PicBasic program flow control commands

Program flow control commands are important in every programming language since they enable the programmer to make a decision and change the flow of the program based on this decision. PicBasic language supports the following program flow control commands:

BRANCH BUTTON CALL FOR…NEXT

GOSUB…RETURN GOTO

IF…THEN

We shall now see what the functions of these commands are and how to use them in programs.

BRANCH

BRANCH offset, (Label0, Label1,…)

When this command is executed, the program will jump to the program label based on the value of offset. Offset is actually a program value and if offset is zero, the program jumps to the first label, if offset is one, the program jumps to the second label, and so on.

Example:

BRANCH B2, (Lbl1, Lbl2, Lbl3)      ‘ If B2     0 then goto Lbl1

‘ If B2     1 then goto Lbl2

‘ If B2     2 then goto Lbl3

BUTTON

BUTTON Pin, Down, Delay, Rate, Var, Action, Label

This command is used to check the status of a switch. The command operates in a loop and con- tinuously samples the pin, debouncing it and comparing the number of iteration performed with the switch closed. The parameters are

Pin           Pin number (0 to 7). PORTB pins only

Down       State of pin when button is pressed (0 or 1)

Delay        Delay before auto-repeat begins (0 to 255). If 0, no debounce or auto-repeat is performed. If 255, only debounce, but no auto-repeat is performed

Rate          Auto-repeat rate (0 to 255)

Var           Byte variable used for delay/repeat countdown. Should be initialised to 0 before use

Action      State of pin to perform goto (0 if not pressed, 1 if pressed)

Label        Program execution continues at this label if Action is true

Figure 4.1 shows the two types of switches that can be used with this command.

For example, the following command checks for a switch pressed on pin 2 (of PORTB) and jumps to Loop if it is not pressed (this command assumes that the port pin will be logic 0 when the switch is pressed, i.e. the figure on the left in Figure 4.1):

BUTTON 2, 0, 255, 0, B0, 0, Loop

Switches that can be used for the Button command

The following command checks for a switch pressed on pin 2 as above, but jumps to Loop if the switch is pressed:

BUTTON 2, 0, 255, 0, B0, 1, Loop

CALL

CALL Label

This command executes the assembly language subroutine named Label. For example, the com- mand calls to assembly language routine with the name calculate.

CALL calculate

FOR…NEXT

FOR index     Start TO End (STEP (   ) Inc)

(body)

NEXT index

This command is used to perform iterations in a program. Index is a program variable which holds the initial value of the iteration count Start. End is the final value of the iteration count. STEP is the value by which the index is incremented at each iteration. If no STEP is specified, the index is incremented by 1. The iteration repeats until index     End and then execution continues with the next instruction following the NEXT. Index can be a byte (0 to 255), or a word (0 to 65535).

In the following example, the two statements enclosed within the FOR…NEXT are executed 10 times.

FOR    B0     1 TO 10

B1     B1     1

B2     B2     1

NEXT B0

or in the following example, the index is incremented by 2 in each iteration.

FOR B0     1 TO 100 STEP 2

B1     B0     2

NEXT B0

GOSUB…RETURN

GOSUB Label

This program calls a subroutine starting at Label. It is like a GOTO command, but here the program returns when the RETURN statement is reached, and continues with the instruction after the GOSUB. The RETURN statement has no parameters. A subroutine has the following characteristics:

●    A label to identify the starting point of the subroutine

●    Body of the subroutine where the required operation is performed

●    RETURN statement to exit the subroutine and return to the main calling program

Subroutines can be nested in PicBasic where a subroutine can call to other subroutines. The nest- ing should be restricted to no more than four levels deep. In the following example, the subrou- tine labelled INC increments variable B1 by one and then returns to the main program. On return to the main program, the statement B2     B1 is executed.

B0     0

B1     1

GOSUB INC      ‘ Jump to subroutine INC B2     B1             ‘ Subroutine returns here

………

………

INC:                                   ‘ Start of the subroutine B1     B1     1      ‘ Body of the subroutine RETURN           ‘ End of the subroutine

GOTO

GOTO Label

This command causes the program execution to jump to the statement beginning at Label. For example,

GOTO Loop

………

………

………

Loop:

IF…THEN

IF Comp (AND / OR Comp) THEN Label

 

This statement is used to perform comparisons (Comp) in a program. If the result of the compari- son is true then the program jumps to the statement at Label, otherwise execution resumes with the statement following IF…THEN.

A comparison can relate to a variable, to a constant, or to other variables. All comparisons are unsigned and the following comparison operators can be used:

           less than

         less than or equal

           equal

         not equal

         greater than or equal

           greater than

Additionally, logical operators AND and OR can be used in a comparison operation. For example, IF B0     10 THEN CALC      ‘ Jump to CALC if B0     10

…………………..

.............................. CALC:

Another example is given below. In this example, if B2 is greater than 40 and at the same time B3 is less than 20 then the program jumps to the statement at label EXT. Otherwise, execution con- tinues with the statement after the IF…THEN.

IF B2     40 AND B3     20 THEN EXT

………..........

……….......... EXT:

It is important to be careful that only a Label can be used after the THEN statement.