Method and apparatus for assisting user of information retrieval systems4754326Abstract In information retrieval system includes a help feature, wherein a help key on a terminal can be actuated and a host computer responds by determining the identity of a page of basic information being transmitted at the time the help key is operated, determining the location of a page of subsidiary information associated with the currently transmitted page of basic information, and transmitting that associated page of subsidiary information to the terminal for display thereby. A system guide aspect enables the system to provide guide information adapted to assist in the use of other data subgroups. A bookmark facility involves the use of a mark request key on the terminal and a mark record device operable in response to the actuation of the mark request key to create a record of the storage location of the one of the pages being transmitted to the terminal for display thereby at the time of such actuation. A search request key on the terminal enable transmission of a search request to the host computer. Claims We claim: Description CROSS-REFERENCES TO RELATED APPLICATIONS
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Subsystem Integrated circuit
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Modem 70A TCM3101
Tone dialer 70B MK5089
ACIA 72, 74, 92 SY6551
Video control logic 94
74123, LM1889
Video memory RAM 98 4416-2
Data Buffer 99 HFE4052B
Address Buffer 100 HC153
Color Map RAM 101 2148
CRT Controller 103 SY6545-1
Multiplexer 105 HC166, HC153
Data Buffer 107 HFE 4052B
Color composite video output 108
LM1886
Data buffer 112 HFE4052B
Non-volatile EEROM memory 114
SY2802E
PLPS ROM 116 SY23256A
PLPS "Scratch" RAM 118
4416-2
Telesoftware RAM 121 4416-2
Multi-tasking executive ROM 123
SY2365A
Session and link ROM 125
SY2365A
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It should be understood that one skilled in the art may employ a plurality of each of the above-identified integrated circuits to serve the appropriate terminal subsystem, in order to provide the desired suitable capacity therefor. For example, a plurality of the ROM and RAM circuits are employed in an implemented embodiment of the system 10, to provide the desired memory storage capacity. III. Keyboard Referring now to FIG. 3, there is shown a membrane panel 128 of the keyboard 85 (FIG. 2). In the upper left hand corner of the panel 128, a set of user command keys are provided and are generally indicated at 130. These keys comprise a CALL key 132, a HANG UP key 134, a LOCK key 136 and a SURPRISE key 138. These user command keys 130 initiate and terminate the use of the terminal 20. At the central upper portion of the panel 128, there is disposed a set of service keys generally indicated at 141 for enabling the user to initiate service functions as identifed on the keycaps. The initiation occurs by a single keypress. The service keys 141 comprise a BANK key 143, a SHOP key 145, a RESERVE key 147, an INFORM key 149, a MAIL key 151 and a SPECIAL key 153. By pressing any one of the service keys, such, for example, as the BANK key 143, the communication path P is established automaticallly from the terminal 20 through the host computer 14 to the desired bank service computer 24. After pressing the BANK key, the user is enabled to perform banking transactions. In the upper right hand portion of the panel 128, there is a row of aid keys generally indicated at 155, and below it, a row of work keys generally indicated at 157. The Aid keys comprise a BROWSE key 159, a MARK key 162, an INDEX key 164, a GUIDE key 166, and a HELP key 168. The work keys comprise a SEEK key 170, a NEXT key 172, a BACk key 174, a REPEAT key 176 and a cancel key 178. A full QWERTY set of keys are generally indicated at 181 and are located at the central portion of the panel 128. To the right thereof, a set of function keys are generally indicated at 183, are each labelled 0-9, together with "*" and a pound sign. They are arranged in four rows and three columns. An ACTION key 185, disposed to the right of the function keys 183, is used to enter data into the terminal 20. To the left of the QWERTY keys 181, there are disposed a set of cursor control keys 187, which enable the movement of the cursor to be controlled by the user. Disposed therebelow, are a pair of keys labelled CLEAR and ADVANCE, indicated respectively at 189 and 191. IV. General Operation Considering now the operation of the system 10 with reference to FIGS. 1-3, the system 10 is initially activated by pushing a power-on switch (not shown) for the terminal. Assuming now for example that the user wishes to perform a banking function, the user pushes the CALL key 132 for the terminal 20. This action causes the modem-dialed 70 to send a telephone number to the central office 45 via the telephone line 62. As a result, a telephone connection , such as the communication path P, is established between the terminal 20 and the host computer 14. By extending this connection to the host computer, it recognizes the connection and establishes an identification sequence. In this regard, the host computer 14 sends a message to the terminal 20 in the form of a session inquiry. The terminal 20 receives the session inquiry message, and then returns a message to the host computer 14 to establish the identity of the terminal. The message includes the model number, serial number, and the manufacturer's identification of the terminal. The manufacturer's identification number and the model number determine what type of terminal is requesting service. In this regard, the call could be coming from any one of a large number of the terminals. The manufacturer's identification indicates that the terminals 20 is one of the terminal associated with the videotex office 12, as contrasted to independent personal computers (not shown) requesting service of the videotex office 12. If no user password is used, the identity of the user is then associated with the particular terminal serial number when the host computer 14 performs a table lookup sequence. In this manner, the user is then logged on to the host computer 14. The user then presses the BANK key 143 (FIG. 3). This action causes the host computer 14 to receive a message from the terminal 20 and an index page is then selected. The information indicative of the index page is then sent to the terminal 20, which in turn displays on the television receiver 83 a menu or index page of banks for selection by the user. The user then pushes one of the number keys of the function keys 183 to select one of the banks identified on the index. Thereafter, the user presses the ACTION key 185 for enabling transmission of the selected number key information to the host computer. This information is transferred to the host computer 14, which in turn, establishes the selected gateway connection 22 from the host computer to the selected bank computer. The gateway connection 22 is the connection extending between the host computer 14 and the bank service computer 24. Alternatively, without the need for the use of a menu page, if desired, the host computer 14 could establish a gateway connection to a particular bank when the user simultaneously pushes the BANK key 143 and the SHIFT key, to in turn connect the host computer directly to the selected bank computer. In this regard, instead of using the index page selection technique, the host computer 14 could also establish the connection to the desired one of banks in which the user has an account. The user presses the number key; e.g. "2 or the bank name", the SHIFT key, then BANK to identify the desired one of the banks to which the user's terminal is to be connected. Once the gateway connection is established to the bank computer, the host computer 14 maintains that connection. In order to perform a disconnect function for disconnecting the gateway connection 22, the bank computer 24 sends information for the terminal to graphically present an index page, offering several selections, one of which would be a QUIT function. Once the user selects the QUIT function to indicate that the banking function is complete, the bank computer 24 sends a message to the host computer 14 for establishing a disconnect cycle of operation. As a result, the host computer 14 then disconnects the gateway connection 22. Alternatively, the gateway connection can be disconnected when the user pushes another service key, for example, the SHOP key 145. When this is done, the host computer 14 disconnects automatically, the bank gateway connection 22, and then commences the cycle of operation for the new service function. Additionally, the gateway connection between the terminal 20 and the host computer 14 is disconnected when the HANG-Up key 134 is pressed. This action immediately causes a disconnect cycle of operation to occur, and the user is not required to perform any additional operations. When the HANG-UP key 134 is pressed, a message is sent from the terminal to the host computer 14 to inform