Operating system and data base having a rule language for condition driven computer operation

Abstract: A programmable computer which operates by executing rules, each including a rule name, and optionally: input parameters, a set of conditions, a set of actions associated with each condition, and a set of exception handlers. A condition is a logical expression which evaluates to a true or false boolean value, while an action is an executable statement. An exception handler contains executable statements. The computer includes text entry and display means for programming of rules. A rule is displayed with each condition and each action having a row of text. The computer generates a yes/no quadrant into which a programmer may enter sequence numbers, to associate ordered sets of actions with individual conditions. Sequence numbers may be entered not associated with any condition, to define a set of default actions. Upon entering a rule, the computer sequentially evaluates the rule's conditions. Upon a condition evaluating to true, the computer executes the set of actions associated with that condition, then exits the rule. If the rule contains no conditions or none evaluates to true, the set of default actions will be executed, and the rule exited. If, during execution of the rule, the computer detects an exception event, such as a missing datum or other execution error, the computer passes to the rule the name of the detected exception event. If the rule contains an exception handler bearing the same name, the computer will execute the statements within that exception handler.

Description: Background of the Invention The present invention relates to high level computer interfaces for data access and program development environments. Description of Related Art The development of applications programs by software engineers is a complicated task. This complication arises in part because of environmental parameters, like the variety of data types, hardware types, operating system types, program auditing techniques, and other details. Computer programming languages have developed to handle all of these environmental parameters, usually by requiring explicit recognition of the parameters in the code. Thus, the typical application programmer must contend with data definitions, editing and validation of input data, selection and ordering of data available to the program, looping constraints on the system for the program, output editing and validation conventions, error processing, program auditing, and other complicated tasks in addition to the basic algorithm for accomplishing the application in mind. These environmental parameters also complicate the running of programs written with high level languages. These programs must be compiled or loaded prior to execution, during which time all the physical resources, data, and rules associated with the application must be bound together. This binding of resources required for a given application at compile time or load time, makes it difficult to implement truly event driven, or data driven, programs. Attempts have been made to make some programming languages interpretive. That is, to allow them to bind certain resources to the program while it is being run. However, these interpretive programs have very limited performance and have not gained widespread acceptance in the industry. Accordingly, there is a need for an operating system, data base, and data access method which will allow application programmers to be free of complications caused by environmental parameters. US-A-4860204, for example, discloses a computer system which caters for interactively entering and executing rules which include a condition in the form of a logic expression having a boolean value of true or false and an associated subsequent action. Summary of the Invention The present invention provides a programmable computer for executing a rule as defined in Claim 1, a method of programming a computer to execute a rule as specified in Claim 16 and a method of executing a rule by a computer according to Claim 21. Other aspects and advantages of the present invention can be seen upon review of the figures, the detailed description, and the claims which follow. Brief Description of the Figures Fig. 1 is an overview block diagram of a data processing system implementing the present invention. Fig. 2 is a schematic block diagram of the virtual data processing machine. Fig. 3 is a conceptual diagram of the transaction processor. Fig. 4 is a schematic diagram illustrating operation of the transaction processor for a rule call. Fig. 5 is a diagram of the rule name hashing method. Fig. 6 is a schematic diagram of the table name hashing method. Fig. 7 is a conceptual diagram of the table access machine with a plurality of servers and buffers. Fig. 8 is a conceptual diagram of the storage in the table access machine. Fig. 10 is a conceptual diagram of the CTABLE. Fig. 11 is a conceptual diagram of a source table and a subview table. Fig. 12 illustrates the operation of the intent list. Fig. 13 is a conceptual diagram of a transaction in the virtual machine. Fig. 14 is a schematic diagram of the table data store access method. Fig. 15 illustrates the layout of a page of data in the table data store. Fig. 16 is an overview of the rule editor components. Fig. 17 shows the TOKENS and the LONG_STRINGS tables used by the rule editor. Fig. 18, 19, 20A-20C, and 21A-21E illustrate operation of the detranslator. Detailed Description I. System Overview Fig. 1 illustrates the basic implementation of the data processing system according to the present invention. This system is implemented as a virtual machine on a main frame computer 10. The main frame computer runs a host operating system, MVS in the preferred system. Under the host operating system, a data base access system, such as IMS 11, can run. At the same time, the data processing system according to the present invention, known as HOS 12, runs under MVS. Coupled with the main frame are a plurality of user terminals 13, 14, such as the Model 3270's. Also, work stations 15 running other operating operating systems, such as UNIX, could be coupled to the main frame 10. Connected with the main frame data server, such as IMS 11, are direct access storage devices 16 and 17 storing corporate data. Also, direct access storage devices 18 and 19 are coupled with the HOS system 12. Fig. 2 shows a conceptual block diagram of the HOS data processing system. The system includes a transaction processor 20, a table access machine 21, and a table data store server 22 which is coupled to direct access storage device 23. The table access machine 21 also drives a screen server 24 which is coupled to a user terminal 25, and an import/export server 26 coupled to other sources of data files 27 and 28. In addition, the table access machine 21 provides an interface to a heterogenous data base system server, such as IMS server 29, which is coupled to direct storage access devices 30. The transaction processor 20 is implemented as a virtual stack machine which executes the object code level representation of rules. Those rules are stored in a rule library 31 associated with each transaction. The rule library typically includes a session manager 39 which provides a screen menu and basic utilities to the programmer. Utilities include rule editor 32, a table editor browser 33, RPW and print utilities 34, query processors, if necessary, 35, and application programs 36, 37, 38 such as are called up for a given transaction. Through the table access machine 21, physical data stored in direct access storage devices 23, 30, 27, 28 are presented to the transaction processor 20 as if it were stored in the table data store system. The servers 22, 24, 26, 29, the table access machine 21, and the transaction processor 20 are all implemented as virtual machines on the host operating system. Thus, in the preferred system, these virtual machines are implemented with system 370 assembler language and perform the functions which are described in detail below. II. Rules Language The rule language is the programming interface for the system. The language comprises an integrated set of database access commands, 4th-generation constructs, and conventional language statements which makes structured programming a natural consequence of developing an application. The rules have four parts as follows: 1. the rule definition 2. conditions 3. actions 4. exception handlers GENERAL FORM OF RULES Table 1 shows a sample rule named LEAPYEAR, which has one argument names YEAR. LEAPYEAR is a function because it RETURNs a value -- either 'Y' or 'N' -- as a result of execution. The rule definition contains the rule header "LEAPYEAR(YEAR);" and would contain local variable definitions, if there were any. The conditions determine the execution sequence. In this rule, there is only one condition, the comparison "REMAINDER(YEAR, 4)=0;". The actions are executable statements in the rule, only some of which will be executed on any particular invocation. The first action, "RETURN('Y');", is executed if the condition is satisfied, and the second action, "RETURN('N');", is executed if the condition is not satisfied. The rule in this example has no exception handlers, so there are no statements in the fourth part of the rule. Table 2 shows a more complex rule. The two GET statements verify that the month referred to by the parameter MM occurs in the table MONTHS and that the day referred to by the parameter DD is less than or equal to the maximum for that month. Failure of a GET statement causes the exception GETFAIL, and the exception handler produces a message and returns the value 'N'. Table 3 shows the table MONTHS, which the rule VALID_DATE refers to. Note the two columns for the numbers of days in a month for leap years and non-leap years. The Table MONTHS MONTH ABBR NAME DAYS LDAYS 1 JAN JANUARY 31 31 2 FEB FEBRUARY 28 29 3 MAR MARCH 31 31 4 APR APRIL 30 30 5 MAY MAY 31 31 6 JUN JUNE 30 30 7 JUL JULY 31 31 8 AUG AUGUST 31 31 9 SEP SEPTEMBER 30 30 10 OCT OCTOBER 31 31 11 NOV NOVEMBER 30 30 12 DEC DECEMBER 31 31 Table 4 shows a rule containing a FORALL statement. The rule COUNT_CARS calculates the number of owners with cars of a given model and year and then prints the result (MODEL and YEAR are fields in the table CARS). The rule has no conditions. The standard routine MSLOG presents the result of the summation in the message log (the symbol g is the concatenation operator). Table 5 shows the table CARS, which the rule COUNT_CARS refers to. The Table CARS LICENSE MODEL YEAR PRICE ASD102 FIESTA 86 7599 BNM029 ESCORT 84 5559 BXT524 TAURUS 88 12099 FDS882 TEMPO 87 10099 GET347 THUNDERBIRD 57 10999 LLA498 FIESTA 87 85059 PFF356 MUSTANG 84 10599 RTY211 TORINO 85 9599 SOT963 LTD 86 15159 TDS412 THUNDERBIRD 88 35299 RULE DEFINITION The rule definition, the first part of a HOS Rule, contains a rule header (obligatory) and a declaration of local variables (optional). The rule header gives a name to the rule and defines its parameters (if any). Parameter passing between rules is by value: a called rule cannot alter the value of a parameter. The data representation of a parameter is dynamic: it conforms to the semantic data type and syntax of the value assigned to it. The scope of a parameter is the rule in which the parameter is defined. Table 6 gives an example of a rule header. Rule Header for the Rule LEAPYEAR LEAPYEAR(YEAR); Local variables are declared below the rule header. The scope of a local variable is the rule in which it is defined and any descendant rules. A local variable can be assigned an arithmetic value or a string and can be used anywhere in an action. The data representation of a local variable is dynamic: it conforms to the semantic data type and syntax of the value assigned to it. Local variables are initialized to null (i.e., zero, the logical 'N', or the null string, depending on the usage). Table 7 gives an example of a local variable declaration. Declaration of Local Variables SUM and RESULT LOCAL SUM, RESULT; CONDITIONS Conditions, which are logical expressions evaluated for their truth value, determine the flow of control in a rule. Conditions are evaluated sequentially, and, if one of them is satisfied, the actions corresponding to it are executable, and no further conditions are evaluated. If there are no conditions (as in the rule in Table 4), then the rule's actions are executable. Table 8 gives some examples of conditions. REMAINDER and SUPPLIER_LOC are functions which return values. Although one of them is a system supplied standard routine and one is a user routine, there is no distinction in the invocation. Conditions CARS.PRICE > INPUT.MIN ; REMAINDER( YEAR, 4 ) = 0; INVOICE.LOCATION = SUPPLIER_LOC(INPUT.SUPP#); The example rules in earlier tables show that the part of a rule that contains conditions also contains a Y/N Quadrant, which displays Y/N ("yes/no") values. The Y/N values coordinate conditions and actions. HOS supplies the Y/N values, however, not the user. The function of the Y/N Quadrant will become clear in the section on actions. ACTIONS An action is an executable statement. Action sequence numbers determine which actions will be executed for each particular condition. The same action can be executed for different condition. A rule, in effect, is an extended case statement. The conditions and actions can be read as follows: CASE: condition 1: actions condition 2: actions ... condition n: actions else: actions END CASE; Consider the rule in Table 9, for example. The conditions and actions in the rule VALID-DATE can be read as the case statement below: The actions available in the rule language are described below. There are two kinds of assignment statement. In simple assignment, a single value is assigned to a field of a table or to a local variable. Table 10 shows simple assignment. Simple Assignment CARS.PRICE = (PRICES.BASE + PRICES.SHIPPING) *TAXES.RETAIL; AMOUNT = PRINCIPAL * ( 1 + INTEREST) ** YEARS; In assignment-by-name, all the values of the fields of the table on the right are assigned to identically named fields of the table on the left. Table 11 shows an example. Assignment-by-name is a convenient way of assigning all the values of fields of a screen table to fields of a data table, or vice versa. Assignment-by-Name ORDERS.* = ORDER_SCREEN.