;
; ***************************************************************************
; ***  Bubble Software Parallax to PIC Source Converter. Copyright 1999.  ***
; ***  http://www.picnpoke.com                 email: sales@picnpoke.com  ***
; ***************************************************************************
;
; GOSUB sub, return address (in w)
; Jumps to the address in the variable sub after storing the return address on a 
; stack in general-purpose RAM. When the destination routine is finished, it 
; jumps to a routine called Return, which fetches the return address off the stack, 
; and jumps to the instruction following the one that invoked Gosub. 

; The Gosub/Return mechanism requires exclusive use of the file-select
; register (fsr). If other parts of the program need to use the fsr, 
; copy it to a variable before altering it and restore it when done. 

	P = pic16c55
	#include <16c55.inc>   ; processor assembler definitions
	_CONFIG _xt_osc & _wdt_off & _protect_off
        reset   start

        org     8
sub     Res      d'1'       ; Destination of Gosub.
temp    Res      d'1'       ; Variables for delays
temp2   Res      d'1'       ; used in demo subroutines. 

; Device data and reset vector
        org     0

start        MOVLW d'0'                 ; All outputs for LEDs
             TRIS 6h
             CLRF 6h                    ; Turn LEDs off.
             MOVLW d'31'                ; Point to end of memory
             MOVWF fsr
             MOVLW One                  ; Destination: One. 
             MOVWF sub
             MOVF pc,w                  ; Save program counter. 
             GOTO Gosub                 ; Gosub One. 
             MOVLW d'15'                ; LEDs on.      
             MOVWF 6h
start_loop   DECFSZ temp                ; Short delay. 
             GOTO start_loop
             DECFSZ temp2               
             GOTO start_loop
             CLRF 6h                    ; LEDs off. 
             GOTO $                     ; Endless loop.         

Gosub        MOVWF indirect             ; Return addr on stack.
             DECF fsr                   ; Decrement stack pointer. 
             MOVF sub,w                 ; Destination address to w. 
             MOVWF pcl                  ; Go there. 

             Return  inc     fsr        ; Increment stack pointer <Microchip instruction>
             INCF indirect,w            ; w = return addr + 1. 
             MOVWF pcl                  ; Jump to return addr. 
; GOSUB (cont)
; Demonstration routines to show successful nesting of 
; three  Gosubs. 
One          MOVLW d'1'                 ; LED on.       
             MOVWF 6h
One_loop     DECFSZ temp                ; Short delay. 
             GOTO One_loop
             DECFSZ temp2               
             GOTO One_loop
             CLRF 6h                    ; LEDs off
             MOVLW Two                  ; Destination: Two. 
             MOVWF sub
             MOVF pc,w                  ; Save program counter. 
             GOTO Gosub                 ; Gosub Two. 
             GOTO Return                ; Return. 

Two          MOVLW d'3'                 ; LEDs on.      
             MOVWF 6h
Two_loop     DECFSZ temp                ; Short delay. 
             GOTO Two_loop
             DECFSZ temp2               
             GOTO Two_loop
             CLRF 6h                    ; LEDs off
             MOVLW Three                ; Destination: Three. 
             MOVWF sub
             MOVF pc,w                  ; Save program counter. 
             GOTO Gosub                 ; Gosub Three
             GOTO Return                ; Return. 

Three        MOVLW d'7'                 ; LEDs on.      
             MOVWF 6h
Three_loop   DECFSZ temp                ; Short delay. 
             GOTO Three_loop
             DECFSZ temp2               
             GOTO Three_loop
             CLRF 6h                    ; LEDs off
             GOTO Return                ; Return.

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             end