it that a disconnect cycle of operation has been selected. Simultaneously, the modem-dialer 70 in the terminal 20 responds to the pressing of the HANG-UP key 134 for disconnecting the telephone connection to the host computer 14. At the same time, the host computer performs a log-off procedure, which includes posting, disconnecting the gateway, if any, and disconnecting the host computer from the telephone connection with the terminal. In general, whenever a key is pressed, two characters of information are generated by the terminal and are sent over the telephone line to the host computer. The first character is an ASCII character code. For example, the code may indicate a lower case "n". The second character is an end of text (EOT) character to indicate to the host computer to enter and to commence the function. The activation characters are treated differently from the regular characters. In this regard, in connection with the activation characters, where there is no naplps protected field, all information is immediately sent to the host computer 14. The CALL key 132 does not generate two characters for the host computer, instead it generates a signal, which is used by the terminal modem. In this regard, the CALL signal initiates a cycle of operation by the modem-dialer 70, which operation includes dialing the telephone number to the closest available multiplex office for establishing the connection therethrough to the host computer 14. It should be noted that the CALL key 132 establishes an automatic dialing function, and the HANG-UP key 134 generates an automatic disconnect cycle of operation. The HANG-UP signal is in the form of two characters which are transmitted from the terminal to the host computer 14. Considering now the LOCK key 136, the LOCK feature enables the user to prevent unauthorized use of the terminal. The LOCK key feature prevents access by unauthorized users to the host computer 14 for utilization of the system 10, by pressing the LOCK key 136, and by subsequently entering a lock password by means of the keys 181 of the keyboard 128. In this regard, the terminal can be temporarily locked to deny access to the system, in the event that the user decides to leave the terminal and go elsewhere for a short period of time. Upon returning, the user can readily unlock the terminal. While the terminal is locked, another person can automatically log-on the host computer 14 by pressing the CALL key 132. However, in so doing, a page is displayed on the television receiver 83, indicating that the terminal is locked and requests the current user to decide whether or not the terminal should be unlocked. If the present user is unable to enter the correct lock password, the host computer 14 disconnects automatically. In operation, in order to lock the terminal, the LOCK key 136 is pressed to generate a two character code, which is sent to the host computer 14. The host computer, in turn, causes the terminal to display a page on the television receiver 83 requesting the user to type in a lock password, and then type it in a second time. The host computer then compares both sets of information. If the two lock passwords do compare and are thus identical, the terminal is locked. The reason for requesting the lock password to be entered twice is that it is not desirable to display the password for unauthorized persons to see. By typing it twice, the host computer then can ascertain whether or not the lock password was correctly entered. Additionally, there is then no need for displaying the lock password to the user to enable the user to determine whether or not the lock password was correctly entered. If the two lock passwords do not compare, or if the user presses another key, the lock program executed by the host computer is terminated. In order to unlock the terminal, the user pushes the CALL key 132, and a page is then displayed on the television receiver 83, indicating that the user has three options. The first option is to hang up by pressing the HANG UP key 134, to initiate a disconnect sequence of operation. The second option is to unlock the terminal, by entering the proper password and user identity information, which is given to the authorized user when first acquiring the terminal. The third option is to enter a different user identification number, in the event that the current user is not the user who locked the terminal. The unlocking procedure commences by a page being displayed, which requests the lock password to be entered. The lock password is then entered once, and if correctly done, the host computer 14 then displays the last page, which was previously displayed before the terminal was locked. The host computer then changes the state to an unlocked condition. In such a condition, the user is free to access the host computer by the easy access method associated with merely pressing the CALL key 132. The third option of using a different user identity information, is used where a different person, other than the subscriber, desires to use the terminal. By selecting the third option, the new user types in a different user identification number, and then his unique password. By logging-on in this manner, the new user is then charged for the videotex service, similar to a credit card telephone call. If the user password is incorrectly entered on three consecutive tries, the host computer 14 causes a special page to be displayed on the screen, indicating that the person currently operating the terminal 20 is not an authorized user, and is attempting to use the system 10 in an unauthorized manner. The host computer 14 can then disconnect from the terminal 20 in such a case. It should be noted that the terminal is not completely locked, in that the security log-on feature is available to any user. In the easy mode, the CALL key 132 is used for automatic log-on. For the secure mode, both the user password, if any, and the user identification information, must be entered, before the user has access to the host computer. When the initial call is made from the terminal 20 to the host computer 14, the terminal identification information is supplied to the host computer, which in turn stores the information in a table to enable the host computer 14 to determine whether a particular user has chosen either maximum convenience or maximum security. Therefore, the terminal identification information is permanently stored in the host computer 14 However, the lock password is changed each time, and after it is used, the host computer 14 discards the information. Should the user decide to press another key, such as the SHOP key 145 or other service key 141, during the locking procedure, the lock program in the host computer 14 is prevented from executing or is "killed". Moreover, the program control in the host computer 14 is automatically transferred to the selected service program. Considering now the other service functions, each time a service key is pressed, two characters of information (the key code) and the end of text (EOT) characters are sent to the host computer 14. Thereafter, the host computer 14 matches the key code with a table to fetch a page from the host computer memory (not shown). Information is then transferred from the host computer 14 to the terminal 20 to cause it to display the page on the television receiver 83. Considering now the MAIL, SURPRISE and LOCK keys and their associated functions, when each one of there three functions are initiated, a page of text material is transferred from the host computer 14 to the terminal 20. A program associated with the page is then given control by the host computer 14. The user then is enabled to use the program and its facilities. Pressing the SURPRISE key 138 initiates the surprise feature, which enables the receiving of a randomly generated message. In this regard, a surprise page s transferred from the host computer 14 to the terminal 20. A program associated with the page is then given control by the host computer 14. The surprise page is displayed on the television receiver, once the SURPRISE key 138 is pressed. For example, the screen may display the "joke of the day". The page of information displayed on the screen would also ask the user if the user would like to see the joke of yesterday. If so, the SURPRISE key 138 is pressed a second time. By pressing it a third time, the joke of the day before is displayed. Therefore, the sequencing of the additional displays is under the surprise program control in the host computer 14. In this manner, the surprise message can be changed in accordance with the surprise program control. The surprise program, therefore, can, if desired, be a randomly generated form of information, and can even be a random generation of numbers for display to create a humorous nonsense message. Additionally, if desired, the surprise message can indicate that a special mail message is available by pressing the MAIL key 151. Alternatively, the surprise program can provide a special message associated with the previous page of