* ; A rule can invoke another rule implicitly through a logical or arithmetic expression that uses functional notation or explicitly with a CALL statement. Table 12 shows the implicit invocation of the function REMAINDER(YEAR,4), which is a standard routine that returns an integer. Implicit Invocation of a Rule R = REMAINDER(YEAR,4); A rule is a function if it contains a RETURN statement, which specifies the result. Table 13 shows examples of RETURN statements. RETURN Statement RETURN('Y'); RETURN( CARS.PRICE - REBATE ); The CALL statement can invoke a rule directly by referring to the rule by its name or indirectly by referring to a field of a table or to a parameter which contains the name of the rule. Note that arguments in CALL statements can be passed by an argument list or by a WHERE clause. A parameter keyword in a WHERE clause must be the name of a parameter in the rule header of the called rule. All parameters must be specified when a rule is called. A HOS database is a collection of tables. Rules access tables on an occurrence basis. When a rule refers to a table, it creates a table template, which serves as a window to the table. Rules enter new information into the table by first placing the new data in the appropriate fields of a table template and then executing either a REPLACE or an INSERT statement. Rules retrieve information from the table into a table template with a GET statement or a FORALL statement. Rules delete information from the table by placing the information in a table template and then executing a DELETE statement (the table template is undefined after a DELETE statement). For a GET, FORALL, or DELETE statement, selection criteria can be specified in a WHERE clause. The WHERE clause contains a predicate composed of one or more comparisons. The predicate is evaluated before data is placed in the table template. Table parameters can be specified in list form or in a WHERE clause (the two forms correspond to the two methods of parameter passing in the CALL statement). The GET statement retrieves the first occurrence in a table satisfying the specified selection criteria. If there are no occurrences that meet the selection criteria, the GETFAIL exception is signaled. The FORALL statement, which is a looping construct, processes a set of occurrences. The body of the loop consists of the statements to be executed for each occurrence satisfying the selection criteria. Nesting of FORALL statements is allowed. A FORALL statement contains a table name, an optional WHERE clause, optional ORDERED clauses, an optional UNTIL clause, and actions. A colon (:) comes after the optional clauses (or the table name, if there are no optional clauses). The actions, which comprise the body of the loop, come on separate lines after the colon. An "END;" clause, on a separate line, marks the end of the FORALL statement. As with all table access statements, parameters can be specified in an argument list or in a WHERE clause. Selection on fields is also specified in the WHERE clause. A WHERE clause in a FORALL statement has the same effect as in a GET statement. A WHERE clause can refer to the current table with an asterisk (*). A WHERE clause can contain the "partial match" operator LIKE, which allows comparison of incompletely specified data strings. Incompletely specified data strings can refer to zero or more unspecified characters with an asterisk (*), and they can refer to one unknown character with a question mark (?). When a FORALL statement is executed, table occurrences are retrieved in primary key order, unless a different order is specified by one or more ORDERED clauses. In the example of Table 18, the occurrences will be presented sorted by descending values of the field PRICE, then by ascending values of the field MODEL, and then by ascending values of the primary key LICENSE (the default for ordering is ASCENDING). Execution of a FORALL statement will terminate under either of two circumstances: (1) all occurrences satisfying the FORALL selection criteria have been processed, or (2) an exception is detected during the execution of the statements comprising the loop. The table template is undefined at the end of a FORALL statement. The INSERT statement adds a new occurrence to a table in the database. No field selection is possible: the WHERE clause can only specify parameter values. Occurrences within a table must have unique primary keys. An attempt to insert an occurrence with a primary key that already exists will cause the INSERTFAIL exception. The REPLACE statement updates an occurrence in the database. No field selection is possible: the WHERE clause can only specify parameter values. If the occurrence does not exist, the REPLACEFAIL exception is signaled. In order to alto the primary key value of the occurrence, it is necessary to DELETE the old occurrence and INSERT the new one. The DELETE statement removes an occurrence from a table in the database. A WHERE clause can specify field selection on the primary key field if the relation specified is equality. No other field selection is allowed. A WHERE clause can specify parameter values, as usual. If the primary key is specified in a WHERE clause, then that occurrence is deleted. If no primary key is specified, then the occurrence referred to by the primary key in the table template is deleted. If the occurrence does not exist in the table, the DELETEFAIL exception is signaled. Screens are the standard user interface. They support input from a keyboard and produce output to a terminal. The DISPLAY statement causes the specified screen to be displayed, and any input that is entered on the screen is available for processing. The UNTIL ... DISPLAY statement, which is a looping construct, displays a screen repetitively. The body of the loop consists of statements which are executed each time the screen is displayed. Inside the body of the loop, any input is available for processing. Two constructs allow looping, the FORALL statement, which can contain an UNTIL clause, and the UNTIL ... DISPLAY statement. The statements between the FORALL part or the UNTIL ... DISPLAY part, which is terminated with a colon (:), and the "END;" clause comprise the body of the loop. The UNTIL clause specifies one exception or two or more exceptions separated by the keyword OR. Looping terminates if an exception is detected. If a loop terminates because of an exception, control passes to new actions as follows: If the exception is specified in an UNTIL clause for the loop, then the actions executed next will be those following the END clause of the loop (control passes to those actions even if there is an ON statement for that exception in the exception handler part of the rule). Upon completion of those actions, the rule is finished executing and control passes to the caller. Execution does NOT resume at the point where the exception was detected. If the exception is not specified in an UNTIL clause for the loop but is specified in an ON statement in the exception handler part of the rule, then the exception will be handled in the usual way: the actions executed next will be those listed in the ON statement. If the exception is not specified in an UNTIL clause for the loop or in an ON statement in the exception handler part of the rule, then the exception will be handled in the usual way: either the exception will be trapped by an exception handler in a rule higher in the calling hierarchy or the transaction will terminate. Five statements control the synchronization of transaction, the COMMIT statement, the ROLLBACK statement, the SCHEDULE statement, the TRANSFERCALL statement, and the EXECUTE statement. The statements COMMIT and ROLLBACK establish transaction synchronization points. All or none of the changes between synchronization points will be applied to the database. Normal termination of a transaction implies COMMIT, and abnormal termination implies ROLLBACK. The SCHEDULE statement allows asynchronous processing by allowing a rule to be queued for execution independently of the current transaction. The rule to be executed must exist when the SCHEDULE statement is executed. The name of a queue can be specified by an optional TO clause. Definition of queues is handled by system parameters and is not done within the rule language. Queuing depends on the normal completion of the current transaction, that is, completion in accordance with the transaction protocol. The TRANSFERCALL statement terminates the current transaction and invokes a new one. Control does not pass back to the calling rule. When the called rule is finished executing, the transaction is complete. Like the CALL statement, the TRANSFERCALL statement can invoke a rule directly by referring to the rule by its name or indirectly by referring to a field of a table or to a parameter which contains the name of the rule. The EXECUTE statement invokes a descendant transaction. Control passes back to the original transaction on completion of the executed transaction. (Note that the CALL statement invokes a rule within the scope of the current transaction, but the EXECUTE statement invokes a rule that starts an independent transaction.) Like the CALL statement, the EXECUTE statement can invoke a rule directly by referring to the rule by its name or indirectly by referring to a field of a table or to a parameter which contains the name of the rule. The SIGNAL statement causes the exception specified within the statement. An ON statement or an UNTIL clause can subsequently detect and process an exception caused by the SIGNAL statement. EXCEPTION HANDLING An exception handler is an ON statement that contains the name of an exception followed by a sequence of actions to be executed in the event that the exception is detected. The ON statement is in effect only during the execution of the actions of the rule in which it occurs. It will trap exceptions generated both in the rule and in any of the rule's descendants (rules which are below the rule in the calling hierarchy). If ON statements in two or more rules at different levels in the calling hierarchy can handle the same exception, the ON statement in the lowest rule handles the exception. System exceptions are hierarchically defined (the hierarchy is presented in the next section). If more than one handler within a rule can handle an exception, the most specific handler will handle it. For example, GETFAIL is lower than ACCESSFAIL in the exception hierarchy. If a rule has both a GETFAIL handler and an ACCESSFAIL handler, and a GETFAIL exception occurs, the GETFAIL handler will be invoked. If the rule has no GETFAIL handler but does have an ACCESSFAIL handler, the ACCESSFAIL handler will be invoked. The GETFAIL handler will handle a GETFAIL exception if it occurs on a GET access to the table when the rule is invoked or if it occurs in the GET statement. Data access exception handlers (handlers for GETFAIL, INSERTFAIL, DELETEFAIL, REPLACEFAIL, ACCESSFAIL, and DEFINITIONFAIL) can limit their scope by specifying a table name, as in the Exception Hierarchy Table. If a table name is specified, the handler will only trap the corresponding exception if it is detected while accessing that table. If no table is specified, the handler will trap the exception regardless of what table is being accessed. The statements comprising an exception handler might cause the same exception. If this happens (and the second exception is not handled somewhere lower in the calling hierarchy), the currently executing handler will not handle the second exception. The rule executor will detect a possible "infinite loop" condition and abort the transaction. The run time environment signals system exceptions to permit an application to recover from an error. System exceptions form a hierarchy of names. The ERROR exception will trap all detectable errors, but the GETFAIL exception will only trap a table occurrence not found on execution of a GET statement. The following diagram shows all of the system exception names with their relative position in the hierarchy. Three levels of exceptions are defined, and an exception will trap any of the exception names that are below it in the hierarchy. The conditions under which each of these exceptions is signaled are described below. ACCESSFAIL - table access error has been detected COMMITLIMIT - limit on number of updates for one transaction has been reached CONVERSION - value contains invalid data for syntax or cannot be converted to target syntax DATAREFERENCE - error in specification of selection criteria has been detected DEFINITIONFAIL - error in definition of a table has been detected DELETEFAIL - key for DELETE statement does not exist DISPLAYFAIL - error in displaying a screen has been detected ERROR - an error has been detected EXECUTEFAIL - an error in the child transaction has been detected GETFAIL - no occurrence satisfies the selection criteria INSERTFAIL - key for INSERT statement already exists LOCKFAIL - there is a lock on an occurrence or a table is unavailable or a deadlock has occurred OVERFLOW - value is too big to be assigned to target syntax REPLACEFAIL - key for REPLACE statement does not exist RULEFAIL - error results from arithmetic computation SECURITYFAIL - permission for requested action is denied STRINGSIZE - size error in assigning one string to another has been detected UNASSIGNED - a field of a table that has not been assigned a value has been referenced UNDERFLOW - value is too small to be represented in target syntax (mostly exponential errors) VALIDATEFAIL - validation exit requested through validation exit key ZERODIVIDE - attempt to divide by zero has been detected EXPRESSIONS, OPERATORS, AND DATA TYPES Conditions, actions, and exception handlers contain expressions. Expressions may contain arithmetic operators and/or a string-concatenation operator. These operators conform with conventional notation, and they obey the precedence given below (exponentiation has highest precedence): ** exponentiation *,/ multiplication, division +,- unary +, unary - +,-,g addition, subtraction, string-concatenation A sequence of arithmetic operators or string-concatenation operators of the same precedence is evaluated from left to right. The Expressions Table shows operators within expressions. Expressions (PRICES.BASE + PRICES.SHIPPING) * TAXES.RETAIL PRINCIPAL * ( 1 + INTEREST) ** YEARS Each element of an expression has a syntax and a semantic data type. The syntax describes how the data is stored, and the semantic data type describes how the element can be used. Syntax The syntax for values of a field of a table is specified in the table definition. The maximum length of the field is also specified in the table definition. Valid specifications for syntax are: B (binary) - valid lengths are 2 and 4 bytes P (packed decimal) - valid lengths range from 1 to 8 bytes, which can hold from 1 to 15 decimal digits, the number of decimal digits is specified in the table definition F (floating point) - valid lengths are 4, 8, and 16 bytes, for a primary key field, or from 1 to 256 bytes for other fields C (fixed length character string) - valid lengths range from 1 to 128 bytes, for a primary key field, or from 1 to 256 bytes for other fields V (variable length character string) - valid lengths range from 3 to 128 bytes, for a primary key field, or from 3 to 256 bytes for other fields, storage is reserved for the length specified, but string operations use the current length Semantic Data Types The semantic data type for values of a field of a table is specified in the table definition. The semantic data type determines what operations can be performed on values of the field. Operators in the language are defined only for meaningful semantic data types. For example, negating a string or adding a number to an identifier are invalid operations (consider adding 3 to a license plate number). Valid semantic data types and their permitted syntax are: I (identifier) C (fixed length character string) V (variable length character string) B (binary) P (packed decimal) S (string) C (fixed length character string) V (variable length character string) L (logical) C (fixed length character string of length 1) Possible values, Y (yes), N (no) C (count) C (fixed length character string) V (variable length character string) B (binary) P (packed decimal with no decimal digits) Q (quantity) C (fixed length character string) V (variable length character string) B (binary) P (packed decimal) F (floating point) The rule language contains the following operators for making comparisons =, H=, <, <=, >, >=. The result of a comparison is always a logical value ('Y' or 'N'). The rule language uses the equal sign (=) as the assignment operator. The rule language contains the following operators for doing arithmetic **, *, /, +, -. The rule language uses a double vertical bar ( g ) as the concatenation operator. The concatenation operator is valid between any two semantic data types, and the result is always a variable length string. To assist the coding of generic routines and utility functions, the rule language permits indirect references to table and field names. The rule COUNT is a generic routine that determines the sum of a field over all occurrences. This rule sums any field of any table without parameters. SYNTAX A complete, formal syntax of the rule language in Backus-Naur Form (BNF) follows. BNF Notation (a) Lower case words enclosed in angle brackets, < >, denote syntactic categories. For example: <start character> (b) In a list of alternatives each alternative starts on a new line. (c) A repeated item is enclosed in braces, { }. The item may appear zero or more times. For example: { <digit> } (d) Optional categories are enclosed in square brackets, [ ]. For example: [ <exponent> ] Character Set All rule language constructs are represented with a character set that is subdivided as follows: (a) letters A B C D E F G H I J K L M N O P Q R S T U V W X Y Z (b) digits 0 1 2 3 4 5 6 7 8 9 (c) special characters @ # $ v H & * ( ) - _ = + ; : ' , . / < > Lexical Elements A rule is a sequence of lexical elements. Lexical elements are identifiers (including reserved words), numeric literals, string literals and delimiters. A delimiter is one of the following special characters v H & * ( ) - + = : ; ' , / < > or one of the following compound symbols ** <= >= H= g Adjacent lexical elements may be separated by spaces or a new line. An identifier or numeric literal must be separated in this way from an adjacent identifier or numeric literal. The only lexical element which may contain a space is the string literal. String literals may span more than one line; all other lexical elements must fit on a single line. Identifiers Rule language identifiers may not exceed 16 characters in length. Numeric Literals The rule language supports the following forms of numeric literals: String Literals A string literal is zero or more characters enclosed in single