and/or the screen or channel, i.e., the go-to screen, to be displayed or tuned, respectively at the user terminal when the key is depressed. In the preferred embodiment, key pad  216  is a hand-held battery operated unit and comprises a total of 18 keys which may be programmably defined for each display screen to cause a particular action to be performed and/or to cause a go-to screen to be displayed. The effect of the go-to screen of the key template is to create a decision tree or hierarchical screen structure, in which each character screen command contains one or more pointers from which one or more other screens may be accessed. 
   An example of the key template is shown in  FIG. 6  for the in-room checkout feature. As shown in the figure, when the checkout feature is initiated by entering channel  20  on key pad  217 , screen  490 , which is the Feature Start screen, appears. Screen  490  includes a number of instructions which direct the user to depress the “1” key of key pad  216  to review charges; the “2” key to check out; and the “No” key to exit the feature. A key template is also associated with screen  490 , and correlates each of keys  1 ,  2  and “No”, i.e., the expected user responses, with particular actions. Thus, in the example, key  1  causes screen  491  to be displayed; key  2  causes screen  501  to be displayed; and “No” causes the video to be brought up. 
   In addition to directing action at the user terminal, a key template is also provided, as shown in  FIG. 6 , to direct certain processing at the system manager location in accordance with the user depressed key. Thus, in the example, depression of key  1  in screen  490  causes the system manager to process the user request and to transfer room charges to the user terminal for display on screen  491 . Depression of key  2  causes the system manager to process the total charges for the room, which are displayed on screen  501  and depression of key  3  causes no action at the system manager. 
   Key templates for the user terminal and system manager are similarly associated with each of the expected key responses of screens  491  and  501 , as shown in  FIG. 6 . 
   There are two basic types of features and, thus, two different types of decision tree structures shown in  FIGS. 7   a  and  7   b . In the “fan-out” type feature ( FIG. 7   a ), the key templates, associated with the screens of the feature, direct the flow of screens so that one of a number of screens located one level below the current screen in the tree structure is displayed responsive to user selection. Once a screen associated with a particular branch of the tree is selected, other screens associated with unselected branches will not be accessed and can, therefore, be considered for deletion, as will be explained. Thus, for example, as shown in  FIG. 7   a , a user viewing screen  60  may directly access any one of screens  71 - 77 , however, once a particular screen, for example, screen  71 , is selected, none of screen  72 - 77  will thereafter be used. Screens  72 - 77  can, therefore, be replaced, as needed, by other screens, for example, screens (not shown) following screen  71  in the tree structure, which are likely to be requested by the user. 
   In contrast to the fan-out feature, in the serial chain type feature ( FIG. 7   b ), the key templates of the associated screens direct the flow of screens so that the user either sequentially progresses from one screen to the next in the chain or skips back and forth along the chain. The in-room checkout feature shown in  FIG. 6  is an example of a serial chain type feature. 
   Besides the key template, a cannibal bit, shown in  FIG. 4 , is also associated with each screen and is used in controlling the storage of screen data in RAM  217 . The cannibal bit has a value of either 0 or 1. The normal state of the cannibal bit is 1, but the cannibal bit is configured as 0 in some screen commands to ensure that the screen is retained in memory. This is particularly useful in dealing with features of the serial chain type. As will be explained, when it is desired to store a screen command in RAM  217  and the screen data area is full, processor  206  will first try to remove screens of type “Other” which belong to a feature other than the feature currently selected. However, if only screens of the current feature exist in memory, processor  206  will scan the list of “go-to” screens for the screen currently being viewed and will delete any screen that is not a direct “go-to.” This has the effect of deleting screens associated with unselected branches of the decision tree and is effective in fan-out type features to remove unneeded screens. If the current feature is a serial chain type feature, it is desirable to maintain all or most of the screens of the feature in terminal memory. But because many of these screens are not directly accessible, i.e., will not be “go-to&#39;s” of other screens, some screens may be deleted. To avoid this situation, the cannibal bit is configured as “0”, which signals processor  206 , as will be explained, to retain screens of the same feature in terminal memory without regard to the direct “go-to&#39;s.” 
   As will be easily understood, the speed at which the system responds to a user request will largely depend upon whether a particular screen is stored locally or whether there is a need to access the central data base at the system manager location. Storage of all screens locally is impractical because this would require too much local storage. However, access is significantly improved by appropriately configuring the local store so that at any given time, the screens most likely to be requested are available in the terminal memory ready for display. 
   As explained above, when a user desires to activate a particular feature, e.g., in-room checkout, he first enters a predetermined channel number via the terminal keyboard associated with the feature in the channel map. This then causes the first screen of the selected feature to be displayed on the television receiver. The user may then follow the instructions on the screen to proceed through a sequence of screens associated with the selected feature. 
   In operation, whenever a user initiates a feature or requests a particular service by depressing a key of the key pad, processor  206  first determines whether the requested screen, i.e., either the Feature Start screen or the “go to” screen associated with the depressed key, is present in RAM  217 . If it is, then the screen is accessed for display. If the screen is not present in RAM  217 , processor  206  issues a screen request to the system manager which responds by issuing an “Add screen” command to processor  206  for the requested screen. 