text displayed on the television screen. In this regard, for example, if the user had previously been performing a SHOP key function, the surprise program would cause a page to be retrieved from the host computer 14 concerning shopping information, such as special items on sale. The surprise program counts the number of times the SURPRISE key is pressed, so that a different page of text can be displayed each time the SURPRISE key is pressed. When the MAIL key 151 is pressed, the host computer 14 sends a page of text to the terminal 20, and the page of text is displayed on the television receiver. The CANCEL key 178, when pressed, interrupts the execution of a program including the flushing of all enroute page transmissions, or terminate page reception. In this manner, the user can make corrections. The ACTION key 185 causes the generation of a two character message which is sent to the host computer 14 indicating that the host computer is to respond to the previously sent information. The HELP key 168, when pressed, causes the transferring of a page of helpful information from the host computer 14 to the terminal 20, or so that the terminal 20 can cause the display of the last page. The helpful page is associated with the previously displayed page of text and contains explanations about the information contained in the previous page. For example, if the previous page requested the subscriber to enter the "SSN" information, and the user does not know what the term "SSN" means, the user presses the HELP key to display the help page. The help page then defines the terms on the previous page and explains that "SSN" means "social security number". In this manner, a user is then informed as to the meaning of the words or requests, previously contained in the last page of information. The user then merely presses the HELP key 168 again, and the last page of text is returned to the television screen. In the system of the present invention, each page of text has an associated help page. However, in certain types of pages, for example, menu or index pages, a generic help page is displayed to inform the user how to interact with the terminal 20 in regard to the last page of text displayed. In order to return to the last page of text displayed, the HELP key 168 is pressed. Alternatively, the BACK key 174 is pushed, and the preceeding page of text is then displayed. When the GUIDE key 166 is pushed, the subscriber is enabled to retrieve a display of information, which display of information is useful to render user guidance in facilitating the operation of the videotex system 10. For example, the guide page is used to explain how to use the terminal 20. The guide page varies for different users. Each user may have a different manner of using his or her terminal. For example, one user may have the convenient mode of operation, whereas another user may have the maximum security mode of access. The guide pages are, therefore, associated with individual types of terminals, and terminal users. In operation, when the user presses the GUIDE key 166, a two character message is sent to the host computer 14, which in turn matches the key code signal in a table to determine a page of information to be displayed. The host computer 14 then returns a message to the terminal 20 and instructs the terminal 20 to display the guide pages. Thereafter, once the user has completed the use of the guide pages, the user then presses either the NEXT key 172, BACK key 174 or the GUIDE key 166. In this regard, the succeeding, preceeding or the page which was displayed immediately prior to pressing the GUIDE key, respectively is then displayed on the television screen. The INDEX key 164, when pressed, causes the last index page to be displayed. The MARK function is started by pressing the MARK key 162. The mark function is similar to a bookmark, in that the user can later retrieve the same page of information as previously viewed on the television screen. In order to mark a page, a coined name, such as "WEATHER" is typed into the terminal 20 for identifying the page. This is a reference operation. Once the name is typed in, the MARK key is pressed. These actions cause the host computer 14 to store the name in the user's personal table in the host computer memory (not shown). The page address information is stored next to the stored name information to provide an indirect reference. In the system 10, only ten pages may be stored, and if an eleventh page is attempted to be stored, a message is displayed on the television screen requesting the user to delete one of the previous pages. A MARK key 162 may be used to bookmark a page, it should be understood that the marked page is not stored in the host memory, but instead, a mark is stored in an appropriate table indicating which page is to be called up at a later date, upon request. In this manner, the desired page may be updated at a later date. In this manner, when the user requests the marked page, the page which is displayed on the television screen is the updated page. Moreover, it should be noted that in order to access a particular service, such as a weather report, the user presses the INFORM key 149, to cause the display of a weather index or menu. The user must then select a particular city to determine the present weather report. If the user has previously marked such a page, all of these operations can be eliminated by merely typing in the word "WEATHER" and pushing the MARK key 162. The weather report for the selected city is then displayed on the television screen. When a question mark key 201 and then the MARK key 162 is pressed, a question mark page is displayed on the screen to illustrate a list of all the names of marks previously stored. In this manner, should the user forget which mark names are stored, the user merely presses the question mark key 201 and MARK key 162 to display the question mark page to refresh the memory of the user. When the BROWSE key 159 is pressed, a series of pages are sequentially displayed concerning a particular subject. In this regard, since the pages for the system 10 are arranged in a tree configuration, each level of the tree can be sequentially displayed by use of the BROWSE key. Once this feature is activated, the pages may be "turned" each time the BROWSE key is pressed. Alternatively, the pages can be turned automatically at a predetermined rate, such as at a rate of once every thirty seconds, or at a desired subscriber rate. The automatic browse feature is initiated by first pressing a rate key such as the S key 203 and then the BROWSE key 159. The pressing of the S key 203, indicates the word "slow" for speed, and a timer rate may also be stated, such as "5" then browse to turn pages at a 5 second rate. Once the BROWSE key 159 is pressed, each one of the index pages for each one of the services is displayed sequentially. Once the desired index page is displayed, such as, for example, the bank index page, the user can then cause the display of additional index pages by making an appropriate selection. The choice made by the user causes an index page to be displayed at a lower level of the tree of pages. As an example, the user may select a shop index, and then decide that the user is not interested in shopping and would like to review the other possible services, which are available. The BROWSE key 159 is then pressed and then the other shop index pages are displayed sequentially. The bank index is then displayed, followed by the information index and so on. The entire number of indices are displayed sequentially, each time the BROWSE key 159 is pressed. Once the user decides to proceed with any given service, such as the bank service, the user then makes a selection as indicated on the bank index page to decide which bank is to be selected. For example, the First National Bank may be selected, and if so, the First National Bank index page would then be displayed. Assuming that the user decides that another bank would be more suitable after the First National Bank index page is displayed, the BROWSE key 159 can then be pressed by the user, and the next bank index page would be displayed. Thereafter, each and every bank index page could be displayed, by repeatedly pressing the BROWSE key 159. In the videotex system 10 there are several index pages which are relational pages. In this regard, unlike an ordinary index page where a choice or selection made by the user results in the display of a single page, a relational index page enables a series of pages to be displayed on the screen, each one of which is related to the original choice made by the user. Once the user enters his or her choice, that choice is communicated to the host computer 14 and stored in appropriate memory therein. Thereafter, an appropriate page is displayed on the television screen. If that page is not acceptable to the user, or alternatively, if the user would like to see additional pages related to that same choice or selection, the user presses the NEXT key 172, and another page bearing