Code Examples

Code Examples

The following is extracted from the example multiply routines given as a sample with MPASM assembler. Most of the comments have been stripped for brevity.

Absolute Code becomes Relocatable Code, Calling File and Relocatable Code, Library Routine.

Absolute Code

LIST P=16C54

#INCLUDE "P16C5X.INC"

cblock H '020'

mulcnd ; 8 bit multiplicand

mulplr ; 8 bit multiplier

H_byte ; High byte of the 16 bit result

L_byte ; Low byte of the 16 bit result

count ; loop counter

endc

mpy clrf H_byte

clrf L_byte

movlw 8

movwf count

movf mulcnd,w

bcf STATUS,C ;Clear carry bit

Loop rrf mulplr,F

btfsc STATUS,C

addwf H_byte,F

rrf H_byte,F

rrf L_byte,F

decfsz count,F

goto loop

retlw 0

;*******************************************************

; Test Program

;*******************************************************

start clrw

option

main movf PORTB,w

movwf mulplr ; multiplier (in mulplr) = 05

movf PORTB,W

movwf mulcnd

call_m call mpy ; The result is in F12 & F13

; H_byte & L_byte

goto main

ORG 01FFh

goto start

END

Since an eight-by-eight bit multiply is a useful, generic routine, it would be handy to break this off into a separate object file that can be linked in when required. The above file can be broken into two files: a calling file representing an application and a generic routine that could be incorporated in a library.

Relocatable Code, Calling File

LIST P=16C54

#INCLUDE "P16C5x.INC"

EXTERN mulcnd, mulplr, H_byte, L_byte

EXTERN mpy

CODE

start clrw

option

main movf PORTB, W

movwf mulplr

movf PORTB, W

movwf mulcnd

call_m call mpy ; The result is in H_byte & L_byte

goto main

Reset CODE H'0lFF'

goto start

END

Relocatable Code, Library Routine

LIST P=16C54

#INCLUDE "P16C5x.INC"

UDATA

mulcnd RES l ; 8 bit multiplicand

mulplr RES 1 ; 8 bit multiplier

H_byte RES 1 ; High byte of the 16 bit result

L_byte RES 1 ; Low byte of the 16 bit result

count RES 1 ; loop counter

GLOBAL mulcnd, mulplr, H_byte, L_byte

CODE

mpy

GLOBAL mpy

clrf H_byte

clrf L_byte

movlw 8

movwf count

movf muland, W

bcf STATUS, C ; Clear carry bit

loop rrf mulplr, F

btfsc STATUS, C

addwf H_byte, F

rrf H_byte, F

rrf L_byte, F

decfsz count, F

goto loop

retlw 0

END