Mike Keitz wrote:
>
> On Wed, 25 Jun 1997 22:29:54 +0200 "Michael Giuggio (Tel (860)763-6861)"
> <Michael.Giuggio@PE.LSI.LEGO.DK> writes:
> >I apologize for the relatively simplistic question re: RTFM material.
> >I am writing my first PIC '84 program.
> >
> >It maps out to 01BE in length in terms of EPROM used for the program
> >(via the xxxx.LST output file from MPASM).
> >
> >When adding YANF (Yet Another Neat Feature) ... (actually fix a bit of
> >bad
> >logic) the length increases past this length (to 01AD for example).
> >
> >The program now does not function.  It appears that the if I have the
> >functioning program and continuously add nops to the program I can get
> >to a
> >point where the program ceases to function
>
> <...>
>
> What has probably happened is the extra code inserted has moved some of
> the targets of computed jumps (usually used to access RETLW tables) out
> of the first 256 program words.  Unless you set the PCLATH register
> otherwise, ADDWF PCL,f MOVWF PCL, etc. will always land in 256 word
> "page" 0 (PC=00XX).  For example, if you have a table at 0xFE:
>
> ; This instruction is at 0xFE now.  Before adding a neat feature, it was
> at ;  0xF0, and the program worked then.
> gettabl addwf   PCL,f
>         retlw   05
> ; Next instruction is at 0x100.  This is bad.
>         retlw   08
>         retlw   03
>         retlw   04
>
> If you call gettabl with 2 in W, expecting it to return 8, suprise.  What
> will actually happen (if PCLATH is still 0) is the PIC will jump to 0000,
> starting the program over as if a reset had occurred.  Not good.
>
> I find it convenient to move all the RETLW tables and other targets of
> computed gotos up to page 3 with an ORG h'300' directive, then set PCLATH
> to 3 while initializing all the TRIS registers, etc. after reset.  The
> tables will then stay put if code is added or removed from the rest of
> the program.  Or you can put all the tables in first, then org the rest
> of the program to 0x100.

Hi!

Try this: it allows you to put your lookup tables wherever you want 'em.
You can even put them ON the edge of the first page and run over to the
second page.

;===================LOOKUP TABLE
LOOKUP
        MOVWF   OFFSET          ;MOVE
        MOVLW   HIGH    BASEADD ;LOAD HIGH BYTE OF BASE ADDRESS INTO W
        MOVWF   PCLATH          ;HIGH BASEADD INTO PC-LATCH
        MOVLW   LOW     BASEADD ;LOAD LOW BYTE OF BASE ADDRESS INTO W
        ADDWF   OFFSET,W        ;ADD OFFSET TO PC LOW
!!-->>  BTFSC   STATUS,C        ;TEST FOR ROLL-OVER IN PC
!!-->>  INCF    PCLATH          ;INCREMENT PCLATH IF ROLL-OVER DETECTED
        MOVWF   PCL             ;LOW BASEADD INTO PCL

BASEADD RETLW   B'01111101'     ;1 - DUD! DUE TO MOD8 INCREMENTING TO 1
        RETLW   B'01111101'     ;1
        RETLW   B'10101010'     ;2
        RETLW   B'01101010'     ;3
        RETLW   B'11100100'     ;4
        RETLW   B'01111000'     ;5
        RETLW   B'10101100'     ;6
        RETLW   B'01101001'     ;7

--
eric van es
vanes@ilink.nis.za
cape town, south-africa