information concerning the same interest as expressed in the intial choice, is then displayed. This process is repeated as long as there is a link to another page having the same interest or choice designation as originally stored. After the last relational index page is displayed, a page is displayed, indicating that there are no additional pages relating to the user's chosen interest. By pressing the NEXT key 172, the original relational index page is displayed. In this manner, the user can then choose another interest to gain access to all pages relating to that interest. By pressing the REPEAT key 176, the same page is then retransmitted displayed by the television screen. In this regard, the REPEAT key is pressed, whenever it is desirable to have a better copy of the page displayed. This is accomplished by causing the host computer 14 to send the same page to the terminal 20 so that another copy of the same page can be displayed. The service keys 141 comprise the BANK 143, the SHOP key 145, the RESERVE key 147, the INFORM key 149, the MAIL key 151 and the SPECIAL key 153. The BANK key 143 enables a gateway to be established to the bank computer such as the service computer 24, for enabling the user to perform banking functions. Similarly, the SHOP key 145 can be pressed by the user to enable a gateway to be established to a particular shop computer. In this regard, the user is able to do his or her shopping by communicating directly with the store's computer. The RESERVE key can be pressed to cause information to be displayed on the television screen concerning the making of reservations for airplanes, theatrical events, or the like. By pressing the INFORM key, news, sports or other information can be displayed on the television screen. In this regard, there is no limit on the number of pages of information that can be displayed. The SPECIAL key is used to initiate a display of information of a miscellaneous nature. In this regard, private data bases may be accessed for additional charges. For example, up-to-the-minute stock reports cou1d be presented for an additional monthly charge. The ADVANCE key 191 is used to cause the user input cursor to move to the next NAPLPS unprotected field displayed on the television screen. By pressing the SHIFT key simultaneously with the ADVANCE key, the cursor moves to the previous field as displayed on the television screen. The function keys 183 can either be used alone, or can be used in combination with the NAPLPS macro information loaded into the terminal memory for each key. When the SHIFT key is used, in combination with the functions keys, the associated NAPLPS macro is activated. When the CALL key 132 is touched followed by touching the ACTION key, an auto-dialing sequence is initiated for connection from terminal to the host computer 14. Alternatively, the user may touch the CALL key 132, then type in a phone number and press the ACTION key, which initiates dialing. Once connection is established with the videotex office, the log-on sequence is started, and a service inquiry is sent to the terminal. The terminal 20 responds by sending its internally stored ID number to the host computer. When pressed, the HANG UP key 134 causes the sending of a "disconnect" code to the host computer, invoking the log-off sequence. The time of log-off is stored for billing. The SEEK key 170, when preceded by a keyword, invokes the keyword search program. If there is only one page with the given keyword, the user is shown that page. If there is more than one, an index page is generated with the available choices displayed. This index page is not stored on the index or back stack. Touching the SEEK key with no preceding keyword causes the index page generated by the previous SEEK operation to be re-displayed. If no SEEK operation had been previously executed, no action occurs. The GUIDE key 166 displays the system guidebook--an explanation of the keys, functions, services and the system, in general. Presssing the key a second time causes the return of the user to the page where he or she was viewing prior to entering the GUIDE operation. Touching the NEXT key 172 causes the display of the next page of a sequence, following a link on the present page. If no link is defined, as may be the case on an index page (where there may only be choices), a message appears on line 20 of the screen, stating that NEXT is an invalid choice. If the link is defined, but the page does not exist, a message appears on line 20, and a message is sent to the production staff informing them of the missing page and of the page with the bad link. Each user's sequence is maintained in a stack of page addresses. When a user touched the BACK key 174, the page previously displayed is shown with the exception of service program pages, overlay pages and generated keyword index pages. The GUIDE and HELP functions have associated therewith, an individual corresponding separate stack for this purpose. The BACK key 174 can be used successively until the stack, which is 63 pages long, is depleted. At that time, the last page is re-displayed, along with an explanatory message on line 20. To the user, the BACK key is the easiest way to recover from an error of pressing the wrong key. Alphabetic characters, number 0-9 and punctuation marks are used for choices or data entry. They do not have associated functions. The normal appearance of the alpha keys 181 is lower case. As in a typewriter, shift, by either pressing the left shift key 182 or the right shift key 184, with a letter displays upper case, or the upper key cap symbol. When used for choices or page selection, upper and lower case letters are treated the same. To obtain the special functions F1-F10, 1-10 numeric keys 183 are shifted. The special functions F1-F10 are not yet specified. These function keys are directly related to the NAPLPS Macros M0-M9. The Macros are capable of transmitting information to the host computer 14 or causing a specific picture to be drawn on the screen. For example; answers to a quiz could be "hidden" from the user by sending them to the terminal using the same color for the text as the background color. Touching one of the special function keys can cause a macro to be executed which can change the color of the text and thus reveal the answers. V. Specific Operation V.(a) The help feature. When the user of this system is in the midst of a search, viewing a terminal video screen display of any information page in the system, touching the Help key on the terminal keyboard sends a message to the host computer which will cause that computer to cease transmitting the current information page and transmit instead a help page associated with the most recently displayed information page. Some information pages require help blocks more than one page in length, in which case the user can use the terminal Next and Back keys to move forward and backward respectively within such a help block. Every information page in the system has a help page or block of help pages associated therewith, although some information pages share a common help page or block of help pages with other pages to which that help page or block is also relevant. If a particular page is of a kind which requires specially tailored help instructions, then that page is likely to have a help page or help pages all its own, or to share such page or pages with a small number of other, similar pages. If a particular information page, on the other hand, is of a kind which does not call for any specially tailored help instructions, even such an information page has at least one help page assigned to it. This is accomplished by providing at least one "generic" help page for the entire system which contains help instructions so general in nature that they are applicable to any information page in the entire system, and by assigning such a page as the help page for each information page in the system which does not have a more specific help page associated therewith. Each help page is displayed on the terminal video screen along with an indication of the identity of the information page to which it relates. In the case of help pages which are associated with more than one information page, the particular information pages so identified is the one which was being transmitted by the host computer to the user's terminal at the time of the help request. In order to accomplish this, the host computer makes a record of the most recently transmitted information page during the time that a help page is being transmitted. This permits the host computer to do three things: to display the identify of the most recently displayed information page as part of the display of the help page; to return to the most recently displayed information page when the user is finished viewing the help page; and to keep a record of the frequency of help requests correlated with the locations of the associated information pages, so that the worst trouble spots can be identified and changes can be made to minimize the difficulties which users are having at those locations in the system. When the user is ready to return to the most recently displayed information page, all he or she need to do is touch the Help key a second time, and the host computer is programmed to respond by consulting its record of the identify of the last-displayed information page, and then to re-display the page so identified. Alternatively, the help page display is also terminated by touching various other keys, e.g. those which are associated with a direct page select, a service area, a keyword search, or hitting the Back key when the help page currently displayed is the first or only page in a help block. Specifically, if while a help page is being displayed the user touches the Hang-up, Lock, Surprise, Bank, Shop, Reserve, Inform, Mail, Special, Mark, Guide or Seek keys, then the help page will disappear from the screen and the particular service program which is associated with that particular key will be started. If the Browse key is touched, there will be an error message displayed on the terminal video screen. If the Index key is touched, the computer will terminate transmission of the current help page and transmit the previously displayed index page. If the Next key is touched, the next help page in the current block, if any, will be transmitted. If the Back key is touched, the previous help page in the current block, if any, will be displayed. If the Help key is touched, or if the Back key is touched and there is no previous help page, the help mode is exited and the last information page re-appears. The Repeat key always repeats the current page, in this case the current help page. The Cancel key aborts the last previous keystroke, if touched before anything irrevocable occurs. The Action key alone is interpreted as a request for the next help page, but if preceded by the typing of a direct select subject, it calls for execution of the direct select program. V.(b) The systemwide guide feature The help instructions offered by the help pages are related to the particular information contained in the information page which was being transmitted at the time the Help key was touched. In some instances, however, there may be a need for a general system overview rather than instructions narrowly focussed on a particular portion of the database. When the Guide key is pressed, the computer responds by transmitting a general system guide page. A second actuation of the Guide key returns the user to the previously displayed information page, just as in the case of the Help key as described above. The Guide page is the same for all areas of the system, and describes the operation of the system in response to all of the various terminal service keys, Bank, Shop, Reserve, Inform, Mail, Special, Surprise and Lock. Thus a user who wishes to step back from the trees and look at the entire forest for a moment, can use the Guide key to call up a system overview, and may as a result decide to switch to a different area of the system. Alternatively, the user has the option to return to the same specific area of the database by simply touching the Guide key again. If, while in the guide state, the user presses the Hang-up, Lock, Surprise, Bank, Shop, Reserve, Inform, Mail or Special key, the host computer takes the user's terminal out of the guide state and proceeds with normal execution of the relevant service program, starting with the main index or menu page for that program. If the Browse key is touched, the computer follows the Browse link for the previously displayed information page, as described in the above-identified related patent application entitled "Method and Apparatus for Retrieving Information Distributed Over Nonconsecutive Pages." If the Mark key is touched, the previously displayed information page is marked, as described below. If the Index key is touched, the appropriate guide index page is displayed; or if there is none the computer exits the guide state and displays the index page which is appropriate to the preceding state. If the Guide key is touched again, the computer exits the guide state and returns to the preceding information page. If the Help key is touched, an error message is transmitted. If the Seek key is touched, the computer exits the guide state and proceeds with normal execution of the associated program (see below). The Next key causes the computer to display the next guide page; and the Back key causes it to display the previous guide page unless there is none, in which case the keystroke is interpreted as a re-touching of the Guide key, causing the computer to exit the guide state and return to the preceding information page. The Repeat key re-displays the current guide page. The Cancel key aborts the previous request, if done in time. And the Action key alone displays the next guide page, but when accompanied by a direct select instruction it causes normal execution of the direct select program. V.(c) The mark feature. When the user of this system finds a page within the database which he or she wishes to mark for subsequent review, this invention provides a way of doing so, and a way of conveniently returning to that page even after the lapse of considerable time. The user first employs the alphanumeric keys of the terminal keyboard to type any desired name for the page and then touches the Mark key. The computer will then respond by creating a mark list, if none has been created previously, and entering the name and storage address of the marked page in that list. The marked page can later be retrieved in the following manner. The user types the page name on the terminal keyboard and then touches the Mark key. The host computer checks to see if the page name is already included in the mark list. If not, the computer assumes that the user is attempting to add a new marked page to the list, and proceeds accordingly (see above). If the page name is already there, however, the computer will find the page name on the list, look up the address of the relevant page, fetch that page from memory, and then transmit that page to the terminal for display on the video screen. If sufficient time has elapsed since that page was marked so that the user cannot remember the page name employed, then he or she is able to consult the mark list directly in order to refresh his or her memory by looking at the list of page names contained therein. To accomplish this, the user simply touches the "?" key followed by the Mark key. When the mark list has been directly accessed in this manner, the user is also permitted to delete any marks no longer required. There must of course be some upper limit on the number of marks which any one user's list can contain. In one particular system devised in accordance with this invention, for example, that limit is ten. If the user attempts to add an eleventh mark to the list, the computer displays a message asking the user to delete any one of the current marks. If, in the course of executing a mark program sequence, an unrelated key is touched, the following will occur. If the Hang-up, Lock, Surprise, Bank, Shop, Reserve, Inform, Mail, Special, Index, Guide, Seek or Back key is touched, the mark program sequence then in progress is aborted and the action ordinarily called for by the new key is executed instead. If the Browse or Next key is touched, an error message is displayed. If the Mark key is retouched, the mark program is restarted. If the Help key is pressed, a help page related to the mark program is displayed. The Repeat key causes the current mark page to be redisplayed. The Cancel key aborts the mark program and causes the previous information to be redisplayed. The Action key is interpreted in terms of whatever alphanumeric instruction preceded it. V.