quotes: <string literal> ::= '{ <character> } ' Single quotes within a string literal are written twice. Thus the following string literal is a single quote ''''. Reserved Words The following list of names are reserved by the system as key words in the rule language. AND ASCENDING CALL COMMIT DELETE DESCENDING DISPLAY END EXECUTE FORALL GET INSERT LOCAL NOT ON OR ORDERED REPLACE RETURN ROLLBACK SCHEDULE SIGNAL TO TRANSFERCALL UNTIL WHERE LIKE Syntax of Rules III. Table Data Store The table data store stores data in relational tables according to the unique table data structure. This structure can best be understood by understanding how tables are built through the table access machine. HOS's logical access method to retrieve and access tables is the Table Access Method (TAM) HOS provides access to other heterogeneous databases. This facility is transparent to the user once the data definition has been done. TAM acts as a traffic cop in conjunction with the transaction processor to access the correct server - resident in other regions The physical access method and data organization is the TDS (Table Data Store) Data Stores are in a B+ Tree relational data structure Since HOS is a transaction system, a user's access to a large amount of data will only affect their dependent region. Defining a TDS table Table Definer is invoked using a command DT <table name> from a work bench menu or a primary command line produced by a session manager-routine. The screen layout of the Table Definer is set out in the Table Define Table. Standard naming conventions can be followed for the table name. The default table type is TDS. Other table types are available such as IMS, IMPORT - EXPORT (sequential) etc. Each table type has its own table definition screen in the session manager. Tables are universal to a system. The Security system on individual tables prevents unauthorized access to the definition and/or data. The syntax specifications of parameters and fields describe how the data is stored. The semantic specifications describe how the data should be used in applications. Event processing can be invoked at data definition time, however the actual rules are coded using the generalized programming language. The event processing is invoked when the data is accessed. General discussion - TDS options The fields of the Define Table - Screen Layout are discussed below. TYPE: The table type specifies the access method. Table Data Store ('TDS') is the default value and is used as the reference template. Each table type has an associated template for display. When the table type is changed, the corresponding template is displayed by pressing any of the function keys or the ENTER key. Valid table types include 'TEM' (temporary), 'IMP' (import), 'EXP' (export), 'PRM' (parameter), 'SUB' (subview) and 'IMS' (IMS) and others defined by a user. UNIT: The user unit the table is associated with is entered in this field. Valid units are provided by the database administration. IDGEN: This informs the system that it is responsible for providing unique primary keys for each occurrence. PARAMETER: The parameter information component is a scrollable area for multiple entries. A maximum of four entries are allowed. This feature allows the system to partition its databases based on a unique field. This mimics a hierarchical structure of data which is more common in the real world than truly relational. NAME The field name which should be unique within the table. TYPE The semantic type - application design control. SYNTAX The internal representation for storage. LENGTH The length in bytes. The system stores its' data as variable length data to optimize storage. DECIMAL If specified, it indicates the number of digits to the right of the decimal point. KEY The valid entry is 'P' for primary, and blank (non-key field). A table must have one field defined as its primary key. The primary key specification effectively makes the field a required one. Each occurrence in the table is uniquely identified by its primary key value. RQD The default value for this filed is blank (not required). Other valid entries are 'Y' for required or 'N' for not required. Inserting or editing an occurrence without proper values in the required fields is not allowed. DEFAULT The default value of the field, this will be input if the filed is left blank when a new occurrence is being added. Semantic Data Type and Syntax All fields of a table are bound to a semantic data type and syntax. The syntax describes how the data is stored while the semantic type describes how a field may be used. Valid semantic data types and their permitted syntaxes are: I - identifier C fixed length character string V variable length character string P packed decimal B binary S - string C fixed length character string V variable length character string L - logical C fixed length character string of length 1 value of "Y" for yes value of "N" for no C - count C fixed length character string V variable length character string B binary P packed decimal with no decimal digits Q - quantity C fixed length character string V variable length character string B binary P packed decimal F floating point Valid field syntaxes specifications B - binary P - packed decimal F - floating point C - fixed length character string V - variable length character string Table Documentation Documentation is associated with all objects including tables. Users can specify a short summary, keywords and a long description to document any tables defined, using a documentation screen as shown in TABLE 34 invoked from the table definer. The summary is limited to one line of information. The keywords specified can be made available for use by the keyword search facility. There is space available to provide a detailed table description. Script formatting commands can be included in the long description. Documentation Subview Tables Subviews provide windows on data. Users can be given a subset of the fields of a table or a subset based on selection criteria. The Table definer is invoked using a DT <subview table name> command from the workbench menu or the command line on the screen. Standard naming conventions are followed for the subview table name. The subview table definer screen is invoked by changing the default table type "TDS" to "SUB", resulting in a definer screen as shown in TABLE 35. The Security system on individual subviews prevents unauthorized access to the definition and/or use. There is no separate space allocated for a subview. All data maintenance is actually carried out on the associated source TDS table. General discussion - SUBVIEWS All fields are the same as described for the 'TDS' table. Fields unique to the subview are described below. SOURCE: Name of the source table whose subview is defined by this table. The source table must exist before its subview can be defined. SELECT: The scope of the selection criteria is to subgroup the number of occurrences selected. If not present, all occurrences will be selected. PARAMETERS: New parameters can be specified in the subview if there is a corresponding field in the TDS table. Source table parameters can be renamed and the source name specified in the SOURCE PARM. ORDERING: This is a scrollable area. Ordering can be specified on a field that is not defined in the subview. Ordering is only used for display purposes. SEQ - A(ascending) or D(descending) Field definition - Subviews: SRC: This is the source indicator field. Fieldnames can be the same, renamed or unique to the subview table. The source indicator field identifies the status of the field in relation to the source table. Valid entries are: Blank Field definition in both source and subview are the same. S = Source: A renamed field is indicated with this entry followed by the field name in the source table D = Derived: A field unique to the subview table is indicated by this entry as a derived field. The source of a derived field ought to be a functional rule which returns a value for this field. Applicational Applicational advantages of this feature allows for table manipulations and ad hoc reporting on a subview table level. In ADHOC processing the derived fields receive values when the table is accessed. for example, when editing the table: ED (table name) Event Rules Event Rules within data definition are commonly made up of a mixture of either validations or triggers. Validations are rules such as, when information is entered into a table, that data must be validated against another table. For example, when maintaining the employee table, the department number has to be in the departments table. This could extend to a series of other tables or external files. Triggers cause actions to happen rather than just verification, such as audit trails, updating of other tables or scheduling of other jobs. With event rules, a user is provided a completely "open" environment, where rigorous checks and controls can be implemented on data. Definition of Event Rules : The event rules section is a scrollable portion Event rules are coded in execution order Event rules can be defined for specific data access codes, such as insert or more generally - get and write Different event rules can be invoked on the same action - Example Write - event rule - Validation Write - Trigger If a data maintenance action is invoked - such as insert or update then write will also be invoked. The event rule that should be executed can be tested from a local library, but would reside in the site library at production time. In the event that a test is required using for example the Table Editor then the workbench function cannot be used if the rule is in a local library. To force the search path to access the local library execute the Table Editor as if it was a rule: EX =====> STE('table name(parameters)') or Command line ===> EX STE('tablename(parameters)') When coding the event rule remember that the occurrence of the table has already been obtained - no data access is required for that table. When coding the event rule the following has to be true: TRIGGERS - subroutines VALIDATIONS - functions returning 'Y', 'N' or a message if not 'Y' IV. Dictionary Data All data within the HOS data processing machine is stored according to the table access machine, either directly in the table data store or virtually in other data storage systems. The actual location and the type of table in which a given occurrence is stored is defined by a plurality of dictionary tables within the table data store. These dictionary tables can be considered metadata in that they are prespecified tables having a structure known to the table data store and to the table access machine so that they can be automatically surveyed in order to build control tables used for accessing occurrences in generic tables. The dictionary tables include the table named TABLE which has a structure. This table includes the name and type of all tables available to a given session in the machine. Also, the dictionary includes a table named FIELDS (table name) which is a table which includes the attributes of all data elements in the system. This table is parameterized on the table name. Therefore, the table access machine generates a view of the table called FIELDS which is limited to the fields of a given table. The dictionary also includes a table called PARMS (table name). This table identifies the parameter associated with each table, if there are any. The dictionary also includes the table called SELECTION (table name), in which is stored a selection string or filter to be applied to accesses to the table. Other dictionary tables include ORDERING (table name) which defines a set of ordering operations to be implemented upon accesses to the table, EVENTRULES (table name) which specifies rules to be executed upon access events to occurrences in the table, @RULESLIBRARY (library name) which stores actual object code for executable rules for the session. The parameter library name is a basic method for dividing up the rules library by a variety of names. The dictionary also includes dictionary tables required for accesses through the servers other than the table data store. For instance, the tables IMSTABFIELDS (table name), IMSSEGFIELDS (db name, seg name), and the IMSACCESS (table name) tables are used by the IMS server. The IMSTABFIELDS table maps the table access method filed name to an IMS field name. The IMSSEGFIELDS table provides the syntax and mapping parameters for an access based on parameters retrieved from the IMSTABFIELDS table. The IMSACCESS table is used by the server to generate actual access sequences for transmission to the IMS data base. Also, the dictionary table includes the tables used by the screen servers. The table SCREENS identifies all the screens that are accessible through the table access machine in a given session. The table SCREENTABLES (screen) provides a screen definition for a window in a position within a screen for the window. The table SCREENFIELDS (screen table) further provides mapping directly onto the screen itself. This dictionary data can be expanded to serve any number of servers, but its structure is hard coded ("meta meta data") in the preferred system. Now that the basic application programmer's view of the system has been described and the representation of data and dictionary data in the system has been provided, an internal specification of the operation of the virtual machines can be understood. V. Internal Representation of Rules Rules are stored in an internal representation that is directly executable by the virtual stack machine. The process of saving a rule in the rule editor involves a translation from its textual source code to virtual machine object code. When the textual representation of a rule is required, a detranslation process converts from the virtual machine object code to text. The obvious advantage of storing only one representation of a rule is that a discrepancy between representation can never arise. This section details the internal representation of a rule. It begins by describing the overall layout of the object code. A detailed description of the various data items, and virtual machine opcodes follows. Examples of the object code corresponding to various kinds of rule statements are included. THE FORMAT OF A RULE Rule object code is stored in the @RULESLIBRARY table. This table is parameterized by library name and has the rule name as its primary key. Conceptually, the object code for a rule may be subdivided into four components: 52 bytes of header information; "n" bytes of code; "m" bytes of static (non-modifiable) data; and "p" bytes of modifiable data. Only the first three of these components are actually stored in the object code. The modifiable area is allocated when the rule is loaded for execution. The Table "Layout of rule object code" shows the detailed layout of the rule object code. The header and code sections contain references to objects in the static data area. These references are two byte binary offsets which are relative to the start of the header. The Parameters, Local Variables, Exception Handler Names, Table.Field Names, Rule names and Constants Sections all belong to the static data area. RULE HEADER The header portion of the object code, which is shown in Table "Layout of rule object code header" contains the name of the rule along with various other values which describe the code, static data and modifiable data areas. The length stored in the header is the number of bytes to the right of the length value. Thus the total length of the rule is the value in Length plus 28 (the number of bytes to the left of and including the Length value). Layout of rule object code header Value Offset Syntax Purpose Rulename 00 Char(16) Name of the rule Date 16 Char(6) Date of translation¦ Time 22 Char(4) Time of translation¦ Length 26 Bin(2) Length of rest of rule Num parms parameters 28 Bin(2) Num. of formal Parm off 30 Bin(2) Offset to local list Num locs 32 Bin(2) Number of local variables Loc off 34 Bin(2) Offset to local list Cond off 36 Bin(2) Offset to conditions Act off 38 Bin(2) Offset to actions Excp off 40 Bin(2) Offset to exception¦ names Function 42 Char(1) Rule is a function (Y/N) TabFld off 43 Bin(2) Offset to t.f names¦ Const off 45 Bin(2) Offset to constants¦ Rule off 47 Bin(2) Offset to rule names Version 49 Bin(1) Object code version# Mod data 50 Bin(2) Size of modifiable data area RULE STATIC DATA This section outlines the contents of the static data area. Certain object names in the static data section contain references to items in the modifiable data area. Each reference is a two byte offset which is relative to the start of the modifiable data area. The values in the header which describe the static data area's layout are identified, when appropriate. Parameters The parameter section of the object code contains the list of names of the formal parameters declared for this rule. Each item in the list is a 16 byte name. "Num parms" contains the number of formal parameters declared for this rule, while the "Parm off" value is an offset to the start of this list. Local variables The local variables section of the object code contains the list of names of local variables declared in this rule. Each item in the list contains a 16 byte name followed by a two byte offset to the modifiable data area. "Num locs" contains the number of local variables declared in this rule, while "Locs off" contains an offset to the start of this list. Exception handler names The exception name section contains the list of names of exceptions handled by this rule. Each item in the list consists of a 16 byte name followed by a pair of two byte offsets. The first two byte offset references the name of the table associated with this exception. The table name is stored in the static data area as a 16 byte name. If there is no associated table name, this offset is zero. The second offset references the code which handles the exception. "Excp off" contains an offset to the start of this list; the list is terminated with a two byte zero. Fields of tables The table.field section of the rule object code is a list of all direct table.field names in the rule. Each item in the list consists of two 16 byte names (one for the table and one for the field) followed by a two byte offset to the modifiable data area. "TabFld off" references the start of the list; the list ends at the "Const off" offset. Rule names The rule name section is a list of all the direct rule (or local variables not declared in this rule) names called by this rule. Each item in the list consists of a 16 byte name, followed by a two byte offset to the modifiable data area. "Rule off" references the start of the list; the list ends at the "TabFld off" offset. Constants The constants section is a list of all the constants in the rule (either literal constants or constants generated internally by the translator). Each item in the list has a six byte descriptor followed by data. The data itself is prefixed by a one or two byte length (one byte if the data is 0..127 bytes in length, two bytes for 128+). The high order bit of CLen is on if CLen is two bytes, otherwise CLen is one byte. The type is a single byte which contains the semantic type of the constant. All constants have an unknown (blank) semantic type. The Syntax is a byte which indicates the representation of the data. Valid syntaxes are B(binary) P(packed decimal) F(floating point) C(fixed char) and V(variable char). The DLen is two bytes and contains the definition (or maximum) length of the Data. CLen contains the current length of the Data. Dec is also two bytes and contains the number of digits to the right of the decimal point. Dec is only meaningful for packed decimal constants. "Const off" references the start of the list of constants; the list terminates at the end of the object code which is given by "Length" + 28. RULE OPCODES The virtual stack machine is based on a stack architecture. Most virtual machine instructions manipulate the items on the top of the stack. All stack items are four bytes in length. Many items are pointers to values which are either fields of a table, rule constants or temporary values built during execution. All values are comprised of a six byte descriptor, followed by a four byte pointer to the actual data. The data itself is prefixed by a one or two byte length. The stack may contain other information, such as that necessary to implement rule call and return. For values that are built during execution, the virtual machine maintains a temporary area. When temporary values are popped from the stack, their storage in the temporary area is freed. All virtual machine instructions contain a single byte opcode. Depending upon the opcode, it may require zero, one or two, two byte operands. For the purpose of describing the semantics of each opcode, let item1 represent the top item on the stack and item2 represent the second item on the stack. Let operand1 and operand2 denote the first and second immediate operands, respectively. Name Opcode #Ops Semantics @ADD 4 0 Add two values together. Pop item1 and item2. Create and push a temporary value of item2 + item1. @SUB 8 0 Subtract one value from another. Pop item1 and item2. Create and push a temporary value of item2 -item1. @MULT 12 0 Multiply two values together. Pop item1 and item2. Create and push a temporary value of item2 * item1. @DIV 16 0 Divide one value by another. Pop item1 and item2. Create and push a temporary value of item2 / item1. @EXP 20 0 Raise one value to the power of another. Pop item1 and item2. Create and push a temporary value of item2 ** item1. @UNM 24 0 Arithmetically negate a value. Pop item1. Create and push a temporary value of -(item1). @CAT 28 0 Concatenate two values together. Pop item1 and item2. reate and push a temporary value of item2 concatenated to item1. @EQ 32 0 Compare two values for equality. Pop item1 and item2. Create and push a temporary value of item 2 = item 1 ('Y" or 'N'). @NE 36 0 Compare two values for inequality. Pop item1 and item2. Create and push a temporary value of item2 -= item1 ('Y' or 'N'). @LT 40 0 Compare to determine if one value is less than another. Pop item1 and item2. Create and push a temporary value of item2 < item1 ('Y' or 'N'). @LE 44 0 Compare to determine if one value is less or equal to another. Pop item1 and item2. Create and push a temporary value of item2 <= item 1 ('Y' or 'N'). @GT 48 0 Compare to determine if one value is greater than another. Pop item1 and item2. Create and push a temporary value of item2 > item1 ('Y' or 'N'). @GE 52 0 Compare to determine if one value is greater or equal to another. Pop item1 and item2. Create and push a temporary value of item2 >= item1 ('Y' or 'N'). @CALL 56 2 Call a procedural rule or builtin. Operand1 is the number of arguments to pass; they have already been pushed onto the stack. Operand2 is an offset to the name of the rule to call; a positive offset is a reference to a name in the static data area. A non-positive offset signifies an indirect rule name, in which case item1 contains the name of the rule to call. On an indirect call, item1 is popped. The caller's environment is saved and execution begins at the start of the called rule. @FN 60 2 Call a functional rule or builtin. Operand1 is the number of arguments to pass; they have already been pushed onto the stack. Operand2 is an offset to the name of the rule to call. The caller's environment is saved and execution begins at the start of the called rule. 64 UNUSED. @RETURN 68 0 Return from a functional or procedural rule. Pop all arguments passed to the called rule and restore the caller's environment. If the called rule was a function, push the return value onto the stack. @UNTIL 72 2 Initialize for the execution of an UNTIL loop. Operand1 is an offset to the opcode to be executed once the loop terminates. Operand2 indicates whether this is an UNTIL .. DISPLAY loop (1) or just an UNTIL loop (0). @PARM 76 1 Push a formal rule parameter onto the stack. Operand1 is the number of the formal parameter. (For a rule with N parameters, the Ith one is numbered (N - I)). Push the parameter value onto the stack. @CONST 80 1 Push a constant onto the stack. Operand1 is an offset to the value in the static data area. Copy the values' descriptor into the temporary area and append a four byte pointer to it. Set the pointer to reference the value's data in the static data area. Push the constant value onto the stack. @TEMP 84 0 Create a temporary copy of a value. If item1 is not a temporary value, create a temporary copy, pop item1 and then push the temporary value. @RFIELD 88 1 Push a field of a table onto the stack. Operand1 is an offset to the table.field name; a positive offset references a name in the static data area. A non-positive offset signifies an indirect table.field, in which case item1 is the name of the table and item2 is the name of the field. On an indirect reference, item1 and item2 are popped. Push the field value onto the stack. As the field is being pushed, a check ensures that the field has a value; if not the exception UNASSIGNED is raised. @WFIELD 92 1 Push a field of a table onto the stack. Operand1 is an offset to the table.field name; a positive offset references a name in the static data area. A non-positive offset signifies an indirect table.field, in which case item1 is the name of the table and item2 is the name of the field. On an indirect reference, item1 and item2 are popped. Push the field value onto the stack. @AEND 96 0 Mark the end of the current code section. Used to separate the conditions from the and the actions from the exceptions. @AEND is NEVER executed by the virtual machine. @SET 100 0 Assign a field of a table a value. Item1 is a field of a table. Item2 is a value. Set data of item1 to data of item2 and pop both items. @ABN 104 0 Assign commonly named fields in tables. Item1 is the name of the source table. Item2 is the name of the target table. Set the data of all commonly named fields in the target table to data of fields in the source table. Pop item1 and item2. @SCHED 108 2 Schedule the asynchronous execution of a rule. Operand1 is the number of arguments to pass; they have already been pushed onto the stack. Operand2 is an offset to the name of the rule to schedule. ???? what is done with these args and queue name ??? Pop all arguments from the stack. Pop item1, which is the name of the queue for the asynchronous event and continue. @DROP 112 1 Branch conditionally to an offset within a rule. Pop item1. If item1 is not 'Y', branch to the offset given by operand1. @NEXT 116 1 Branch unconditionally to an offset within a rule. Branch to the offset given by operand1. @BRC 120 1 Branch conditionally to an offset within a rule. Check the return code set as a result of the last @TAM opcode. If it is non-zero, branch to the offset given by operand1. @FORI 124 0 Initialize for the execution of a FORALL loop. @TAM 128 1 Call TAM to perform a table access request. Operand1 contains the number of arguments on the stack. Build a parameter list from these arguments, and call TAM. If the access was other than a FORALL or UNTIL..DISPLAY, pop the arguments from the stack. If the access was a FORALL or UNTIL .. DISPLAY and the return code is zero (end of occs.), pop the arguments, otherwise leave the arguments on the stack. Save the return code from the TAM call. @TAMP 132 1 Insert a reference into a TAM selection string or parameter list. Item1 is a value which is to be inserted into the selection string or parameter list. Item2 is the selection string or parameter list. Operand1 is an offset in item2 where the reference to item1 is to be inserted. If necessary, create temporary copies of both item1 and item2. Insert the reference to item1 in item2 and pop item1. (item2 will be popped during the course of executing the @TAM opcode). @TAMN 136 1 Copy a table name into a TAM table name argument. Item1 is a table name which is to be passed to TAM. Item2 is a 16 byte blank character string. Operand1 is an offset in item2 where the table name is to be copied. If necessary, create temporary copies of item1 and item2. Copy item1 into item2 and pop item1. (item2 will be popped during the course of executing the @TAM opcode). @SIGNAL 140 0 Raise an exception. Item1 is the name of the exception which is to be raised. Pop item1 and raise the exception. 144 UNUSED. @ACALL 148 1 Call an action from the conditions. Push the offset to the next instruction (the "return address") and branch to the offset given by operand1. @ARETURN 152 0 Return from an action (to the conditions). Item1 is the offset of the next opcode to be executed. Pop item1 and branch to that offset. @EXEC 156 2 Execute a procedural rule. Operand1 is the number of arguments to pass; they have already been pushed onto the stack. Operand2 is an offset to the name of the rule to execute; a positive offset is a reference to a name in the static data area. A non-positive offset signifies an indirect rule name, in which case item1 contains the name of the rule to execute. On an indirect execution, item1 is popped. The environment of the current transaction is saved and a new transaction begins at the start of rule being executed. 160 UNUSED. @SETL 164 1 Assign a local variable a value. Item1 is the value to be assigned. Operand1 is the local variable. Set data of operandl to data of item1 and pop item1. @WARG 168 2 Convert a list of arguments which are passed by name to a list of arguments passed by position. The stack consists of parameter name/parameter value pairs. Operand1 is the number of name/value pairs. Operand2 is an offset to the name of the rule being passed there args; a positive offset is a reference to a name in the static data area. A non-positive offset signifies an indirect rule name, in which case item1 contains the name of the rule. On an indirect reference, item1 is popped. Pop all name/value pairs and push and values in positional order. @NOT 172 0 Logically negate a value. Pop item1. Create and push a temporary value of -(item1). 176 UNUSED. 180 UNUSED. @XCALL 184 2 Transfercall a procedural rule. Operand1 is the number of arguments to pass; they have already been pushed onto the stack. Operand2 is an offset to the name of the rule to transfercall; a positive offset is a reference to a name in the static data area. A non-positive offset signifies an indirect rule name, in which case item1 contains the name of the rule to transfercall. On an indirect transfercall, item1 is popped. The environment of the current transaction is discarded, and a new transaction begins at the start of the transfercalled rule. RULE MODIFIABLE DATA The virtual machine performs run time binding of references to objects named in the static data area, specifically rules, local variables and fields of tables. Binding involves a search for the object and occurs when an object is first referenced. The binding is remembered by storing the object's address (or "thread") in the modifiable data area. Subsequent references to a bound object may simply use the thread since the address of an object is constant for an entire transaction. The size of the modifiable area is calculated when a rule is translated from source to object code. This size is stored as the "Mod data" value in the rule header. The first time a rule is called during the course of a transaction, its modifiable data area is allocated and set to contain zeros. As the rule is executed, references to fields of tables, local variables and rules are bound by saving their addresses at the designated offset in this rule's modifiable area. The modifiable data area of a rule is deallocated when the transaction terminates. The modifiable data area is also used to save the names of indirect rule and indirect table.field references. As described in the Rule opcodes section, an indirect reference is denoted by a non-positive offset as an operand to the @RFIELD, @WFIELD, @CALL, @EXEC, @XCALL or @WARG opcodes. The non-positive offset indicates that the name of the rule or table.field being referenced is on the virtual machine stack. This name is saved in the modifiable data area at the absolute value of the offset along with its thread. RULE CODE SEQUENCES Now that we have seen the various virtual machine instructions available, it is time to explore how they are put together to implement the various language features. Conditions and Actions The code generated for the condition section of a rule, amounts to sequences of "calls" (@ACALL) to the actions to be executed (the action list) when each condition is satisfied. These @ACALL opcodes may be "guarded" by code to evaluate a condition and an @DROP opcode which branches to the next condition if this one is not satisfied. An unguarded sequence of @ACALL opcodes may arise when there are no further conditions to evaluate. The @ACALL opcode pushes the offset to the next instruction onto the stack and then branches to the action. The last opcode in an action is an @ARETURN which uses the saved offset to return to process the next action for the condition. An @RETURN opcode follows the last @ACALL in each sequence to return from the rule. The main benefit of "calling" actions from the conditions is that the code for an action need only be generated once, even