Program Control Instructions

The execution of a PL/B program begins at the first executable instruction. It continues sequentially until a Program Control Instruction alters the sequence, the program terminates or it is interrupted to execute another program.

Exceptions	Describes program exception handling.
FUNCTIONs	Provides an overview of functions.
Trappable Events	Describes trappable events.
BRANCH	GOTO one of several execution labels based on an index.
BRANCHF	GOTO one of several execution labels based on a function key.
BREAK/CONTINUE	Exit the inner most LOOP/REPEAT structure.
CACHELIST/CACHELISTEND	Control caching of PLFORMs.
CALL/CALLS	Save current execution address and call an execution label.
CHAIN	Terminate currently executing program and start another program.
CLEAREVENT	Discard the next event on the event queue.
DEBUG	Enter the debugger if debugging is active.
DSCNCT	Perform a CHAIN to the ANSWER program.
ENTRY	Designate the end of function parameters.
EVENTCHECK	Check to determine if any objects have been acted upon.
EVENTCLEAR	Clear an event from the event queue.
EVENTCOUNT	Retrieve the number of events currently on the event queue.
EVENTINFO	Retrieve information regardig an event on the event queue.
EVENTQUEUE	Manipulate the event queue.
EVENTWAIT	Wait on an object event to occur.
EXCEPTCHECK	Test for an exising exception handler.
EXCEPTCLEAR	Clear an exception handler.
EXCEPTDISABLE	Suspend an exception handler.
EXCEPTDO	Trigger an exception handler.
EXCEPTENABLE	Restore an exception handler.
EXCEPTSET	Set an exception handler.
FLAGRESTORE	Restore the flags from a variable.
FLAGSAVE	Save the flags in a variable.
FOR	Structured logic for looping.
FRESTORE	Restore the flags from an internal flag stack.
FSAVE	Save the flags in an internal flag stack.
FUNCTION	Defines a function.
FUNCTIONEND	End a function definition.
GOTO	Go to an execution label.
IF/ENDIF	Structured logic for testing that also allows ELSE and ELSE IF instructions.
LFUNCTION	Defines a local function.
LOADMOD	Load separately compiled subroutine.
LOOP/UNTIL/WHILE/REPEAT	Structured logic for looping that also allows UNTIL and WHILE instructions.
LROUTINE	Local routine that may be called with parameters.
NOBREAK	Continue execution into next CASE, WHEN, or DEFAULT statement.
NORETURN	Discard return address from last CALL, TRAP, or EVENTWAIT instruction.
PAUSE	Wait for a period.
PERFORM	CALL one of several execution labels based on an index.
PERFORMF	CALL one of several execution labels based on a function key.
PROCEDURE	External routine that may be called with parameters.
RETCOUNT	Retrieve the number of return address on the return stack.
RETURN	Return execution to the next address on the return stack.
ROUTINE	External routine that may be called with parameters.
SELECT	Define a multi-way branch.
SETFLAG	Set the state of the condition code and function key flags.
STOP/DETACH	Stop program execution. Return to MASTER program or to the OS.
SWITCH	Define a multi-way branch.
TRAP	Set an event trap to an address. Call the address if the event occurs.
TRAPCLR	Clear one or more event traps.
TRAPRESTORE	Clear and restore saved traps.
TRAPSAVE	Save all traps that are set.
TRAPSIZE	Returns the size of the variable required for the TRAPSAVE instruction.

Some Program Control Instructions have the ability to alter the flow conditionally based upon Flags. These flags are set to TRUE or FALSE and are checked as follows:

	GOTO	LABEL IF OVER
	GOTO	LABELX IF NOT EOS

PL/B supports the conventional method of program control, (GOTO, CALL, RETURN) and structured code (IF, ELSE, ENDIF, LOOP, REPEAT, etc.). These two types may be intermixed within the same program.