(d) The seek feature. Pages of information in the system have associated therewith respective lists of keywords, ranging in quantity from zero in some cases up to some maximum number such as ten in a typical system. In addition, each keyword of each information page has appended thereto a qualifier word. When a user types any such keyword on the terminal keyboard and touches the Seek key, the host computer responds by looking up the keyword in the list, and then generates an ad hoc index or menu page (or pages if more room is required) to suit the occasion. The index page (or first index page) so generated is then transmitted to the terminal for video display. The index displays the keyword in question, and also presents the user with a range of choices among all the information pages in the system which have that particular keyword associated therewith. In order to facilitate the user's choice among these alternatives, each of the options has displayed in conjunction therewith the related qualifying word. Suppose, for example, that the keyword is "Vacations" and suppose also that the host computer locates "Vacations" on its keyword list and finds that there are three information pages in the database which are related to that keyword. Moreover, one of the pages has associated with that keyword entry the qualifier word "Africa," a second one has the qualifier word "Italy," and the third has the qualifier word "England." The computer would then generate an ad hoc index page reading as follows: ##STR1## and it would transmit this page to the terminal for video display. The displayed index page indicates to the user that there are three vacation-related information pages to choose from, and that the choice can be made by pressing the appropriate one of the numeral keys 1, 2 or 3. This choice is facilitated by the further information that the first page relates to vacations in Africa, the second to vacations in Italy, and the third to vacations in England. The user then makes a selection and touches the appropriate numeral key, whereupon the host computer obtains the storage address of the chosen page from the keyword list, fetches that page, and transmits it to the terminal for display. Afterward, the ad hoc index page is not added to the standing inventory of system index pages, but neither is it immediately erased. Rather, it is stored in a temporary slot until the user performs the next keyword search using the seek key and a new keyword, at which time the preceding ad hoc index page is erased and replaced by the new ad hoc index page. But if, before that time, the user should touch the Seek key without first entering a new keyword, the computer assumes that he or she wishes to repeat the previous search using the same keyword as last time. Accordingly the still-stored ad hoc index page is fetched and re-displayed. Thus the user can return any number of times to the subject of current interest without re-entering the same keyword. It is only when the user's interest shifts to another keyword that the previous ad hoc index page is erased. If during a Seek program sequence the user touches the Hang-up, Lock, Surprise, Bank, Shop, Reserve, Inform, Mail, Special, Index, Guide, Help or Cancel key, the host computer will abort the Seek program and respond to the new key in the usual manner. If the Browse or Mark key is touched, an error message is displayed. If the Seek key is re-activated, the keyword search program starts over, with a new keyword if one was previously typed. If the Next key or the Action key is touched, then the next page of the ad hoc keyword index (if there is one) is displayed. If the Back key is touched, then the previous page of the ad hoc keyword index (if there is one) is displayed. If the Repeat key is touched the computer will re-display the current ad hoc index page. In one embodiment of the invention the qualifier words associated with a given keyword and a given information page are chosen from the other keywords associated with that same page. Thus, in the example given above, "Africa," "Italy," and "England" are keywords associated with the information pages designated 1, 2 and 3 respectively in the ad hoc index page shown above, along with the keyword "Vacations" which they all have in common. The keywords associated with any given information page in the database are preferably not stored as part of that page, since that would increase the page size and require one more disk access for various database maintenance activities. It is preferable to maintain a separate file in which the storage addresses of the information pages are linked to the keywords associated with those pages. Such a file is schematically illustrated at 350 in FIG. 3C. In addition, a sorted index of all the keywords for all the information pages is maintained for searching purposes, as schematically illustrated at 351. Both these files are serviced by an update program 352. When a keyword search is generated by means of the seek key, the host computer looks up the chosen keyword in file 351, and after finding it determines from file 351 the addresses at which the related information pages are stored. The computer then generates the ad hoc index page, as illustrated schematically at 353. That page is then transmitted to the user's terminal for video display in order to prompt a choice among all the information pages related to that keyword. In addition, the host computer associates with the index page 353 a list of storage addresses 354 correlated with the information pages referred to in the index page 353. After the user has made a choice among these information pages, the host computer consults the address list 354 to determine the storage address of the chosen information page, fetches it from that address, and transmits it to the user terminal for video display. It will now be appreciated that the user assistance features disclosed herein help to humanize the operation of videotex, teletext, and other information retrieval and computer service systems. V. Specific Operation V(a) The lock feature. A lock feature in accordance with the present invention is used to prevent unauthorized access to the system, and is preferably associated with a specific user identity so that it not only prevents unauthorized persons from gaining access altogether, but also prevents each authorized user from gaining access under any other user's identity. At the time a user wishes to use the lock feature, he or she is asked to enter into the terminal keyboard a one-to-six-character password which will later be used to unlock the system so far as that individual's identity is concerned. In order to prevent others from observing the password on the video screen as it is being entered, the system is programmed not to display the password characters on the video screen as they are typed in. But this runs the risk that the lack of visual confirmation might permit the inadvertent entry of the wrong password, which would be unknown even to the authorized user who typed it, and therefore would lock that user out of the system without any way to re-enter. Therefore the user is required to type the desired password twice, and only after both attempts have produced the same combination of characters does the host computer accept the password as a valid one on the basis of which to lock and unlock the system to the particular user. Thus, when the user touches the Lock key, the host computer transmits the following Lock Page to the terminal for video display: "LOCK PAGE "Enter the locking password in both blank boxes below. For your protection they will not be visible. ##STR2## "Touch Action when finished." If while this Lock Page is displayed, the user inappropriately touches one of the other service keys, one of the following will occur. If the key touched is the Hang Up, Lock, Surprise, Bank, Shop, Reserve, Inform, Mail, or Special key, then the Master Service Program will intercept the Lock Service Program, abort that program, and begin normal execution of the Service Program associated with the last-touched service key. The terminal will remain unlocked. Even touching the Lock key will leave the terminal unlocked, but will restart the Lock Service Program. In the key touched is the Browse or Mark key, an error message will be displayed. If the key touched is the Incex, Guide, Seek, Next or Back key, the Lock Service Program will be aborted and the particular program associated with the last service key touched will be executed. If the Repeat key is touched, the computer will redisplay the Lock Page (above). If the Cancel key is touched, the Lock Service Program is aborted and the Cancel Page is displayed. If the Action key is touched before the password is entered in both boxes, the computer treats that as the entry of an invalid password, and deals with it as explained below. If the Action key is touched after duplicate entry of the password, then the lock sequence is consummated. Normally, the user positions the cursor successively in each box depicted on the video screen, types in the same password in each place, and then touches the Action key. The two entries are then compared by the host computer to determine whether they are the same. If they are not, the user is asked to repeat the procedure until they match. When they do match, the password is accepted and that particular user identity becomes locked until subsequently unlocked. As soon as the lock sequence is consummated in this manner, the following Terminal Locked Page is transmitted to the terminal for video display: "Terminal Locked. Please choose what to do next a. Hang up b. Enter password to unlock c. Enter a new user ID" Thus, if the user leaves the terminal on, goes out of the room and then returns, he or she will find this page still displayed, and must choose one of these three alternatives in order to resume use of the system. Also, if the user should disconnect the telephone connection entirely and re-establish it at a later time, he or she will reach a point in the log-on sequence at which the same page will again be displayed, and it will then be necessary to choose one of the options so offered. If a. is chosen, the phone connection is terminated. This permits the user, if the break is going to be a long one, to disconnect from the