when the action is a member of more than one condition's action list. An example is set out in Table E. Assignments The code generated for an assignment statement consists of the postfix representation of an expression followed by either an @SETL (assigning to a local variable) or an @SET (assigning to a field of a table). Table F, shows a reference to an identifier G. At the time the rule ASSIGN is translated, it is not known whether G is a local variable defined in one of ASSIGN's ancestors, or a function which takes no arguments. The translator generates code assuming that G is a function and when the @FN opcode is executed, a check is made to determine if G is actually a local variable. Rule Calls Arguments to a rule may be passed by position or by name (using a WHERE clause). The code generated for rule calls when parameters are passed by position is very straightforward. First each argument is evaluated and made a temporary on the stack. This is followed by the @CALL (or @FN for a functional rule) opcode which takes the number of arguments and the name of the rule as its operands. When the @RETURN opcode in the rule being called is executed, all the arguments passed to the called rule are popped. If the called rule is a function, the return value is left on the top of the stack. When arguments are passed by name, the code generated is slightly more complicated. Each argument is evaluated and made a temporary, just as it is when parameters are passed by position. However, the name of the formal parameter which each argument corresponds to, is also pushed, immediately after the argument. This results in parameter name followed by parameter value on the stack for each argument. Immediately preceding the @CALL opcode is the @WARG opcode. @WARG massages the name/value pairs on the stack into the same form as when arguments are passed by position. This allows the @CALL opcode to be executed unaware that the arguments were actually passed by name. An example of code generated by a rule call is shown in Table F. Table Accesses The Table Access Method, or TAM, is used to implement various language statements which access the database. The interface to TAM is quite simple: an @TAM opcode and N (between one and five) arguments on the run time stack. The code generated for table access statements first pushes the necessary arguments on the stack, and then calls TAM. After returning from TAM, the @TAM opcode pops its arguments (unless we are in the midst of performing an UNTIL .. DISPLAY or a FORALL) and saves the code returned from TAM. The first argument pushed onto the stack is always the TAM operation code. It tells TAM what kind of table access is being performed. The TAM operation code is the only required argument. Each of the others are optional and are only passed as needed. The second argument is the name of the table or screen being accessed. This name is always padded out to be 16 characters in length. The third argument contains the list of parameter values for the table. Each element in this list is a four byte pointer to a value. Elements in the parameter list are inserted by the @TAMP opcode. The fourth argument, which contains the selection that is to be applied to the table, is the most complicated to describe. Essentially it contains a postfix representation of the WHERE clause. It is comprised of one or more terms which are joined by logical operators. Each term begins with a field reference, is followed by an expression (made up of field references, values and arithmetic operators) and ends with a relational operator. Each field reference is 17 bytes long: a one byte binary code indicating that this is a field reference followed by the name of the field. Each value is a one byte binary code for value, followed by a four byte pointer to the value. Every operator is a single byte binary code, representing the particular operator. As with elements in the parameter list, the @TAMP opcode is used to insert references to values in the selection string. The fifth argument contains any ordering that is to be applied to occurrences retrieved. Each term in the ordering string contains a 16 byte field name followed by either "A" or "D" to indicate whether that field should be ordered in Ascending or Descending fashion. An example of the table access code is shown in Table H. Indirect references Indirect references may be used to specify a rule name (in the CALL, TRANSFERCALL or EXECUTE statements), a table or screen name in the various table access statements, table and field names on the left hand side of an assignment, or in an expression. The code generated for indirect references, first evaluates the reference by pushing it onto the stack. The opcode which uses the indirect references has a non-positive offset as one of its operands. The non-positive offset indicates that the name is on the run time stack (rather than in the static data area as is the case for direct references). An example of indirect reference code is set out in Table I. Indirect table and screen names for table access statements behave somewhat differently than described above. After the value of the table or screen name is pushed onto the stack, a special opcode (@TAMN) is generated. It pops this value and copies it into a temporary value on the top of the stack. VI. Transaction Processor Fig. 3 is a block diagram of the transaction processor which implements the virtual stack machine for execution of the object code level rules, as described in the preceding section. The virtual stack machine is a software module 100 which manages execution of the rules. When a session begins, the virtual stack machine 100 retrieves a plurality of rule libraries 101 and establishes a set of buffers 102. In addition, an opcode table 103 is established and a stack 104 is set up. The buffers include a data management area 102A, a working area 102B, and a temporary area 102C. In execution, the virtual stack machine generates an instruction pointer across line 105 to the rule libraries 101. A stack pointer is generated and supplied across line 106 to the stack. The virtual stack machine 100 communicates with the opcode table 103 across line 107, and the buffers 102 across line 108. Data access instructions are passed to the table access machine across line 109. The virtual stack machine executes the algorithm as follows: 1. Fetch opcode pointed to by the instruction pointer in the rule; 2. Lookup opcode in opcode table 103 to retrieve the host system routine identified by the opcode; 3. Dispatch the host system routine; 4. Advance the instruction pointer; 5. Return to the fetch opcode step. Figure 4 provides an example of execution by the virtual stack machine for calling a rule named "CHECKTRAIL". In this example, the CHECKTRAIL rule includes two parameters, including 'TORONTO' and PERSON. The rule object code is stored in the rule library at the location 120. The instruction pointer will be advanced from the top code @CONST 'TORONTO' to the last object @CALL 2, offset, where the offset is indicated by the line 121. The offset identifies the position within the static data area of the rule where the rule name CHECKTRAIL with offset 122 is stored. The offset 122 stores a location in the modifiable area of the rule at which the value for the CHECKTRAIL object code is stored. The value points across line 123 to the location in the rule library 124 at which CHECKTRAIL can be found. The stack 125 is impacted as follows: First, in response to the @CONST opcode, the value of the variable TORONTO is placed in the working area of the vertical stack machine. In response to the @TEMP opcode, a temporary copy of the TORONTO parameter is stored. The stack is loaded with an offset 126 to the value 127 for the temporary copy 128. Next, the parameter 'PERSON' is moved into the working area of the virtual stack machine and a temporary copy is made. The stack is loaded with a pointer 129 to the value 130 of the temporary copy of the constant PERSON 131. Next, the @CALL 2, offset rule is executed. If the offset in the @CALL opcode points to the name and offset in the static data area of the rule to be called, the value which identifies the location in the rule store of the rule to be called is stored in the modifiable data area at the location 132 identified by the offset 122. The instruction pointer in the virtual stack machine is then moved to the location in the rule store 124 at which CHECKTRAIL is found, and CHECKTRAIL is executed. The rule libraries 101 shown in Fig. 3 consist of a plurality of rule libraries. When a @CALL opcode is executed, a search for the appropriate rule is carried out by rule name. The rule can be found as rule object code in one of the libraries, as a built in function or sub-routine (e.g., the ENTER key sub-routine which responds to user input), a customer defined external routine which is written perhaps in a different source language like Cobol or PL1, or it could be a local variable reference, rather than a call to an actual rule. Thus, the rule name search proceeds first through the list of local variables in the rule from which the statement was executed, second, through the local rules library which is set up by the user for a given session, next, through an installation rules library which is a standard set of routines established by a customer for a given execution and programming environment, fourth, through a built in routines library which includes all the standard functions of the operating system, fifth, through a system rules library which stores the HOS system rules which are utilized in the data management and other built in operations, and sixth and finally, through a library of external routines. When a rule is called, from a prior rule, such as a session manager, the stack of the prior rule is saved in a context save area in the stack before execution of the subsequent rule is begun. The stack pointer returns to the prior rule upon execution of a return statement. The data management area 102A in the virtual stack machine contains the tables through which rule name searches are carried out. These tables are shown in Fig. 5. For the rule names, a rule name hash table 150 is generated, which is 127 entries long. Each entry in the rule name hash includes a pointer to a first rule which hashes into that entry. For instance, a pointer 151 to the CHECKTRAIL rule may be the first entry. The CHECKTRAIL rule will store a pointer 152 to a second rule which corresponds to that entry in the hash table (e.g., the SHORT rule shown in the Figure). The SHORT rule then includes a pointer 153 to a subsequent entry down to the end of the list which hashes into a given location in the rule name hash table. If a called rule is not located in the hash table, then it must be retrieved through the table access machine and inserted into the rule library for the session. Fig. 6 shows the table definition built in the data management area in the virtual stack machine. The table name is first hashed into a table name hash table 160. Then entry in the hash table 160 points to a table list 161 which is searched to find the name of the table "FRUIT" of interest. This entry includes a first pointer 162 to a parameter list 163 and a second pointer 164 to a field list 165 for the table. This field list includes a pointer 166 to the row buffer 167 for the occurrence of the table which has the field of interest. The virtual stack machine maintains buffer for all data accesses and local variables. So, whenever a rule program says GET CARS WHERE LIC_PLATE = '878BBC' ; the VSM has to acquire the dictionary information for the table CARS and establish a buffer for it. Likewise, when a rule program says CUSTOMER.NAME = INPUT.NAME ; without having issued a data access command, the virtual stack machine must acquire the dictionary information for the table CUSTOMER and establish a buffer for it. As soon as a buffer for a particular table has been established, it is henceforth available for all rules in the current transaction. The virtual stack machine keeps track of which fields and which tables have been assigned a value, either through a GET or FORALL command or through an assignment statement. The virtual stack machine acquires the dictionary to populate its data management area exactly as any application program would, by issuing a set of access commands: GET TABLES WHERE NAME = TABLE NAME ; FORALL FIELDS (TABLE NAME) ; FORALL PARMS (TABLE NAME) . The uniform view of dictionary data and other data are strictly preserved above the table access machine. No component of the system above the table access machine sees unnormalized dictionary data. VII. The Table Access Machine The table access machine is illustrated in block diagram form in Figure 7. This machine or method consists of a host assembler software module 200 which communicates with the virtual stack machine across line 201. This module 200 establishes buffers 202 including a data management area buffer 202A and a working area 202B. In addition, the table access method module 200 provides an interface 401 for a plurality of servers including a TDS server 203 for the native table data store, an IMS server 204 for data stored within an IMS data base associated with the host machine, a screen server 205 which provides an interface to display systems coupled to the host machine, import/export server (not shown), and other servers which are required for retrieval of data through the table access method 200 to the virtual stack machine. Coupled with the servers across interface 402 is a selection evaluator module 206 which interprets selection strings at the server level. The Table Access Method (TAM) is the component of the system, which manages all data access in the system, and effectively is the owner of the dictionary tables and repository of all other tables used by a session. A call to TAM has the following information: 1. Operation 2. address of data and return areas 3. Table name 4. Parameters 5. Selection string 6. Ordering string The operations are: A : First request of a FORALL B : Subsequent request of a FORALL I : Insert G : Get with key N : Get without key R : Replace D : Delete S : Display screen C : Commit O : Rollback T : Transaction end On the first request for a particular table, TAM will acquire a CTABLE from the Table Data store. The CTABLE is a control block consisting of the dictionary data in unnormalized form. The CTABLE is used for processing by both TAM and all the data servers, of which TDS is one. The CTABLE contains the equivalent data as in TABLES, FIELDS, PARMS, SELECTION, ORDERING and EVENTRULES. Note that TAM acquires the CTABLE for a table as soon as it is given a call like: GET TABLES WHERE NAME = tablename; Normally, this means that a component above TAM wants to build a buffer for <tablename> and therefore immediately after the GET request will issue a FORALL FIELDS(tablename); and a FORALL PARMS(tablename); These requests are serviced by TAM out of the CTABLE. Besides the CTABLE, TAM sets up data structures for data rows, where the last row accessed are maintained, as well as an Intentlist of pending updates to the data servers. TAM manages positioning within the data bases. The data servers like TDS process requests from TAM one by one, without keeping any information from request to request. Only the LOCK MANAGER (within TDS) keeps information for the duration of a transaction. In the case of a parameterized table, TAM maintains a buffer and a position for each table accessed within the table set. When a REPLACE, INSERT or DELETE request is received by TAM, it will ascertain that the request is possible by asking the server, if the occurrence is there (DELETE and REPLACE) or is not there (INSERT), and will acquire the necessary exclusive lock on the occurrence. Thereafter TAM will place the REPLACE, INSERT or DELETE occurrence in the Intentlist (work to do list), until it receives a COMMIT, ROLLBACK or TRANSACTION_END request. TAM performs the conversions between subviews and logical views, consisting of: Data conversions between different representations Projections (subview contains fewer fields than logical view) Selection (subview contains fewer rows than logical view) Ordering (different from logical view) Derived or calculated fields A subview is effectively a window into a logical view, which never materializes in storage, only appears in buffers during the execution. To the user and VSM, however, there is no apparent difference between a subview and a logical view table. Derived fields are fields of a subview, which are calculated during the GET or FORALL processing. Validation and