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Subroutines

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Subroutines Defined

Subroutines are small programs inside the main program performing a well-defined self-contained task.

In other programming languages subroutines are given various names
- Procedure: Pascal
- Function: C, C++, Java, C#, PHP
- Method: Most object oriented languages
- Subroutines: In BASIC they are actually called subroutines

They perform small, well defined tasks like
- providing a time delay
- reading a port
- writing to a port
They are especially useful because
- they can be re-used many times within a program
- they can be re-used in completely different programs
- it's possible to build a library of useful subroutines for use in future programs
Subroutines for computer code are like subsystems for hardware.
- each subroutine is relatively simple and can be tested separately from the main program.
- subroutines are useful for team-work because different team members can be responsible for designing and testing different subroutines.

Examples

A Time Delay

Method One - Simple but not Recommended

Use several NOP commands to create the delay.

CTRL+Click here to run the simulator.

Type in the code below or copy and paste it.
Click the "A" button to assemble the code (translate it into binary).
Click the "S" button to step through the program.

; ===== MAIN PROGRAM =====
START:
        NOP
        NOP
        NOP
        CALL    DELAY
        NOP
        NOP
        JMP     START

; ===== TIME DELAY =======
DELAY:
        NOP
        NOP
        NOP
        NOP
        RET
; ========================

START:	This labels address zero.
NOP	This does nothing for one clock cycle.
CALL DELAY	This copies 7 (the address labelled by DELAY) into the program counter. This also copies 4 (the return address) onto the stack.
DELAY:	This labels address seven.
RET	This copies 4 (the address saved on the stack) into the program counter.
JMP START	This copies 0 (the address labelled by START) into the program counter.

Method Two - A Fixed Time Delay

CTRL+Click here to run the simulator.

Type in the code below or copy and paste it.
Click the "A" button to assemble the code (translate it into binary).
Click the "S" button to step through the program.

; ===== MAIN PROGRAM =====
START:
        NOP
        NOP
        NOP
        CALL    DELAY
        NOP
        NOP
        JMP     START

; ===== TIME DELAY =======
DELAY:
        MOVW    0x10
REP:
        SUBW    0x1
        JPZ     DONE
        JMP     REP
DONE:
        RET
; ========================

START:	This labels address zero.
NOP	This does nothing for one clock cycle.
CALL DELAY	This copies 7 (the address labelled by DELAY) into the program counter. This also copies 4 (the return address) onto the stack.
DELAY:	This labels address seven.
MOVW 0x10	This copies hexadecimal 10 into W, the working register. This sets the delay.
REP:	This labels address eight.
SUBW 0x1	This subtracts one from W and stores the answer back into W.
JPZ DONE	If the status register (SR) zero flag is set, this will jump to address 0xB. The zero flag will be set when W has finally counted down to zero.
JMP REP	This will set the program counter to the address labelled by REP (address eight).
DONE:	This labels address 0xB.
RET	This copies 4 (the address saved on the stack) into the program counter.
JMP START	This copies 0 (the address labelled by START) into the program counter.

Method Three - A Variable Time Delay

CTRL+Click here to run the simulator.

Type in the code below or copy and paste it.
Click the "A" button to assemble the code (translate it into binary).
Click the "S" button to step through the program.

; ===== MAIN PROGRAM =====
START:
        NOP
        NOP
        MOVW    0x15    ; Set the delay
        CALL    DELAY
        NOP
        NOP
        JMP     START

; ===== TIME DELAY =======
DELAY:
        SUBW    0x1
        JPZ     DONE
        JMP     DELAY
DONE:
        RET
; ========================

START:	This labels address zero.
NOP	This does nothing for one clock cycle.
MOVW 0x15	This copies hexadecimal 15 into W, the working register. This sets the delay.
CALL DELAY	This copies 7 (the address labelled by DELAY) into the program counter. This also copies 4 (the return address) onto the stack.
DELAY:	This labels address seven.
SUBW 0x1	This subtracts one from W and stores the answer back into W.
JPZ DONE	If the status register (SR) zero flag is set, this will jump to address 0xA. The zero flag will be set when W has finally counted down to zero.
JMP DELAY	This will set the program counter to the address labelled by REP (address eight).
DONE:	This labels address 0xA.
RET	This copies 4 (the address saved on the stack) into the program counter.
JMP START	This copies 0 (the address labelled by START) into the program counter.

Method Four - Use TMR and PRE

Full details are on the Timers and Interrupts page.