system and stop the running of phone and user charges, yet upon the user's return he or she will find the system locked as to that individual's user identity upon re-establishing the telephone connection to the system. The log-on procedure will proceed no further than the re-display of the Unlock Page without the user of the password to unlock. If c. is chosen, only the current user identify is disconnected, and the host computer re-transmits the Log-on Page to the terminal for video display so that the same or another user can attempt to enter the system under another user identity. If another user comes along at this point, he or she can enter the system from the same terminal while that terminal is not occupied by the first user. If that second individual's user identity is unlocked, he or she can enter immediately and use the system. If the second individual's user identity is also locked, then that individual will also have to go through the unlock sequence in order to enter the system, but of course the password used will be the second individual's own password. When the second individual gives the terminal back to the first individual, the latter will find that the terminal is still locked to that individual's user identity notwithstanding the fact that it might not be locked to another individual. Moreover, an individual as to whose user identity the system is currently locked will find that even if he or she attempts to enter the system through an entirely different terminal that the host computer will still require a valid password to permit access to the system under that user identity. Thus the password concept is terminal-independent, and locks the entire user identity regardless of the physical terminal used or the geographical location from which access is sought. In some cases the user identity concept might be employed not to segregate different individuals, but to segregate different fields of activity by a single individual. Therefore, it is possible that such an individual might be authorized to enter the system under more than one user identity, and the two identities can be locked or unlocked independently of each other. If b. is chosen, then the computer transmits the following Unlock Page: "UNLOCK PAGE Please enter unlocking password ##STR3## Touch Action when finished." At this time the user must position the cursor in the box, type out the previously selected password and then touch the Action key in order to unlock the particular user identity. If the password is incorrect, the user can try again. On the third unsuccessful try, a help page is displayed. If the user does not succeed in unlocking the user identity even with the help of that page, for example if the password has been forgotten, then he or she must then put in a telephone call for human assistance. If, while the Unlock Page is displayed, the user touches an inappropriate service key, one of the following responses will occur: If the Hang Up, Lock, Surprise, Bank, Shop, Reserve, Inform, Mail, Special, Browse, Index, Guide, Help, Seek, Next or Back key is touched, the computer will send an error message to the terminal for video display. If the Cancel key is touched, the Unlock Page (above) will be re-displayed. If the Action key is touched before the correct password is entered, that will be treated as the entering of an invalid password and dealt with as described above. If the Action key is touched after the correct password is entered, then the user identity will be unlocked and normal use of the system can then re resumed. The program sequences described above are summarized in FIG. 3B. Start #1 is the entry point employed when the user is already in the system, and wishes to take a break, using the lock feature for security purposes. Pressing the Lock key (step 300) starts the lock program sequence. Then (step 301) the Lock Page is displayed and the program is primed to accept a proper password input and go on to step 302, or if any other event occurs, to abort the lock sequence and exit via arrow 303. Step 302 includes locking of the user identity, display of the Unlock Page, and storage of the selected password for subsequent comparison with the password which is presented when the user attempts to unlock. At this point the program exits to step 304. Step 304 immediately displays the Unlock Page, and constitutes the start of the unlock sequence at the end of a short break during which the telephone connection has not been disrupted. Start #2, on the other hand, is the entry point for a user returning from a long break during which the telephone connection has been broken, and wishing to resume use of the system. In the latter situation, the user starts with step 305, in which the Log-On Program is employed to re-establish the telephone connection, and the host computer checks to determine whether the particular user identity under which access is sought is presently locked or unlocked. If it is unlocked, the log-on sequence proceeds normally, as represented by exit arrow 306. If it is locked, on the other hand, then the program proceeds as represented by arrow 307, i.e. it returns to the Unlock Page (step 304). The latter presents three options. If option a. is chosen, the program exits via arrow 308, hanging up the telephone. If option c., the other user identity, is chosen, then arrow 309 indicates that the subsequent course depends upon whether the other user identity chosen is itself locked or unlocked. Finally, if the unlock sequence is pushed to its culmination via option b., the current user identity will either be locked or unlocked depending upon the user's success in supplying the correct password (see steps 310 and 311). FIG. 3C shows the same program in conventional flowchart form and in considerably greater detail. This figure is also labeled in such a way as to permit its correlation with the line numbers of the appropriate module from a master program listing which is attached hereto as Appendix A. The latter is a full print-out of all the program sequences in the herein-described information retrieval and service system. A collection of program flow charts corresponding to FIG. 3C is presented in FIG. 3A. The lock feature is of maximum utility in connection with those users who have elected maximum convenience instead of maximum security. By means of the lock feature, even these users, whenever a special need arises, can elect to secure their user identities for any desired period of time against use by unauthorized individuals such as baby-sitters or children when they go away from home, or by such individuals as co-workers or visitors in the office environment. And users who have elected maximum security can further strengthen their security measures by means fo the lock feature, since in that case an unauthorized user would have to know both the user identity code and the lock password in order to break into the system under that individual's identity. Moreover, both types of users can, if they wish, use a different password each time they lock up, so as to further minimize the probability of a security breach. The additional feature of not displaying the password on the screen at selection time also increases the impregnability of the system, but the requirement of a matching double entry effectively minimizes the possibility that typographical errors in password selection might prevent authorized re-entry. V(b). The relational page feature. Some information retrieval systems facilitate searching by storing a series of information pages relating to a common subject in consecutive storage addresses so that these pages can be scanned in succession. But in such systems there is a problem as to how to deal with pages of information which are relevant to more than one subject. Ordinarily it would be necessary to store duplicates of such pages in connection with each such subject. But that substantially increases the memory capacity requirements of the data bank, and it also multiplies the difficulties of updating such pages when the information they contain becomes obsolete. These disadvantages become readily apparent upon considering FIG. 3D, which illustrates the search path for a prior art system. The system so represented requires duplicative storage and maintenance of certain pages of information which are relevant to a plurality of search categories, in order to permit retrieval of those pages under any of the relevant categories. The search path is in the form of a branching tree, which starts with an index or menu page 312 offering a choice of vacation interests identified by search key numbers 0 through 9. Typical search paths for choices 0, 1, 5 and 6 are illustrated. Note that the Hotel Africa is relevant to three of these choices, because it is located in Africa (choice 0), it has a golf course (choice 5), and it has a swimming pool (choice 6). Similarly, the Hotel Egypt appears twice, and the Hotel Italian and Hotel Pizza each appear three times. Only the Hotel Sand appears a total of only once in