   Aside from requesting a screen from the system manager when the processor determines that a screen requested by a user does not currently reside in memory, processor  206  may also prefetch screens into memory prior to their actual use. This prefetch function is tied to user viewing so that when a user views a screen, screens directly accessible by it, as defined by key template go-to&#39;s, and not already stored in memory, will be requested. It is also desirable once a user request has been completely serviced and the user is no longer waiting for a screen, e.g., because he is watching television or because he has left the room, to reconfigure terminal memory to an initialized state. Processor  206  will, thus, check a Special screen list maintained in RAM  217  to determine whether there are Special screens which are not already stored in memory, and if there are, these screens will be requested. The channel map will also be searched to identify the screen numbers of all Feature Start screens, a comparison will be made with those screens existent in terminal memory, and any Feature Start screens not present in memory will also be requested. 
   The basic mechanism for adding screens to RAM  217  is through the “Add screen” command and its companion “Conditional add screen” command. The “Add screen” command is issued by the system manager to processor  206  responsive to a screen request by the user terminal or when the system manager has a new screen to add to the configuration. The “Conditional add” is issued when the system manager wishes to address a screen globally to all of the interactive terminals in the system, and may also be used to address a screen to a particular terminal. 
   In operation, when the “Add screen” command is issued, processor  206  determines whether vacant space is available in RAM  217  for storing the incoming screen. If space is available, the new screen is stored in the first empty location, and the Priority List is updated so that the memory address of the new screen is appropriately positioned in the list in accordance with the relative priority of the screen. Otherwise, if the RAM is full, it is necessary to remove screen data from RAM  217 , and the algorithm described below is used for this purpose. 
   In general, the removal method will depend upon the current screen i.e., the screen currently being displayed, and the feature with which it is associated. Thus, all “Other” screens, beginning with the screen having the lowest priority as reflected by the Priority List, are first analyzed for possible deletion by checking whether a particular candidate screen is associated with the current feature. Once a screen is found which is not part of the current feature scenario, that screen is replaced by the new screen. If the user is currently watching television, i.e., is not currently requesting any system service, there is no current feature, so that processor  206  will replace the first “Other” screen in the Priority List, i.e., the “Other” screen with the lowest priority, with the new screen. 
   If the user is currently viewing a screen and all of the “Other” screens in RAM  217  are associated with the current feature, processor  206  examines the cannibal bit of the incoming screen. If the cannibal bit is 1, processor  206  searches the “Other” screens, again beginning with the screen having the lowest priority, to locate screens not directly accessible from the current screen, i.e., screens that are not direct “go-to&#39;s” of the current screen. If a non-go-to screen is located, that screen is replaced by the incoming screen. 
   If all of the “Other” screens are “go-to” screens of the current screen, or if the cannibal bit is 0, processor  206  will then search the “Feature Start” screens and will choose the screen lowest in priority for removal, except that processor  206  will not remove any screen with a 0 priority and also will not remove the “Feature start” screen of the current feature. 
   If no “Feature start” screen is found which can be removed, a similar search is made of the Special screens, and the Special screen having the lowest priority will be removed. Again, no screens of 0 priority will be removed. 
   If no suitable screen can be found for removal, another search is performed depending upon whether the incoming screen was requested by the user terminal. This will generally mean that the screen is essential for continued operation. Thus, if the new screen is the result of a screen request, processor  206  will replace the lowest priority screen in memory, disregarding screens with a 0 priority, with the new screen. 
   The other mechanism for adding screens to RAM  217  is the “Conditional add screen” command which allows the system manager to address a screen globally to all of the interactive terminals in the system or to a particular terminal. The add command is “conditional” in that each terminal determines whether or not it will accept the new screen and, if the screen is accepted, will make room for the screen in accordance with the algorithm described above. 
   In determining whether a particular screen will be accepted, processor  206  first determines whether the screen corresponds to, i.e., has a same screen number as, a screen currently residing in memory. If it does, then the old screen is replaced with the new screen data. 
   If the user is watching television, i.e., is not requesting a system service, at the time the “Conditional add” command is issued, processor  206  will also accept and store all “Feature start” screens and all “Special” screens. This ensures that those screens critical to system operation as well as those screens likely to be needed and requested first are available for immediate display. 
   If, on the other hand, the user is viewing a screen at the time the “Conditional add” command is issued, the criteria for accepting the incoming screen are different. In particular, if the terminal is currently waiting for this particular screen, the screen will be accepted. Otherwise, “Special” screens and “Feature start” screens with a priority of 0 will be accepted, and “Other” screens will be accepted only if the new screen has the same feature number as the screen currently being viewed and is either a direct “go-to” of the current screen or has a cannibal bit of 0. 
   This invention has been described in detail in connection with the preferred embodiment. However, it will be easily understood by those skilled in the art that other variations and modifications can be made within the scope of the invention as defined by the appended claims.