trigger rules are executed on the basis of EVENTRULES information. In all 3 cases, TAM issues a (recursive) call to the virtual stack machine to execute a rule. The table buffer, on which the validation or trigger is scheduled is shared by the recursion levels, as is the intent list. TAM implements the concept of temporary tables, which exist only during a transaction (in the TAM data areas), but does not materialize in any data server like TDS. TAM performs all the necessary sorting for both subviews and for FORALL requests with ordering which is different from the ordering maintained by the data server. Data access requests are directed to the appropriate server on basis of the information in the TABLES table for the particular table. Figure 8 is a diagram of the TAM buffer storage area. The TAM buffer storage area includes an anchor storage area 250 which includes the table index. This table index points to a set of CTABLES 251 which buffers the CTABLE for each of the tables T1 - T3 and so on. In addition, the buffer pointer for a given table T1 points to a set 252 of buffers for the table T1. This set 252 includes an index 253 for triggers and parameters for table 1. This index points to the actual data occurrence buffers 254, 255, 256, and so on, as required. In addition, the index page 253 points to other T1 information, such as selection strings and the like, in page 257. In addition, the intent list for table T1 is maintained in pages 258 and 259, as required. The anchor storage 250, in addition, points to free buffers in a set 260, which includes a plurality of single linked empty buffers. The buffering is sequential and forward. The table anchors are cleared at a transaction end in response to the "T" opcode. Fig. 10 illustrates the CTABLE formation in the table access machine. The CTABLE is a table of characteristics of a corresponding table stored in a buffer 280. This buffer is generated from raw data 281 which is retrieved from the dictionary tables 282 in the table data store. These dictionary tables are in turn maintained in the balanced storage structures 283 of the table data store just as all other data in the system. Thus, the metadata in a CTABLE is stored in the relational, object-oriented data base upon which the transaction is executing. However, "meta-meta-data" is hard coded. That is, the dictionary structure is established for the user and cannot be changed. Thus, there must be the minimum set of dictionary tables for a given transaction to successfully complete. A given CTABLE is bound for the duration of a transaction. Thus, after it has been retrieved, upon the first call to a portion of a table, all subsequent calls to that table are served by the CTABLE in the transaction storage area of the table access machine. Fig. 11 is a representation of a source table (T) and a sub-view table (S). A sub-view is a map of some rectangle within the source table. Thus, as can be seen in Fig. 11, the sub-view includes the first and third columns of rows 2, 3 and 5 of the source table T. This sub-view is implemented by the table access machine with one set of data buffers, but two CTABLES. A sub-view is built for each occurrence at interface time. Derived fields, however, are generated only from the source table. Thus, in response to a request in the rule that needs access to the sub-view table S, the table access machine will build a control table for both the source table T and the sub-view table S. The access will occur in the table data store to the source table T. Thus, a source table page will be returned to the table access machine. The table access machine then buffers the page, converts it to the logical subview, and returns the first occurrence in the sub-view to the executor. Triggers can be executed from sub-views as event rules and otherwise in response to entries in the CTABLE for a given table. When a rule is called, the calling rule execution environment is saved, TAM sends the triggered rule to the executor for interpretation. The triggered rule can also issue table operations which will cause the executor to recurse to the table access machine. Upon completion of the triggered rule, control is returned back to the calling rule. Parameterized tables are implemented in the access model, but not at the occurrence level. Thus, they are executed in a manner similar to sub-views. These parameterized tables allow concatenated keys, improved concurrency, save storage space in the virtual machines, and simplify archiving. They are executed by building a parameter table in the table access machine out of the dictionary in the table data store. This control table is built in response to the "P" call to the table data store from TAM. The table access machine then transfers the parameter occurrences to the executor for entry in the table lookup indexes. TAM will retrieve data from the table data store from the source table. It builds a buffer for the source table and then will perform the parameter selections prior to sending occurrences to the executor. Also, sub-view selection can specify a source parameter name on RHS of the operator. Temporary tables and intent lists are created by the table access machine with the same code. The temporary tables also allow selection. These tables are searched in a linear manner. However, access can be random. Fig. 12 shows the code and insertions in an intent list for a given operation. See the intent list stores the intent to store the value k=5 and the value k=4. The FORALL statement executed at the end invokes the sort to establish the insertion. Other operators like replace, delete, and get use the intent list in a similar manner. Fig. 13 is a diagram of transactions. This object-oriented operating system, according to the present invention, is transaction oriented. Thus, the executor determines a transaction, and the table access machine operates with recognition of the transaction. This allows operations to be broken down into small pieces for concurrency and integrity. Also, it can be user transparent or implicit, except for transfer call and execute functions. At transaction start, as shown in Fig. 13, a first rule is CALL at point 300. This rule is executed until a second rule is called at point 301. This second rule is implemented by the executor with recursive calls to the table access machine. The second rule may call a third rule at point 302. This third rule executes until it completes operation and issues a return to the second rule at point 303. The second rule then completes execution and issues a return to the first rule at point 304. The first rule then completes execution and ends. Conceptually, the first rule could be considered a session manager in which it presents menu screens to an operator. The operator then invokes a second rule to perform a given application. This second rule may include any number of transaction levels as required by the given application. The table access machine also executes a two-phase lock protocol for objects in the data base. All locks are cleared at transaction end and cannot be made "weaker" during a transaction. The locks are requested by the table access machine and granted, or not, by the native access method server, such as the table data store or the server of other types of data sources. The table access machine also builds selection strings for transfer to a selection evaluator which is associated with the table data store server or other native data store server. Selection evaluator is called by a server. The server supplies a row from a table, along with a selection string. There are no data definitions passed to the selection evaluator. The selection evaluator carries out the selection operation. Results of the selection are returned to the table access machine by the appropriate server. To access the various tables, screens, and other subsystems in the subject data processing system, object code routines in the VSM build @TAM arguments, which are forwarded to TAM on the VSM <---> TAM interface. TAM communicates various requests to the Servers which perform the actual accessing of the tables, screens, etc. The Servers may in turn forward requests to the Selection Evaluator, which performs evaluations and returns responses to the Servers. The Servers then return data or answers to TAM, which forwards them to the VSM object code routines. TAM <--> Server Interface TAM communicates with the various entities of the computing environment via Servers. There are Servers for screens, IMS, import/export, native table data store, and other entities. TAM sends a request to a Server, followed by one or more data strings. A data string begins with a 1-byte length indicator if the string is less than 128 bytes in length (bit 0 clear), or a 2-byte length indicator (bit 0 set) if the string is greater than 127 bytes in length. Table 64 lists the valid Server requests which TAM may make: (Valid TAM --> Server Requests) Code Name C Ctable request R Release locks G Get N Next P Next Parameter Tables 65 - 69 give detailed information about the data contents of the request and return communications for each of the request codes: TAM --> Server C request: Byte Offset Length Contents 0 2 Message length 2 4 Transaction ID 6 1 Request code 7 3 Type of data 10 variable Tablename, UserID, and GroupID data strings Server --> TAM C return: Byte Offset Length Contents 0 2 Message length 2 2 Return code 4 4 Transaction ID 8 variable CTable data string TAM --> Server R request: Byte Offset Length Contents 0 2 Message length 2 4 Transaction ID 6 1 Request code Server --> TAM R return: Ryte Offset Length Contents 0 2 Message length 2 2 Return code 4 4 Transaction ID TAM --> Server G request: Byte Offset Length Contents 0 2 Message length 2 4 Transaction ID 6 1 Request code 7 1 Lock 8 variable Tablename, Parameters, and Key Value data strings Server --> TAM G return: Byte Offset Length Contents 0 2 Message length 2 2 Return code 4 4 Transaction ID 8 variable Table Row data string TAM --> Server N request: Byte Offset Length Contents 0 2 Message length 2 4 Transaction ID 6 1 Request code 7 1 Lock 8 variable Tablename, Parameters, Last Row, Selection, Keyname Ordering, and Keyname ¦op¦ Value data strings Server --> TAM N return: Byte Offset Length Contents 0 2 Message length 2 2 Return code 4 4 Transaction ID 8 variable Table Rows data string TAM --> Server P request: Byte Offset Length Contents 0 2 Message length 2 4 Transaction ID 6 1 Request code 7 1 Lock 8 variable Tablename, Last Parameter Set, and Selection data strings Server --> TAM P return: Ryte Offset Length Contents 0 2 Message length 2 2 Return code 4 4 Transaction ID 8 variable Parameter Sets data string Additionally, an Update (synchronization) message may be sent by TAM. Table 70 gives detailed information about the data content of that message and its return answer: Server --> Selection Evaluator Interface TAM may send to the Servers various messages which include information which must be forwarded to the Selection Evaluator for processing. The Selection Evaluator expects the following registers to contain the indicated data: Register Contents 15 address of entry point into Selection Evaluator 13 address of standard save area 01 address of parameter list The parameter list contains, at the given byte offsets, the following information: Byte Offset Contents 0 address of selection string 4 address of table row to be compared 8 address of 4080-byte work area A selection string consists of a 2-byte length field followed by one or more selection terms in postfix notation. Each selection term begins with a 1-byte opcode, and can be grouped into the categories listed in Table 71: (Selection Terms) Term Type Function Operator 1-byte arithmetic, relational, or logical opcode Field reference 1-byte opcode ('44') followed by a 6-byte field descriptor and a 2-byte field number Offset reference 1-byte opcode ('4C') followed by a 6-byte field descriptor and a 2-byte offset of the field in the row. Used for Temporary, Import, Screen, and IMS tables. Value 1-byte opcode ('48') followed by a 6-byte value descriptor and a value. Values begin with a 1-byte (length < 128, bit 0 clear) or 2-byte (length > 127, bit 0 set) length prefix. Detailed information about the contents of the field and value descriptors, and selection string term opcodes are given in Tables 72 and 73, respectively: (6-byte Field and Value Descriptors) Byte Offset Length Contents 0 1 semantic type 1 1 data syntax 2 2 maximum data length 4 2 number of decimal places (Hexadecimal Opcodes) Opcode Function (Arithmetic) '04' + (addition) '08' - (subtraction) 'OC' * (multiplication) '10' / (division) '14' ** (exponential) '18' - (unary minus) '1C' ¦¦(concatenation) (Relational) '20' = (equal) '24' ¬= (not equal) '28' (less than) '2C' <= (less than or equal) '30' > (greater than) '34' >= (greater than or equal) (Logical) '38' & (and) '3C' ¦ (or) '40' ¬ (not) (Other) '44' R (reference to i'th field in row) '48' V (value) '4C' O (reference to field at offset in row) '54' @ (parameter replaced in selection and always evaluates to true) 'BC' LIKE (relation) The LIKE relational operator has two operands: a field name and a pattern. The result of the relation is true if the value of the field in the row matches the pattern. The pattern may include the wildcards: '*' (any string of zero or more characters) and '?' (any one character). The table row passed by address to the Selection Evaluator may be in either of two formats: Expanded Variable Length Row format (used with Temporary, Import, Screen, and IMS tables), or Variable Length Row format (used elsewhere). Variable Length Row format begins with a 2-byte row length designator, which gives the length of the entire row including the designator itself. Appended to the row length designator are one or more fields, the first field being a "key" field. Each such field begins with a 1-byte field length indicator if the field length is less than 128 bytes excluding the length indicator (bit 0 clear), or 2-byte field length indicator if the length is greater than 127 bytes (bit 0 set). The value of the field is represented in the given number of bytes. Blanks are removed from the right end of all fields containing syntax characters. If a requested field is not present in the row, the field is assumed to have a null value. Expanded Variable Length Row format differs slightly, in that no row length designator is used. Also, fixed numbers of bytes are reserved for each field's value, with the length indicator for each field merely indicating how many of those bytes are actually used. The final difference is that if a requested field is not present in the row, the row is not selected unless the field is being compared with a null string. Selection Evaluator --> Server Interface Upon return, the Selection Evaluator will pass, to the Servers, the following return codes in register 15: Return Code Reason 0 row matches selection criteria 4 row does not match selection criteria 8 error detected If an error is detected, the Selection Evaluator will return, in the work area, the data shown in Table 74. (Selection Evaluator Return) Byte Offset Length Meaning 0 2 error code where: Code Meaning 1 overflow 2 underflow 3 conversion 4 division by zero 5 result of exponential operation is imaginary 2 2 error message length indicator 4 variable error message text The error message text may contain strings of the form "%Fnnnn", where "nnnn" is a four-digit field number for Variable Length Row format or a four-digit field offset for a fixed length format row. For example, an error message may read 'TYPE ERROR, FIELD "%F0010" COMPARED WITH TYPE IDENTIFIER' and the calling entity may then substitute the appropriate field name into the string in lieu of "%F0010". Thus, a table access can be summarized as beginning with a table access statement in the object code of the executor. The object code thus includes the code which builds the @TAM arguments, and constants to be used by such arguments. This code can be rebuilt individual form at the source level by the rule editor. The table access statement in object code is then translated into a @TAM opcode format. This includes the opcode, table names, parameters, selection strings, ordering information onto the stack. The values are stored in descriptor format, and based on data management data structures. The accesses are made using field names. Next, the TABLE ACCESS MACHINE INTERFACE is executed, which the opcode, table name, parameters, selection strings, and ordering are set out in machine code format. The server then performs a necessary communication with the native data