;**********************************************************************
;   This file is a basic code template for object module code         *
;   generation on the PIC16F873A. This file contains the              *
;   basic code building blocks to build upon.                         *
;                                                                     *
;   Refer to the MPASM User's Guide for additional information on     *
;   features of the assembler and linker (Document DS33014).          *
;                                                                     *
;   Refer to the respective PIC data sheet for additional             *
;   information on the instruction set.                               *
;                                                                     *
;**********************************************************************
;                                                                     *
;    Filename:      xxx.asm                                           *
;    Date:                                                            *
;    File Version:                                                    *
;                                                                     *
;    Author:                                                          *
;    Company:                                                         *
;                                                                     * 
;                                                                     *
;**********************************************************************
;                                                                     *
;    Files required: P16F873A.INC                                     *
;                                                                     *
;                                                                     *
;                                                                     *
;**********************************************************************
;                                                                     *
;    Notes:                                                           *
;                                                                     *
;                                                                     *
;                                                                     *
;                                                                     *
;**********************************************************************


    list        p=16f873a   ; list directive to define processor
    #include    <p16f873a.inc>  ; processor specific variable definitions
    
    __CONFIG _CP_OFF & _WDT_OFF & _BODEN_OFF & _PWRTE_ON & _RC_OSC & _WRT_OFF & _LVP_ON & _CPD_OFF

; '__CONFIG' directive is used to embed configuration data within .asm file.
; The labels following the directive are located in the respective .inc file.
; See respective data sheet for additional information on configuration word.

;***** VARIABLE DEFINITIONS (examples)

; example of using Shared Uninitialized Data Section
INT_VAR     UDATA_SHR   0x71   
w_temp      RES     1       ; variable used for context saving 
status_temp RES     1       ; variable used for context saving
pclath_temp RES     1       ; variable used for context saving


; creating copies of interupt context saving variables in bank1
INT_VAR1    UDATA_SHR   0xF1
w_temp1     RES     1       ; variable used for context saving 


; example of using Uninitialized Data Section
TEMP_VAR    UDATA_SHR   0x20 ; explicit address specified is not required
temp_count  RES     1        ; temporary variable (example)


;**********************************************************************
RESET_VECTOR    CODE   0x0000 ; processor reset vector
    nop                       ; nop  for icd
    pagesel start
    goto    start             ; go to beginning of program


INT_VECTOR      CODE   0x0004 ; interrupt vector location

INTERRUPT

    movwf   w_temp          ; save off current W register contents
    movf    STATUS,w        ; move status register into W register
    bcf STATUS,RP0          ; select bank0
    movwf   status_temp     ; save off contents of STATUS register
    movf    PCLATH,w        ; move pclath register into w register
    movwf   pclath_temp     ; save off contents of PCLATH register


; isr code can go here or be located as a call subroutine elsewhere

    bcf STATUS,RP0          ; select bank0
    movf    pclath_temp,w   ; retrieve copy of PCLATH register
    movwf   PCLATH          ; restore pre-isr PCLATH register contents
    movf    status_temp,w   ; retrieve copy of STATUS register
    movwf   STATUS          ; restore pre-isr STATUS register contents
    swapf   w_temp,f
    swapf   w_temp,w        ; restore pre-isr W register contents
    retfie                  ; return from interrupt

MAIN_PROG       CODE

start

    nop                     ; code starts here (example)

; remaining code goes here

    goto $

    END                       ; directive 'end of program'

