Hi Ronald,

The "addlw 0 - x" instruction is a quick and dirty way of doing an explicit
subtraction from "w".

  addlw 0 - x

is equivalent to:

  w = w + ( 0 - x )
    = w - x

"sublw" is an instruction I try to avoid whenever possible.  That's because:

  sublw  x

is equivalent to:

  w = x - w
    = x + (( w ^ 0x0FF ) + 1 )          ;  This is how the PIC handles sub

I, personally, find this very hard to keep straight in my mind (especially
with what it does to the Status bits).  I also find few instances where
subtracting the contents of w from a constant useful.

Actually, I lie, there is one *very* useful application for "sublw" and that
is to negate the value in "w" and that's done by:

  sublw 0                               ;  Negate the value in "w"

I find there are two real areas you have to understand well in the PIC.  The
first is how PC changes work (goto and tables using PCLATH) and subtraction.

Good Luck,

Myke

>Myke, Keith,
>
>thanks for your help sofar.
>
>While you were cooking up your solutions I devised the following possible
>solution.
>What I did not mention is that inloc and outloc are buffer pointers that
>cycle over the values 0x0c and 0x24.
>
>My solution, if it works which I don't know yet:
>
>                                                        ; if inloc is
within bufmax places of outloc then hold input
>                                                        ; if out > in then
if (out - in) < 5 hold
>                                                        ; else if out < in
then if (in - out) > 19 hold
>          movf  InLoc, W
>          subwf OutLoc, W
>          btfss _c                       ; if C = 1 then inloc > outloc
((out - in) in W)
>            goto        inGTout
>          sublw 5
>          btfsc _c
>            goto        hold
>            goto        RestoreIntStatus        ; else ready
>inGTout:                                        ; test if in-out > 19
>           movf OutLoc, W
>           subwf        InLoc, W
>           sublw        D'19'
>           btfsc        _c              ;
>             goto       RestoreIntStatus        ; else ready
>hold:
>        data_off                                ; no more data please
>
>
>One question about Myke's solution:
>In PIC assembler, using "w" as "temp", this would be:
>
>  movf  outloc, w
>  subwf inloc, w
>  btfss STATUS, C               ;  Skip Next if we don't have a Negative
>   sublw 0                      ;  Make the Negative Positive
>
>>>>>  addlw 0 - 4                   ;  Subtract 4 from the absolute value
>  btfsc STATUS, C
>   goto X                       ;  Value is Greater than 4...
>
>
>I don't understand the addlw 0 - 4 line. It is supposed to substract.
>Shouldn't it read        sublw 4
>
>
>Ronald
>
>

Do you ever feel like an XT Clone caught in the Pentium Pro Zone?