store to return the identified occurrences. The selection evaluator is invoked by the server when appropriate. VIII. Dynamic Binding The dynamic binding carried out in the virtual stack machine can be summarized as implementation with three major steps. First, the rule object code is implemented with a modifiable data area. Second, the executor builds hash tables for rules and tables by which objects to be bound can be quickly found. Third, at a log on of a session, the virtual stack machine buffers fixed data, including rules from system and installation libraries, and the fixed table definitions for the dictionaries and the like. Objects in a rule which are not bound include rules or data stored in tables. When a rule calls another rule by a name, the rule will include an offset to a position in the modifiable data area of the rule at which the value of the rule to be called can be stored. The executor then searches for the rule by name through the hash tables and its buffers. If it is not found in the hash tables, then a call to the table access machine is executed to insert the buffer and the proper values in the hash tables. When the buffer for the rule to be called is found by the executor, then the value of the buffer is inserted into the modifiable portion of the rule. Similarly, when an object is called by a table and field name by a rule, that table is found by the virtual stack machine through hash tables or libraries. The value for the occurrence of the table in which the called field is found is then stored in the modifiable portion of the rule at the specified offset. All objects in the data base can be bound at execution time using these formats. In fact, the rule binding is the same as the table.field binding, except that the table in which a rule is found is determined by searching a predetermined set of libraries as described above. At log on of a session, a good number of the data tables from the table data store and other storage systems associated with the host can be retrieved into the virtual stack machines buffers to minimize the calls through the table access machine and the native data stores during execution of a rule. IX. Table Data Store The table data store is the native access method for the object-oriented operating system according to the present invention. Data in this system is stored in a balanced structure where each table is ordered on the primary key within the table. The basic structure of the access method implemented by the table data store is shown in Fig. 14. The table data store consists of a table index page 500. The table index page includes a list of table names with pointers to a parameter index page 501 where appropriate. Other entries in the table index page point directly to a primary key index 503. Entries in the parameter index page 501 point to a primary key index 502. The primary key index is a balanced binary tree 507 based on the B+ tree organization ordered on the primary key (where index 502 is the top of the tree). The bottom of the B+ tree points to the actual link to data pages which store occurrences for the table. The parameter index 501 or the table index page 500 can point also to a secondary key index 508. Secondary key index 508 points to lower sets 509 of indexes on a secondary key based on the B+ key structure. These indexes point to the same linked data pages 510 as the B+ tree on the primary key 507. The page layout for all page types is shown in Fig. 15. The page includes a 32 byte header 520. The header consists of the page number 521, a previous page pointer 522, a next page pointer 523, a page type indicator 524, and a date value 525. A time stamp 526 is included, as well as a transaction I.D. 527, a check point number 528, the number of rows in the page 529, and the size of each row in the page 530. A data area 531 consists of 4,051 bytes of raw data. At the end of the page, a 13 byte record definition area is reserved for VSAM INFORMATION 532 used by the host system. A record within the page stores a row of the table in the format as follows: LL/l/value/l/value/l/value...l/value/l/value, where LL is equal to a 2 byte length of the row, l is equal to a 1 or 2 byte length for the value, and the value is the actual data entry or a null value. A primary index record is of the following format: value/pointer. A secondary index record is of the format: secondary key value/primary key value/pointer. A group index entry is of the format: parm/parm/.../pointer. The B+ tree automatically balances the data structure. Whenever more than two entries in a given level of the tree are created, the B+ tree automatically implements an independent level of the tree according to well know techniques. Accordingly, searches for occurrences in a table by the primary key are carried out very quickly. For searches on non-ordered fields, an entire table must be retrieved to the server for selection or the table access machine for creating a sub-view or the like. X. Translator/Detranslator The creation and modification of rules is accomplished through the interaction of three components: an editor, a translator and a detranslator. The rule editor is an interactive tool which allows a user to make textual changes to a rule's contents. The translator converts a rule's textual image into "virtual machine code", which may be executed by HOS's virtual machine. The detranslator performs the inverse transformation: it produces a textual representation of a rule from its virtual machine image. When a rule is edited, the detranslator transforms its virtual machine code into a tokenized representation of the source. The composer then takes the tokenized form and builds the line by line textual representation of the rule. This is what is displayed to the user by the editor. As changes are made by the user, the scanner performs lexical analysis of the lines that are updated. Lexical analysis converts the lines into their tokenized form, ready for input to the translator. When the user saves a rule, the translator parses it and, if valid, generates the virtual machine code for the rule. Once the rule is saved in its executable form, the rule's text is discarded. Fig. 16 provides an overview of the translator/detranslator system. It operates by scanning the rule source code 600 which translates it into a tokenized source code 601. The translator then generates the virtual machine code 602. Virtual machine code is detranslated to a tokenized source code 603 when required by a user for editing. It is supplied back to the user by a composer as rule source code. The tokenized source code consists of data stored in a TOKENS table built by the detranslator and parameterized by logical line. A lexical analyzer is used to modify the TOKENS table. This table consists of the following fields: INDEX-the primary key; STRING-the text of the token; TYPE-the type of token; LEN-the character length of the token; DEC-the number of digits to the right of the decimal points of the token; TABREF.MARK-indicates whether this token is part of a table.field reference. Tokens which are too long for the TOKENS table are stored in the LONG_STRINGS table. This includes tokens greater than 16 characters in length, quoted strings, and large numbers. This table is comprised of the following fields: INDEX-primary key; STRING-test of token; TYPE-type of token; LEN-character length of the token; DEC-the number of digits to the right of the decimal point. This LONG_STRING table is identified in the TOKENS table by inserting in the TYPE field "long string", and by inserting an index to the LONG_STRINGS table in the LEN field. A typical TOKENS table and LONG_STRINGS table are shown in Fig. 17. The logical line is shown at 1000, and the tokenized representation of the line is shown in the tables 1001, 1002. The translator converts tokenized representation of the rule into virtual machine code. It performs context free syntax checking and some context sensitive syntax checking. Further, it generates the threaded object code suitable for run time binding. The algorithm structure is written in rules of the object-oriented operating system. It performs a top down recursive descent translation algorithm in which the language syntax is hard coded in conditions. The translator converts the infix form of the rules language to the postfix form of the object language suitable for execution on the stack machine. Finally, it saves information about rule data. The translator generates the rule object code in the format described above. The modifiable data area is actually allocated only at run time. A rule may contain a variety of data types including constants that are literally in the program or generated by the translator, exception handler names, global (unbound) names, such as a rule or local variable, and table.field names. The translator maintains a table for each kind of data item called CONSTANTS, EXCEPTIONS, SYMTAB, and TABLE_REFS, respectively. It also maintains a table known as RELOCATION which allows dummy references to data items to be resolved by the translator. The CONSTANT table contains the following fields: INDEX_PRIMARY KEY; STRING_TEXT OF CONSTANT; TYPE-semantic type (always "S"); SYNTAX-internal representation; LEN-number of bytes; DEC-number of digits to the right of the decimal point; REAL_ADDRESS-offset to the constant. Thus, the object code format of a constant can be generated from the CONSTANTS table. The SYMTAB table contains information about rules, such as local variable, parameters, and global unbound names directly or indirectly called in the rule. The field of the SYMTAB table include: IDENT-primary key (identifier name); KIND-parameter, local or global; REF-the offset to the data item (negative if an indirect name); INDEX-unique within the kind (key). Again, the parameters and locals and globals in the object code format can be generated from the SYMTAB table. The TABLE_REFS table contains information about the table.field references. The fields include the following: INDEX-primary key; TABLE_NAME-name of the table (quote if indirect); FIELD_NAME-name of the field (quote if indirect); REAL_ADDRESS-the offset to the data item (negative if either name is indirect). Again, it can be seen that the table.field references within the object code can be generated from this table. The RELOCATION table contains information about "dummy" references to data objects. It is used to resolve these references once data objects have been allocated. The RELOCATION table includes the following fields: OCCUR_ADDR-the offset to the dummy reference; TYPE-the kind of dummy reference (constant, global, ...); TABLE_INDEX-the index to the corresponding table; OFFSET-this contains the offset to the object. Once all data objects have been allocated, the translator traverses the RELOCATION table and resolves all references to these objects. The EXCEPTION table contains the following fields: EX_NUM-the primary key; IDENT-the exception name; FORALL#-the loop number if within an until clause, otherwise zero; OFFSET-the offset decode for the handler; TABLE NAME-the associated table name if any; TABLE OFFSET-the offset to the name of the table in object code. The exception handler names are generated from the EXCEPTIONS table. If the handler has an associated table name, it can also be generated from this table. The detranslator converts the virtual machine code to tokenized representation of the rule language. The algorithm structure is written in the rules within the object-oriented operating system. It converts the post-fix form of the object level representation to the in-fix form of the rule language. The detranslator is table driven. Most of the object code structure is encoded in tables which are referred to by the detranslator. The detranslator uses several stacks for the conversion from post-fix to in-fix. The detranslator is essentially a stack machine. Rather than executing post-fix code, it transforms it into its tokenized representation. Each opcode causes the manipulation of the detranslator internal stacks. Once the opcode for the left hand side of a statement is reached, contents of the stack or stacks are moved to the tokens table. The detranslator maintains three stacks which operate in harmony and are identified in corresponding tables. The first is the DET.STACK table. Objects are pushed onto this stack as each opcode is visited. It is a marked stack, as stack elements between the markers are moved as one element. The DET.STACK table includes the following fields: INDEX-primary key; TYPE-the detranslation type (parameter, constant, table.field reference, ...); OFFSET-the offset to the data item in the static data area; ITEM-symbol text; SUBTYPE-this is the translator type (operator, reserved word, ...). The next table is known as the DET_PRED_ST table. This table contains the precedence of an item of the DET.STACK. The fields include the following: INDEX-primary key; PRECEDENCE-the item's precedence. The final stack is identified in the DET_WORKAREA table. Items are placed in this table from the DET_STACK table. It helps during conversion of post-fix to in-fix. Items are transferred from the DET_WORKAREA back to the DET_STACK. It contains the same fields as the DET_STACK table. An example of detranslation is shown in Figs. 18 - 21E. The source for the detranslation is L=1+2; the object is as follows: 1:@CONST 1, 2:@CONST 2, 3:@ADD, 4:@SETL L. The first step is shown in Fig. 18. The value "1" is inserted into the DET_STACK with the marker X indicating a boundary element. The DET_PRED_ST is filled with the precedence 10 of the first data element. The DET_WORKAREA is empty. The next step is shown in Fig. 19. In this step, the value constant 2 is entered as an element of the DET_STACK and its precedence is set on the DET_PRED_ST as a value 10. The work area remains empty. In the next step, the @ADD operator is encountered. In this step, the constant 2 is moved to the work area and its precedence removed from the stack. Next, the "+" operator is added to the work area stack as part of the element including the constant 2. Finally, the constant 1 is moved from the stack to the work area as part of the same element as the constant 2 and the "+". These steps are sequentially shown in Figs. 20A, 20B, and 20C. In Fig. 21A, the contents of the work area are transferred back to the main stack. In Figs. 21B, 21C, 21D and 21E, the TOKENS table is filled. This happens in response to the @SETL opcode which inserts the first and second tokens as shown in Fig. 21B. Thus, the fields of the tokens table for the first token is filled with the value L, an identifier I. It is a one character string with zero positions to the right of the decimal point. It is not a part of the table.field reference. The next entry 2 is the equal sign. It is an operator with one character length with no points to the right of the decimal point and not part of a table.field reference. The contents of the stack are then reordered into the work area to the in-fix form as it appears in the rule language as shown in Fig. 21C. Finally, the balance of the TOKENS table is build from the work area entries as shown in Fig. 21D. The 1 is taken from the top of the work area and inserted as the third entry in the TOKENS table. The "+" operator is inserted as the fourth entry. The constant 2 is inserted as the fifth entry, and the ";", which is the end of the line, is inserted as the last entry. After this occurs, the stack, the precedence table and work area are cleared as illustrated in Fig. 21E. This translator/detranslator pair provides the means of maintaining only one single version or representation of any given program on the computer. Only the object code version of a program is stored on the secondary storage medium. The source code version is only generated on demand by the detranslation from the object code. The object code version includes all information necessary to assist the detranslation process. This provides significant maintenance cost reductions because no effort is spent to secure synchronization of source and object code versions of programs. The need for secondary storage is reduced as only the object code version need be stored. Further, several translator/detranslator pairs could be implemented so that the same program can be represented differently for different programmers. Use of this technique would allow implementation of the rule language in a variety of different spoken languages like French, German or English, different graphical or diagrammatic representation of program structures, or in different communication media, for instance, visual or sound.

