please dont rip this site

PIC Microcontroller Memory Method

Table lookups

Mike Keitz says:

...There are two situations where "paging" of the program memory is an issue. For CALL or GOTO, the pages are 800h instructions long [on the more advanced PIC processors]. Most short programs don't need to be concerned with them. [HOWEVER] For writing to PCL, the pages are only 100h instructions long. So [any] 256-value table definitely crosses one of those pages. And the program will crash when the index into the table gets large enough if your table-access code doesn't set up PCLATH properly.

[Here is some code that sets up PCLATH correctly]

   movlw  high   (TABLE_START)
   movwf  pclath
   movf   index,w
   addlw  TABLE_START
   skpnc
    incf  pclath,f
   movwf  pcl
TABLE_START
   retlw  ...

If the table is called such that W is the index:

   addlw  TABLE_START
   movwf  temp
   rlf    known_zero,w
   addlw  high(TABLE_START)
   movwf  pclath
   movf   temp,w
   movwf  pcl
TABLE_START
   retlw ...

Andrew Warren of Fast Forward Engineering - San Diego, California https://geocities.restorativland.org/SiliconValley/2499/ says:

...write table-lookup code [with] automatic page-crossing checks built in.

I generally do it like this:

        ADDWF   PCL

    TABLE1:

        DT      "Test"

        IF ((HIGH ($)) != (HIGH (TABLE1)))
            ERROR "TABLE1 CROSSES PAGE BOUNDARY!"
        ENDIF

P.S. By the way, another common way that a newly-inserted CALL can screw up previously-working code is if the called routine changes the PCLATH register... Or if your previously-working code expected the W register or STATUS flags to remain unchanged across the portion of the program where you inserted your CALL.

Rich Leggitt says:

...something like this should work for arbitrary table of any length located anywhere in the program space.
                ...
                movlw high string       ; point to a string
                movwf look_hi           ; in reality, a macro...
                movlw low string
                movwf look_lo
                call process            ; go process it
                ...

; subroutine to process string at look_hi/look_lo
process         call lookup             ; get a byte (this is the magic)

                ; here, do something with byte in W
                ; also, return if end of string
                ; otherwise...

                goto process            ; do it again

; Jump to address in look_hi/look_lo, which presumably is an RETLW.
; Note pointer post increment.
; Equivalent to: W=*look_ptr++
lookup          movf look_hi,w          ; set PCLATH
                movwf PCLATH
                movf look_lo,w          ; and get PCL
                incf look_lo,f          ; but post inc
                skpnz
                incf look_hi,f
                movwf PCL               ; ok, now jump


See Also:

Interested:


file: /Techref/scenix/lib/mem/tables.htm, 3KB, , updated: 2020/2/15 22:15, local time: 2024/3/29 03:05,
TOP NEW HELP FIND: 
35.153.134.169:LOG IN

 ©2024 These pages are served without commercial sponsorship. (No popup ads, etc...).Bandwidth abuse increases hosting cost forcing sponsorship or shutdown. This server aggressively defends against automated copying for any reason including offline viewing, duplication, etc... Please respect this requirement and DO NOT RIP THIS SITE. Questions?
Please DO link to this page! Digg it! / MAKE!

<A HREF="http://www.massmind.org/Techref/scenix/lib/mem/tables.htm"> Table lookups</A>

After you find an appropriate page, you are invited to your to this massmind site! (posts will be visible only to you before review) Just type a nice message (short messages are blocked as spam) in the box and press the Post button. (HTML welcomed, but not the <A tag: Instead, use the link box to link to another page. A tutorial is available Members can login to post directly, become page editors, and be credited for their posts.


Link? Put it here: 
if you want a response, please enter your email address: 
Attn spammers: All posts are reviewed before being made visible to anyone other than the poster.
Did you find what you needed?

 

Welcome to massmind.org!

 

Welcome to www.massmind.org!

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  .