Apple is a PL/I dialect programming language created by General Motors Research Laboratories for their Control Data Corporation STAR-100 supercomputer.
Apple is a subset of full PL/I, but has been extended to integrate with the Associative Programming Language (APL â not to be confused with APL (programming language) )
An <code>OFFSET</code> variable identifies the relative location of a based variable in the containing file. A <code>DESCRIPTOR</code> variable resembles a pointer variable, but also contains the length of the associated based variable in addition to its address.
The <code>LOCK</code> statement puts a program into "locked status", where all asynchronous events will be queued. A corresponding <code>UNLOCK</code> statement puts the program in "unlocked status", capable of processing asynchronous events. All queued events will be processed.
Apple uses the PL/I <code>EVENT</code> 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 <code>SIGNAL</code> statement, rather than the PL/I standard assignment to the<code>COMPLETION</code> pseudovariable. An event can be put into a delay state by the <code>DELAY</code> pseudovariable, and recognition will remain deferred until the program resets the delay state.
Events can be associated with "ON-Units", by the <code>ON EVENT</code> statement, similar to PL/I standard for conditions. The <code>REVERT</code> 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 <code>ONPTR</code> builtin function, which returns the address of an "Event Completion Block", and sets the event to complete.
The <code>REGISTER</code> storage class and the <code>INLINE</code> builtin subroutine allow access to STAR hardware features.
A scalar arithmetic variable can be declared <code>REGISTER [register-specification]</code>. <code>register-specification</code> 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 <code>INLINE</code> 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);
The <code>LITERALLY</code> 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;
Some features of standard PL/I are not included in Apple. The most significant are: