Associative programming features
Other features
AnOFFSET variable identifies the relative location of a based variable in the containing file. A DESCRIPTOR variable resembles a pointer variable, but also contains the length of the associated based variable in addition to its address.
The LOCK statement puts a program into "locked status", where all asynchronous events will be queued. A corresponding UNLOCK statement puts the program in "unlocked status", capable of processing asynchronous events. All queued events will be processed.
Asynchronous processing
Apple uses the PL/IEVENT data type to implement asynchronous processing.
An event variable can be associated with an external action, such as a keypress at the user's terminal, by a system call. Each event has a "delay state" and a "completion state" associated with it. When the event associated with the action occurs, the event becomes complete, and remains complete until the program accesses the information associated with the event. Events can also be marked complete with a SIGNAL statement, rather than the PL/I standard assignment to theCOMPLETION pseudovariable. An event can be put into a delay state by the DELAY pseudovariable, and recognition will remain deferred until the program resets the delay state.
Events can be associated with "ON-Units", by the ON EVENT statement, similar to PL/I standard for conditions. The REVERT statement removes the association. When the event is recognized (complete and not delayed) the On-Unit is executed. This ON-Unit can access system information about the event using the ONPTR builtin function, which returns the address of an "Event Completion Block", and sets the event to complete.
Hardware access
TheREGISTER storage class and the INLINE builtin subroutine allow access to STAR hardware features.
A scalar arithmetic variable can be declared REGISTER egister-specification/code>. register-specification can be an unsigned integer constant 0..255 to specify one of the computer's hardware registers. If it is omitted the compiler will assign a register.
The INLINE builtin subroutine inserts an arbitrary machine-language instruction into the compiled code. Except for the function code (operation code) and subcode of the instructions all operands can be numeric constants, variables, or arithmetic constants.
Example:
CALL INLINE("F8", 5, 0, SOURCE, 0, "20", 0, TARGET);
Text replacement
The LITERALLY specification allows the programmer to specify replacement text to be substituted at compile time. The syntax is:
%DECLARE <identifier> LITERALLY (<parameter-list> ) <character-constant> ;
If <parameter-list> is not specified the compiler replaces all occurrences of <identifier> throughout the program with <character-constant>.If <parameter-list> is specified, the replacement character string is formed by replacing all occurrences of the parameter name with the corresponding argument.
Example:
%DECLARE BITS LITERALLY(A1,A2) 'A1*A2*64';
I = BITS(J,B);
will generate the statement:
I=J*8*64;
Omitted PL/I features
Some features of standard PL/I are not included in Apple. The most significant are:
* The CONTROLLED and AREA storage attributes are not supported. FILE and FILESET have many of the characteristics of AREA.
* List-directed and data-directed input and output are not supported.
* Record input-output statements, including READ, WRITE, REWRITE, LOCATE, and DELETE. DELETE is used as a synonym for FREE when deleting ENTITY variables.
References
{{Reflist
External links
APL: a language for associative data handling in PL/I
PL/I programming language family
Programming languages
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