all four search paths. Thus the information pages for the many hotels which have multiple relevance must be duplicatively stored, i.e. once for each search category to which they are related. Each duplicate occurrence requires a separate operation to update the information contained therein when it becomes obsolete. The expense of this additional storage capacity and update labor is quite substantial. In accordance with the search feature of the present invention, the search path is designed to avoid entirely the need for such duplicative storage and maintenance. Instead of storing the actual page of information physically in each of several search chains, each information page is stored only once. But the information pages each include pointer data for each subject to which the page relates, and the pointer data designates the storage address of the next page in relational, not numerical, order which relates to each such subject. In the case of pages which are relevant to more than one subject, its storage address will be included in the pointer data of more than one other page. In addition, the host computer makes a temporary record of the subject category currently being pursued by the searcher. Each time that the searcher asks for the next page in relation to that interest, the host computer does not call up the next page in numerical address order, but instead consults the pointer data for the current page, and for the current subject category, in order to determine the physical storage address of the next page in the relational sequence, and then fetches that page from that storage location and transmits it to the user's terminal for video display. This type of search path, as illustrated in FIG. 3E, is not a simple branching tree structure, but it is convoluted because paths relating to different subject categories often converge upon the same physical storage location. Here again the search path begins with a menu 313 offering a number of choices. The user is requested to press a number key indication his subject choice, and follow that by pressing the Action key to initiate the search procedure. At that point the host computer makes a record of which menu option has been chosen, and interprets all subsequent keystrokes in terms of that choice until otherwise instructed. Note that the search paths for choices 0, 5 and 6 all converge on the information page relating to the Hotel Africa, because of its multiple relevancy, described above. But that page is stored in only one physical location, and need only be updated once when the information contained therein becomes obsolete. Search paths 0 and 6 both continue on to the same page, i.e. the one concerning the Hotel Egypt, because that hotel also has multiple relevance in that it is in Africa and also has a swimming pool. But search path 5 diverges to the page dealing with the Hotel Italian because that hotel, not the Hotel Egypt, is the next one having a golf course. The Hotel Italian is excluded from the above-mentioned search path 0 because it is not in Africa. The Hotel Italian does come up in search path 6 because it does have a swimming pool, but it appears only after the hotel Egypt in that search category. At each step of the search paths illustrated in FIG. 3E when the searcher has finished perusing the current page, he or she merely presses the Next key and the system will respond by transmitting the next page in relational order, not the next page in storage address order. Some of the information entries may occupy more than one page, however, as in the case of the Hotel Pizza in FIG. 3E. In such a case, the next relational page is stored in the next memory address in numerical order. Eventually all search paths culminate in a common path-ending page, after which the search route returns to the starting point to permit the user to search another category offered by menu 313. But if one of the pages illustrated in FIG. 3E should be accessed by a keyword searched or direct selection, instead of by a choice of one of the options offered on the menu page 313, then subsequent operation of the Next key is given an entirely different significance by the host computer. Under those circumstances the computer is programmed to interpret Next key actuation as a request to transmit the next page in numerical order of storage address, rather than as a request to send the next page in relational order. The alternative embodiment of this invention illustrated by FIG. 3F is almost the same, except that the menu and the information pages all instruct the user to progress to the next page in relational order at each step of each search path by pressing the current menu choice number key again, instead of by means of the Next key. This arrangement leaves the Next key available to select the next page in numerical address order, or the next menu or the like. An additional advantage of this embodiment is that it avoids the need for the host computer to store a record of the menu option which the user is currently exploring during the entire time that the search path for that option is being transversed. But with either embodiment of the invention, the need for duplicate storage and updating of information pages is entirely avoided. Studies have shown that improvements of the order of 90% in storage capacity requirements and updating labor can be expected as a result. VI. Terminal Firmware Presentation videotex level features Screen resolution is 256 horizontal pixels by 200 vertical pixels (PLPS display area). A status line positioned below the unit-coordinate display area is provided for keyboard echo when the terminal 20 has no unprotected fields and local echo is in effect. The host computer 14 is not allowed to write into or affect the Status Line. The color of the border surrounding the PLPS unit display area is black (blanked beam). The terminal 20 employs a 16 by 9 bit color map for selection of 16 concurrent colors from a total of 512 possible colors. Blinking is performed by color map manipulation. Blink is implemented in color modes 1 and 2 only. The terminal 20 operates in a 7 or 8-bit NAPLPS environment. The host computer 14 initializes the terminal 20 for the 8-bit environment. A memory pool of 4 K-bytes is available for allocation to the support of DRCS, macro-PDIs, unprotected fields, and other memory intensive functions. The worst case DRCS storage requirements are computed by the following formula: w*h/8 bytes where w=width and h=height of the character cell size, and where h and w units are pixel counts for a screen resolution of 200.times.256. A contiguous RAM area of 16 K-bytes are available for telesoftware usage (code and data). NAPLPS Standard Conformance The terminal 20 provides the level of funtionality defined by the "General Service Reference Model (SRM) for Videotex," Appendix B of an ANSI X3L2.1 document, entitled "Videotex/Teletext Presentation Level Protocol Syntax (North American PLPS)," published Draft Proposed American National Standard, Oct. 1, 1982. Both functionality and specific implementation parameters are defined in that appendix. On power-up, NAPLPS attributes default as specified in Table 30 of that same reference. The terminal 20 accepts NAPLPS data encoding generated by conventional AT&T Frame Creation Terminal (FCT) (not shown). Any data base from a gateway system 10 is converted by the system 10 to conform to the foregoing standards. Telesoftware Requirements Downloaded software from the host computer 24 is supported in a manner as hereinbelow described. In this mode of operation, integrity of the terminal 20 can not be guaranteed, since the downloaded code can inadvertently destroy the PLPS RAM environment. According to the encoding scheme, the downloaded software is coded in absolute machine language, corresponding to the microprocessor 81 for the decoder 69. This software is transmitted from the host computer 14 in 8-bit binary format, to minimize transmission time. With reference to the callable firmware services, the downloaded code has the facility to read the keyboard 71, write to the television screen via the PLPS protocol, read or write the non-volatile memory 114, auto dial and disconnect, communicate with the host computer 14, and make use of a timing facility. Concerning hardware access, downloaded code has access to video memory, the color memory map, and the microprocessor bus extender. The bus extender provides the address, data, control, and status signals necessary to support external memory expansion (not shown), and/or a peripheral controller (not shown). No hardware write protect of the non-volatile memory 114 is implemented. ROM firmware called from telesoftware does not allow the terminal 20 ID field to be altered. If necessary, writing the terminal 20 ID (after the first time) is done via telesoftware, by accessing the non-volatile memory 114, without the assistance of firmware. Operational Features The following operational descriptions of the keyboard/display features are not addressed by the for | ||||||