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 buf
max places of outloc then hold input
                                                        ; if out > in then if (o
ut - 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