;
; ***************************************************************************
; ***  Bubble Software Parallax to PIC Source Converter. Copyright 1999.  ***
; ***  http://www.picnpoke.com                 email: sales@picnpoke.com  ***
; ***************************************************************************
;
; LOOKDOWN key, table
; Searches a table for an element that matches key. If a match 
; is found, Lookdown returns its position in the table in the 
; variable index and a 0 in the w register. If no element matches key, 
; Lookdown returns a 1 in the w register as an error code. 

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

        org     8
key     Res      d'1' ; Element to find. 
index   Res      d'1' ; Position of key in table. 

        org     0

start        MOVLW d'0'                 ; Output to show index on LEDs.
             TRIS 6h
             MOVLW 'G'                  ; Search for byte corresponding to 
             MOVWF key
             CALL Lookdown              ; ASCII G and return index no. 
             MOVF index,w               ; Show index on LEDs hooked to RB.
             MOVWF 6h
             GOTO $                     ; Endless loop. 

; The table to be searched must include the number of searchable elements 
; as element no. 0 (the very first element in the table). 
Table        ADDWF pcl                  
             RETLW d'10'                ; No. of searchable elements. 
             RETLW 'A'                  ; Table of ASCII characters. 
             RETLW 'B'
             RETLW 'C'
             RETLW 'D'
             RETLW 'E'
             RETLW 'F'
             RETLW 'G'
             RETLW 'H'
             RETLW 'I'
             RETLW 'J'

; Locate table element corresponding to key. 
; Return with the corresponding element number in index. Key is not
; altered by Lookdown. Note that when the element is found, :done
; decrements index so that the first element = 0, second = 1...
; just as with the PBASIC instruction. 

Lookdown     CLRW                       ; Retrieve element 0. 
             CALL Table                 
             MOVWF index                ; Copy table length to index. 
Lookdown_loop
             CALL Table                 ; Retrieve element. 
             SUBWF key,0                ; W = key - W <Microchip instruction>
             BTFSC status,z             
             GOTO Lookdown_done         ; If w = 0, we have a match. 
             DECF index                 ; If not, try next lower element.
             MOVF index,w               ; Put index into w for table. 
             BTFSS status,z             ; If zero, element is not in table. 
             GOTO Lookdown_loop         ; Try next lower element
             RETLW d'1'                 ; Error code: element not in table. 
Lookdown_done
             DECF index                 ; index=index-1 to match PBASIC.
             RETLW 0h                   ; Element found: return a 0 in w.

             
             
             end