Claims: A programmable computer for executing a rule, said computer comprising: means for storing the rule, the rule including, i) a rule name for the rule, and a set of parameters through which the computer provides input data to said rule, ii) a set of conditions in the form of logical expressions each having a boolean value of true or false, one of said boolean values being a first boolean value and the other being a second boolean value, and iii) a set of actions in the form of executable statements; iv) means for associating each given condition from said set of conditions with a given ordered subset of actions from said set of actions; and said computer further comprising: means, coupled to said means for storing, for sequentially evaluating said set of conditions for the respective boolean values of said conditions in said set during execution of said rule by said computer, and for indicating said boolean values; and means, responsively coupled to said means for evaluating, for executing, in order, a first subset of actions in response to indication of said first boolean value for the given condition associated with said first said subset of actions by said means for evaluating. The computer of Claim 1 wherein: said rule further includes a set of exceptions, each exception including an exception name coupled with a set of exception actions; and said computer further comprises: means, coupled to said means for executing, for detecting exception events during execution of said rule and for returning, responsive to detection of a given exception event, an exception name identifying the given exception event; and means, coupled to said means for storing and responsive to return of a given exception name by said means for detecting, for executing, upon said given exception name being the name of an exception in said set of exceptions, the set of exception actions coupled with said given exception name. The computer of Claim 2, wherein said means for associating comprises delineation of conditions and actions by object coded rule instructions including: each condition, which has associated actions, including a series of instances of a first instruction (@ACALL), execution of each of which causes execution to jump to a given action associated with the current condition; each action terminating in a second instruction (@ARETURN), execution of which causes execution to return from execution of the given action to a point at which the given action was invoked; a fifth instruction (@SIGNAL), execution of which creates an exception event and specifies a name of the created exception event. The computer of Claim 2, wherein said computer includes means for operating a virtual stack according to object coded instructions stored in an Opcode Table within said computer. The computer of Claim 2 further comprising: text entry means for entering text of said rule, including text of said rule name, parameters, conditions, actions, and exceptions; means for grammatically evaluating said text of said rule, and indicating grammatical correctness or incorrectness of said rule; means, responsive to indication by said means for grammatically evaluating of grammatical correctness, for generating an executable representation of said rule; wherein said means for grammatically evaluating evaluates said text of said rule according to the grammar which is defined in Backus-Naur Form according to the Syntax of Rules Table of the above detailed description. The computer of Claim 1 further including: text entry means for entering text indicating said rule name, set of parameters, set of conditions, and set of actions; display means, coupled to said text entry means, for displaying, i) said text indicating said rule name and set of parameters as a row of text, ii) text indicating each condition from said set of conditions as a row of condition text and the respective rows of condition text as a column, and iii) text indicating each action from said set of actions as a row of action text and the respective rows of action text as a column; means, coupled to said text entry means, for generating and displaying a yes/no quadrant of boolean indicators arranged in yes/no rows each uniquely associated with one row of condition text and the corresponding condition, and in yes/no columns each including exactly one indicator of said first boolean value, said exactly one indicator being in a yes/no row uniquely coupling one of said conditions with the given yes/no column such that each condition is uniquely coupled with exactly one yes/no column; and means, coupled to said means for generating, for accepting and displaying action sequence numbers in an action quadrant arranged in action rows each uniquely associated with one row of action text and its action, and in action columns each uniquely associated with one yes/no column and the condition coupled with that one yes/no column, each given action sequence number associating, through its action row and action column, a given action with a given condition, the action sequence numbers within each action column defining the subset of actions associated with the given condition, and the order of execution of the subset of actions being defined by the relative magnitudes of the action sequence numbers in the given action column. The computer of Claim 6 further comprising: said text entry means being further for entering text indicating a set of exceptions, each exception including an exception name coupled with a set of exception actions, and said display means is further for displaying text of said set of exceptions; means, coupled to said means for executing, for detecting exception events during execution of said rule and for returning, responsive to detection of a given exception event, an exception name identifying the given exception event; and means, coupled to said means for storing and responsive to return of a given exception name by said means for detecting, for executing, upon said given exception name being in said set of exceptions, the set of exception actions coupled with said given exception name. The computer of Claim 1, wherein said means for associating comprises delineation of conditions and actions by object coded rule instructions including: each condition, which has associated actions, including a series of instances of a first instruction (@ACALL), execution of each of which causes execution to jump to a given action associated with the current condition; each action terminating in a second instruction (@ARETURN), execution of which causes execution to return from execution of the given action to a point at which the given action was invoked. The computer of Claim 8 wherein said object coded rule instructions further include: a third instruction (@AEND), which is not executed, but which separates said set of conditions from said set of actions. The computer of Claim 8, wherein said computer includes means for operating a virtual stack which holds data including a first datum (D1) and a second datum (D2), and executes object coded arithmetic instructions upon data in said virtual stack, said arithmetic instructions comprising: a sixth instruction (@ADD), execution of which pops two data from said virtual stack, performs addition upon said two data, and pushes the sum onto said virtual stack (D2 + D1); a seventh instruction (@SUB), execution of which pops two data from said virtual stack, performs subtraction upon said two data, and pushes the difference onto said virtual stack (D2 - D1); an eighth instruction (@MULT), execution of which pops two data from said virtual stack, performs multiplication upon said two data, and pushes the product onto said virtual stack (D2 * D1); a ninth instruction (@DIV), execution of which pops two data from said virtual stack, performs division upon said two data, and pushes the quotient onto said virtual stack (D2 / D1); a tenth instruction (@EXP), execution of which pops two data from said virtual stack, performs exponentiation upon said two data, and pushes the result onto said virtual stack (D2 A D1); an eleventh instruction (@UNM), execution of which pops one data from said virtual stack, and pushes the unary negative of said one data onto said virtual stack (-(D1)); and a twelfth instruction (@CAT), execution of which pops two data from said virtual stack, performs concatenation upon said two data, and pushes the concatenated result onto said virtual stack (D1D2). The computer of Claim 8, wherein said computer includes means for operating a virtual stack which holds data, and executes object coded relational instructions upon data in said virtual stack, wherein: to execute a given relational instruction, said computer pops a first datum (D1) and a second datum (D2) from said virtual stack and compares said first datum to said second datum according to a relation specified by said given relational instruction, and pushes onto said virtual stack a boolean indicator of the result of the comparison, said relational instructions comprising: a thirteenth instruction (@EQ) comparing said second datum to said first datum for equality (D2 = D1); a fourteenth instruction (@NE) comparing said second datum to said first datum for inequality (D2 -= D1); a fifteenth instruction (@LT) comparing whether said second datum is less than said first datum (D2 < D1); a sixteenth instruction (@LE) comparing whether said second datum is not greater than said first datum (D2 <= D1); a seventeenth instruction (@GT) comparing whether said second datum is greater than said first datum (D2 > D1); an eighteenth instruction (@GE) comparing whether said second datum is not less than said first datum (D2 >= D1). The computer of Claim 8, wherein said computer includes means for operating a virtual stack which holds data including a first datum (D1) and a second datum (D2), and further executes object coded logical instructions upon data in said stack, said logical instructions comprising: a nineteenth instruction (@AND), execution of which pops first and second data from said virtual stack, and pushes onto said virtual stack a boolean indication of whether both first and second data have values of true (D2 & D1); a twentieth instruction (@OR), execution of which pops first and second data from said virtual stack, and pushes onto said virtual stack a boolean indication of whether either first or second data, or both, has a value of true (D2 v D1); a twenty-first instruction (@NOT), execution of which pops a first datum from said virtual stack, and pushes onto said virtual stack the boolean inverse of said first datum (HD1). The computer of Claim 8, wherein said computer includes means for operating a virtual stack and maintains a database having tables, and includes a table access method TAM which accesses a table named in a TAM parameter list, and said computer further executing object coded table control instructions upon said database, said table control instructions comprising: a twenty-second instruction (@WFIELD) identifying a given field within a given table, execution of which pushes said given field of said given table onto said virtual stack; a twenty-third instruction (@RFIELD) identifying a given field within a given table, execution of which, i) pushes said given field of said given table onto said virtual stack if contents of said given field contain an assigned value, but ii) causes an exception event to occur, if contents of said given field contain no assigned value, signalling said computer that although said given field has no assigned value an attempt was made to use the contents of said field; a twenty-fourth instruction (@SET) identifying a given field within a given table and specifying a value, execution of which assigns said value to said given field of said given table; a twenty-fifth instruction (@ABN) giving a field name and a value, execution of which assigns said given value to each field having the given name, in all tables which include a field having the given name; a twenty-sixth instruction (@TAM) specifying a table access request, execution of which calls the table access method to perform the specified table access request; a twenty-seventh instruction (@TAMP) specifying a reference parameter, execution of which inserts the specified reference parameter into the TAM parameter list; and a twenty-eighth instruction (@TAMN) specifying a table name, execution of which inserts the specified table name into said TAM parameter list. The computer of Claim 1 further including: said means for storing further storing a plurality of the rules where in each rule of said plurality of rules said set of conditions is an ordered set of conditions in the form of boolean expressions, said ordered set of conditions including a last condition which is a default condition which is not in the form of a boolean expression and where each condition has a correspondence with one said set of actions; a virtual stack; means for pushing data onto and popping data off from said virtual stack, wherein said data may be parameter data to a rule and may be null data; calling means, coupled to said means for pushing and popping, for calling a next rule for execution (upon @CALL); input means, responsive to said calling means, for popping said first datum from said virtual stack into a first input parameter of said next rule, any current rule becoming a previous rule and said next rule becoming a current rule; said means for evaluating, depending upon contents of said current condition, performing, i) a comparison of said first datum to a value (upon @EQ, @NE,@LT, @LE, @GT, or @GE), ii) an invocation of said calling means to evaluate a rule as a next rule, execution to return to the current rule upon completion of the next rule (upon @CALL), and/or iii) performance of arithmetic (upon @ADD, @SUB, @MULT, @DIV, or @EXP); said means for executing sequentially executing the set of actions which corresponds to the current condition contingent upon the current condition evaluated to said first boolean value, wherein execution of each action may include, depending upon contents of said each action, i) performance of arithmetic (upon @ADD, @SUB, @MULT, @DIV, @EXP), ii) invocation of said means for pushing and popping to push or pop a datum onto or from said virtual stack, iii) performance of relational operations (upon @AND, @OR, or @NOT), iv) invocation of said calling means to evaluate a rule as a next rule, execution to return to a current point of action execution within the current rule upon completion of the next rule (upon @CALL), and/or v) invoking said calling means, to terminate execution of the current rule and return execution to a previous rule (upon @RETURN) ; means, coupled to said means for evaluating and said means for executing, for indicating said default condition as the current condition if all other conditions in the set evaluate to said second boolean value, to stimulate execution by said means for executing of the set of actions corresponding to said default condition, said default condition always evaluating to said first boolean value; and means, responsive to said means for evaluating, for invoking said calling means, to terminate execution (upon @RETURN) of the current rule and return execution to a previous rule after one condition in the rule has evaluated to said first boolean value and that condition's actions have been executed. The computer of Claim 14 wherein exception events occur in said computer, and wherein each rule further includes 4) an optional plurality of exception handlers each adapted to handle a given exception event, each exception handler having a name and an optional set of exception actions in the form of executable statements similar in content to the actions in the sets of actions which correspond to the rule's conditions, said computer further comprising: means for detecting an exception event during execution of said actions or evaluation of said conditions; means, responsive to said means for detecting, for checking whether any exception handler in the current rule is adapted to handle the detected exception event; means, responsive to said means for checking, for invoking execution, by said means for executing, of the exception actions of the given exception handler which the checking indicated as being adapted to handle the detected exception event. A method of programming a computer to execute a rule, comprising the steps of: first entering a rule name for the rule, and a set of parameters through which the computer may provide input data to the rule; second entering for the rule a set of conditions in the form of logical expressions each having a boolean value of true or false, one of said boolean values being a first boolean value and the other being a second boolean value; third entering for the rule a plurality of sets of actions in the form of executable statements; first indicating a one-to-one association of each individual set of actions with an individual condition, to specify that upon a given condition evaluating to said first boolean value, the computer is to execute the individual set of actions associated with said given condition. The method of Claim 16 further comprising the steps of: fourth entering a set of exception names; fifth entering a plurality of sets of exception actions in the form of executable statements; second indicating a one-to-one association of each individual set of exception actions with an individual exception name, to specify that upon said computer detecting an exception event and returning an exception name, if the returned exception name is in said set of exception names, said computer is to execute the set of exception actions associated with the returned exception name. The method of Claim 16, wherein: said first indicating step permits one subset of said actions to not be associated with any condition including said default condition, to specify that said computer is never to execute said one subset of actions, to permit said one subset of actions to remain in said rule without affecting execution of said rule. The method of Claim 16 wherein: said step of second entering enters a list of conditions in the form of logical expressions to be evaluated sequentially for their truth value during execution of said rule, said logical expression returning either a true value or a false value; said step of third entering enters a list of actions in the form of executable statements, each coupled with one or more corresponding conditions in said list of conditions, to be executed in response to the evaluation of said corresponding conditions; and said method further comprises the step of: defining in the computer a yes/no quadrant in which for a given condition, the yes/no quadrant includes a column for each sequence of one or more actions to be executed in response to an evaluation which returns a true value, and a column for each sequence of one or more actions to be executed in response to an evaluation which returns a false value, and each action having a sequence number in the column of the given condition indicating its position in the sequence of actions. The method of claim 19 further including the step of: fourth entering an exception name followed by a sequence of actions to be executed in the event that the exception event identified by the exception name is detected by the computer during the execution of a rule. A method of executing a rule by a computer, said rule including: 1) a rule name, 2) a set of conditions in the form of logical expressions each having a boolean value of true or false, one of said boolean values being a first boolean value and the other being a second boolean value, 3) a plurality of sets of actions in the form of executable statements, the individual sets of actions having a one-to-one association with individual conditions; said method comprising the steps of: starting evaluation of said set of conditions at a first current condition; evaluating the boolean value of said current condition; if said current condition evaluates to, i) said first boolean value, executing the set of actions associated with said current condition, and exiting execution of said rule, ii) said second boolean value, making a next condition current, contingent upon there being conditions in said set of conditions which have not yet been evaluated, otherwise exiting execution of said rule; and returning to said evaluating step. The method of Claim 21 wherein said rule further includes: 4) a set of exception handlers, each exception handler having an exception name and a set of exception actions in the form of executable statements, said method further comprising the steps of: detecting an exception event during execution of said rule; specifying to said rule a name of the detected exception event; determining whether the specified name of the detected exception event matches the name of any of said exception handlers; executing the set of exception actions of an exception handler whose name matches that of the detected exception event, if any match is determined to exist; and exiting execution of said rule. The method of Claim 21, wherein said computer includes means for operating a virtual stack which holds data and addresses, and: said substep i) of executing the set of actions associated with said current condition further comprises, for execution of each action in said set, a) executing a first branching instruction (@ACALL) which pushes onto said virtual stack an address of an instruction immediately following said first branching instruction, and which specifies an address of a first action instruction of a given individual set of actions to which execution by computer then branches, b) executing a second branching instruction (@ARETURN), which pops from said virtual stack said address of said instruction immediately following said first branching instruction, to which address execution by said computer then returns; said first and second branching instructions permitting a single copy of a given action to be in the sets of actions associated with a plurality of conditions. The method of Claim 23 wherein said substeps of exiting execution of said rule include the step of: executing a third instruction (@RETURN), execution of which causes execution to return from said rule to a point of execution at which said rule was invoked; and the actions associated with the current condition are called by a series of successive first branching instructions (@ACALLs), which series is followed immediately by said third instruction (@RETURN).

Inventors: CHONG DANIEL T, KNUDSEN HELGE, PLAZAK ZBIGNIEW, ROBERTSON MICHAEL, TAUGHER JAMES, YAFFE JOHN, CHONG, DANIEL T., KNUDSEN,HELGE, PLAZAK, ZBIGNIEW, ROBERTSON,MICHAEL, TAUGHER,JAMES, YAFFE,JOHN
Applicants: AMDAHL CORP, AMDAHL CORPORATION
Application year: 1990
Main classification: G06F9
IPC codes: G06F9, G06F17