WHILE/ENDW Example

WHILE/ENDW Example

Directives highlighted in this example are:

• while

endw

Program Functional Description

This example shows the usefulness of directive while to perform a loop while a certain condition is true. This directive is used with the endw directive.

Commented Code Listing

list p=16f877 ;Select the device.

#include <p16f877.inc> ;Include standard header file

;for the selected device.

variable i ;Define the symbol 'i' as a

;variable.

reg_hi equ 20 ;Assign value 20H to label

;reg_hi.

reg_lo equ 21 ;Assign value 21H to label

;reg_lo.

ORG 0000 ;The following code will be

;programmed in reset address 0.

goto start ;Jump to an address whose label

;is 'start'.

shift_right macro by_n ;Beginning of a macro, which

;shifts register data n times.

;Code length generated after

;assembly, varies depending upon

;the value of parameter 'by_n'.

i=0 ;Initialize variable i.

while i< by_n ;Following 3 lines of assembly

;code are repeated as long as

;i< by_n.

bcf STATUS,C ;Clear carry bit.

rrf reg_hi ;reg_hi and reg_lo contains

rrf reg_lo ;16-bit data which is rotated

;right through carry.

i+=1 ;Increment loop counter i.

endw ;End while loop. The loop will

;break here after i=by_n.

endm ;End of 'shift_right' macro.

org 0010 ;My main program starts at 10H.

start ;The label 'start' is equal to

;10H.

shift_right 3 ;Shift right 3 times the 16-bit

;data in reg_hi and reg_lo. This

;is an example. A value 8 will

;shift data 8 times.

goto $

end